Infrared Observations of the Spiral Galaxy NGC 891. Queen s University

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1 Infrared Observations of the Spiral Galaxy NGC 891 by Cynthia Whaley A thesis submitted to the Department of Physics, Engineering Physics & Astronomy in conformity with the requirements for the degree of Master of Science Queen s University Kingston, Ontario, Canada August 27 Copyright c Cynthia Whaley, 27

2 Abstract This thesis is a detailed, multi-waveband study of the inner 14 kpc of the famous spiral galaxy, NCG 891. The primary data have come from the Infrared Space Observatory s Camera. These data are images of the galaxy in 9 different mid-infrared wavebands. We have supported these data with archived data from the Spitzer Infrared Array Camera in 4 similar wavebands. Surface brightness contour maps of the galaxy were created and examined to determine where the mid-infrared emitters are located with respect to the galactic plane. We have determined that the main mid-infrared emission, due to warm dust and PAHs, lies in a thin disk of width 7-8 pc, but has faint emission that reaches up to about 2.3 kpc into the halo. The infrared spectral energy distribution (SED) for four environments in NGC 891 were created from the above mentioned wavebands as well as measurements from Spitzer s Multiband Imaging Photometer (3 Far-Infrared wavebands), the Two Micron All Sky Survey J, H, and K near-infrared wavebands, and the Sub-millimeter Common User Bolometer Array 450 and 850 µm bands. These spectra were fit with a SED model created by Frederic Galliano, and the physical properties of these environments were computed. The maps and SED show that while there is a relatively large amount of dust in NGC 891 s halo, there is a depletion of PAHs beyond 2.3 kpc from the mid-plane. This is only the fourth galaxy to date that has PAH emission discovered in the halo, and it is the first in which the SED has been modeled for the halo. ii

3 Acknowledgments First I would like to thank those who helped to supply the NGC 891 data for this thesis. I thank Dr. R. Swaters for supplying us with the HI FITS file, Dr. George Bendo for supplying the MIPS FITS files, and Dr. Manolis Xilouris for the SCUBA 450 & 850 µm images. I thank Dr. Frederic Galliano for contributing the SED model and colour corrections, as well as for his help with all of my questions. I also wish to thank Dr. Suzanne Madden for her scientific input. Most importantly, I thank my supervisor Dr. Judith Irwin for her help all along the way with this thesis. Her teaching and guidance have been invaluable. On a personal level, I wish to thank my family; My mother, Dr. Iris Jackson, my sister, Elisabeth Whaley, my father, Dr. Charles Whaley, and my life partner, Vincent Piette. Their loving support has been a large factor in my success. Cynthia Whaley Queen s University August 27 iii

4 Contents Abstract ii Acknowledgments iii List of Tables vii List of Figures ix Chapter 1 Introduction Spiral Galaxies PAHs Dust NGC The Disk The Halo Organization of Thesis Chapter 2 Observations ISO s ISOCAM Spitzer s IRAC Spitzer s MIPS Published Observations SCUBA MASS iv

5 2.5 Summary of Observations Chapter 3 Data Reduction ISOCAM Data Reduction Procedure Estimating Error IRAC Data Reduction More Modifications to the Data Chapter 4 Results: Distribution of IR Emission Images and Contour Maps ISO Contour Maps Supporting ISOCAM results: Spitzer s IRAC Far Infrared Emission: Spitzer s MIPS and SCUBA Looking for Correlations Between MIR Emission and Other Wavelengths Surface Brightness Profiles Minor Axis Profiles Major Axis Profiles Chapter 5 Results: The Infrared Spectral Energy Distribution The IR Spectrum Modeling the Spectrum Sources of emission Sources of Excitation How do they interact? The Modeling Results Chapter 6 Discussion The MIR Maps Features Seen in All MIR Images: Halo Features Seen in All MIR Images: Disk The 3.6 and 4.5 µm Wavebands v

6 6.1.4 The 5.8, 6.0 and 6.8 µm Wavebands The 7.7 µm Waveband The 6.7 & 14.3 µm Wavebands Correlations in the Maps The Minor Axis Profiles The SED Mass of the Dust Fraction of PAHs Fraction of ionized PAHs The Parameter α The Minimum and Maximum Heating Intensities The Extinction Parameter Mass of the old stars The Big Picture Chapter 7 Summary and Conclusions Major Findings The IR distribution in NGC The SED of NGC Future Work Appendix A List of Abbreviations 119 Bibliography 121 vi

7 List of Tables 1.1 Basic Galaxy Parameters of NGC Observing and map parameters. See explanation in text, Section 2.1, for each parameter IRAC Observing Parameters MIPS Observing Parameters MASS Isophotal Bandpasses and Fluxes for 0 Magnitude (Cohen et al. 23) Statistics on average total images for 1998 data. All values are in mjy/pixel, for 6 6 pixels Statistics on average total images for 1997 data. All values are in mjy/pixel, for 6 6 pixels statistics of the sky regions before sky subtraction Sky Statistics After Sky-subtraction Background Statistics for Sky-subtraction of IRAC Maps Shifting of ISO Data Dominant Source of Emission for Each Waveband ISO, IRAC & MIPS Image Statistics MIR Fluxes for SN 1986J Exponential and Gaussian Fit Parameters Characteristic Scale Height for Each Waveband dynamic range for minor axis profiles vii

8 4.7 distance above the galactic plane for which the emission is 3σ dynamic range comparison Dynamic Ranges and Distance from the Galactic Plane for Figure Regions of NGC 891 Chosen for Spectral Analysis Flux Measurements for Centre of NGC 891 and Corresponding Luminosity Flux Measurements of South Disk of NGC 891 and Corresponding Luminosity Flux Measurements of North Disk of NGC 891 and Corresponding Luminosity Flux Measurements of the Halo of NGC 891 and Corresponding Luminosity Modeling Parameter Results for Different Regions of NGC Mass of PAHs in the 4 Measured Regions of NGC Composition and Extent of NGC 891 s Halo A.1 List of Abbreviations A.2 Bibliography Abbreviations viii

9 List of Figures 1.1 NGC 891 Digitized Sky Survey (DSS) optical image from Skyview The spectral energy distribution of NGC 891, obtained from the NASA Extragalactic Database (NED) Diagram of a spiral galaxy viewed from the edge-on (from Stephane Courteau s class notes, 26). Sizes given are typical approximations for the Milky Way Comparison MIR spectrum of the central position of the galaxy M 83 from the ISOCAM Circular Variable Filter observations of Vogler et al. (25). The major features are labeled and the bands used in the ISO observations of NGC 891 are indicated at the top Drawing of typical PAH molecules. Green are carbon atoms, and yellow are hydrogen atoms, (NASA, 25) NGC Contours of 6.7 µm emission overlaid onto an optical image. (See Irwin & Madden 27 for contour levels.) Here 10 corresponds to 0.5 kpc Modeled optical properties of the neutral and cationic PAHs by Draine & Li (27). a is the effective radius of the PAHs, and Q abs (a,λ) is their absorption efficiency. It is defined such that the cross-section of a PAH is πa 2 Q abs (a,λ). Ionized PAH absorption is in blue, and neutral PAH absorption is in red Diagram of ISOCAM (Blommaert et al. 23) ix

10 2.2 An example of the PSF that came with the ISO data. This graph is for the 11.3 µm band. The full width at half maximum (FWHM) of the graph gives the resolution. The PSF graphs for the other wavebands are similar, and thus not shown Some examples of the different on-source pointings of ISOCAM on NGC 891. In this example, from 1 to 4, we see the pointing go from the southern end of the galaxy, to the centre regions of the galaxy, and finally to the northern end of the galaxy. Thus, the different pointings allow data from the entire galaxy to be collected and later mosaicked to make a complete picture of the whole galaxy. The white arrow points along the dead pixel column Spectral transmission response functions for the ISO wavebands. The x- axis is wavelength in µm, and the y-axis is the fraction of light that can pass through the filter, with 1 being complete transparency and 0 being completely opaque Spectral transmission response functions for the ISO wavebands. The x- axis is wavelength in µm, and the y-axis is the fraction of light that can pass through the filter, with 1 being complete transparency and 0 being completely opaque An illustration of an ISO data cube An example of the LW dark frame, after a 2.1 s integration time. Note the evident line pattern (Blommaert et al. 23) Sky statistics for each waveband (1997 data set) that were used for the skysubtraction. The x-axis is the intensity value in mjy/pix and the y-axis is the fraction of total pixels that have that intensity value Sky statistics for each waveband (1997 data set) that were used for the skysubtraction. The x-axis is the intensity value in mjy/pix and the y-axis is the fraction of total pixels that have that intensity value x

11 3.5 Sky statistics for each waveband (1998 data set) that were used for the skysubtraction. The x-axis is the intensity value in mjy/pix and the y-axis is the fraction of total pixels that have an intensity value that falls in each 0.1 mjy/pix bin ISO emission contours (left) of the wavebands that select PAH emission, and their respective error maps (right). The crosses denote foreground stars and the triangle denotes the location of SN1986J. The contours are 0.9, 0.013, 0.017, 0.022, 0.04, 0.06, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, & 0.87 mjy arcsec 2 for 6.0 µm, 0.8, 0.013, 0.017, 0.021, 0.035, 0.05, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0, & 2.5 mjy arcsec 2 for 7.7 µm, and 0.019, 0.028, 0.037, 0.046, 0.07, 0.1, 0.2, 0.3, 0.5, 0.7, 0.9, 1.1, 1.28 mjy arcsec 2 for 11.3 µm ISO emission contours overlayed on a grey scale representative of the intensity of emission (left) of the wavebands that select the continuum, and their respective error maps (right). Symbols as in Figure 4.1. Contours are 0.5, 0.7, 0.9, 0.011, 0.02, 0.03, 0.05, 0.07, 0.1, 0.15, 0.2, 0.3, 0.4 mjy arcsec 2 for 4.5 µm, 0.011, 0.017, 0.023, 0.028, 0.05, 0.07, 0.1, 0.2, 0.3, 0.5, 0.7, 0.94 mjy arcsec 2 for 6.8 µm, 0.012, 0.018, 0.024, 0.03, 0.05, 0.07, 0.1, 0.2, 0.3, 0.4, 0.5, mjy arcsec 2 for 9.6 µm, and 0.02, 0.03, 0.04, 0.05, 0.07, 0.09, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 mjy arcsec 2 for 14.9 µm Emission contours overlayed on a grey scale representative of the intensity of emission (left) of the ISOCAM wide bands, and their respective error maps (right). Symbols as in Figure 4.1. Contours are 0.8, 0.011, 0.015, 0.019, 0.03, 0.05, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.38 mjy arcsec 2 for 6.7 µm and 0.015, 0.022, 0.03, 0.037, 0.05, 0.1, 0.3, 0.5, 0.7, 0.9, 1.15 mjy arcsec 2 for 14.3 µm ISO contours overlayed on the DSS optical image. Symbols and contours as in Figure xi

12 4.5 IRAC contour maps overlayed on a grey scale representative of the intensity of emission. Here the contours are 0.73, 0.011, 0.015, 0.018, 0.03, 0.05, 0.07, 0.15, 0.25, 0.3, 0.4, 0.5 mjy arcsec 2 for 3.6 µm, 0.47, 0.71, 0.95, 0.012, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4 mjy arcsec 2 for 4.5 µm, 0.5, 0.75, 0.01, 0.012, 0.02, 0.04, 0.06, 0.1, 0.3, 0.5, 0.7, 0.77 mjy arcsec 2 for 5.8 µm, and 0.59, 0.81, 0.011, 0.013, 0.03, 0.05, 0.1, 0.3, 0.7, 1.1, 1.6 mjy arcsec 2 for 8.0 µm MIPS contour maps (created from data obtained from George Bendo) overlaid on the DSS optical image for spatial reference. Contours are 0.01, 0.015, 0.02, 0.025, 0.035, 0.05, 0.1, 0.5, 1, 1.5, 2, 2.9 mjy/sr for 24 µm, 0.073, 0.11, 0.18, 0.3, 0.5, 1, 2, 3, 5, 7, 10, 15, 17.8 for mjy/sr 70 µm, and 0.41, 0.62, 0.82, 0.01, 0.03, 0.04, 0.05, 0.07, 0.1, 0.15, 0.2, 0.25, 0.32 mjy/sr for 160 µm. Recall the spatial resolution is 6, 18 and 40 for the 24, 70 and 160 µm wavebands, respectively ISO 4.5 µm contours (symbols and contours as in Figure 4.2) overlayed on 2MASS K band image ISO 7.7 µm contours (symbols and contours as in Figure 4.1) overlayed on SCUBA 450 µm image (top) and MIPS 24 µm image (bottom). Maps on the right highlight the halo emission Minor axis radial surface brightness profiles for wavebands containing PAH emission (red). With Exponential (green) and Gaussian (blue) fits. For an example we have zoomed into the 7.7 µm profile to better see the fits in the wing region. This example is representative for all of the ISO wavebands Minor axis radial surface brightness profiles for wavebands containing continuum emission (red). With Exponential (green) and Gaussian (blue) fits Zoom-in of the 7.7 µm minor axis profile. The X denotes the position and intensity at which the intensity is 3σ Minor axis profiles of NGC 5907 and NGC 891 in 6.7 µm. The x-axis is in arcseconds, with zero being the centre of the galaxy. Flux density has been normalized to compare widths only xii

13 4.13 Minor axis surface brightness profiles for 3 of the IRAC wavebands (red). For the similar ISO wavebands, their minor axis profiles have been overlayed (green) Minor axis surface brightness profiles for 7.7 µm (red), 2MASS K band (green), 450 µm SCUBA band (blue) and 24 µm MIPS band (purple). The bottom is a zoom-in of the wing region. The X s denote the 3σ level for each profile. All of these data are smoothed to Minor axis surface brightness profiles for 7.7 µm (red), H I emission from Swaters (1997) (green) and 850 µm SCUBA band (blue). All of these data are smoothed to Major axis surface brightness profiles for the ISO wavebands that select PAH emission. Positive values on the x-axis refer to the northern half of the galaxy, zero is at the centre of the galaxy, and negative values are to the south. The dashed region refers to the data that have been affected by the dead pixel column Major axis surface brightness profiles for the 4.5 µm ISO waveband. This is the distribution of the old stars.positive values on the x-axis refer to the northern half of the galaxy, zero is at the centre of the galaxy, and negative values are to the south Major axis surface brightness profiles for the ISO wavebands that select mainly dust continuum emission (except 14.3 µm which does contain emission from the 12.7 µm PAH band). Positive values on the x-axis refer to the northern half of the galaxy, zero is at the centre of the galaxy, and negative values are to the south. The dashed region refers to the data that have been affected by the dead pixel column xiii

14 5.1 Top: CVF Spectrum of NGC 891 taken with CAM04 of ISOCAM. Bottom: Low resolution spectrum of NGC 891 created from ISOCAM and IRAC images. Both are for the centre of the galaxy, but the top one covers a larger area of the galaxy than the bottom one. The boxes indicate the width of the filters and the points denote the flux measured NGC 891 with red boxes indicating the locations in which the flux was measured to create the spectra Spectrum created from the mean galactic flux of NGC 891 s centre. Data are from 2MASS, ISO, IRAC, MIPS and SCUBA. The x error bars represent the filter wavelength range, and the points represent the fluxes measured in each waveband. Data found in Table Spectrum created from the mean galactic flux of a region in NGC 891 s southern disk. Data are from 2MASS, ISO, IRAC, MIPS and SCUBA. The x error bars represent the filter wavelength range, and the points represent the fluxes measured in each waveband. The data are found in Table Spectrum created from the mean galactic flux of a region in NGC 891 s northern disk. Data are from 2MASS, ISO, IRAC, MIPS and SCUBA. The x error bars represent the filter wavelength range, and the points represent the fluxes measured in each waveband. The data are found in Table Spectrum created from the mean galactic flux of a region in NGC 891 s halo. Data are from 2MASS, ISO, IRAC, MIPS and SCUBA. The x error bars represent the filter wavelength range, and the points represent the fluxes measured in each waveband. The data are found in Table Dust/PAH SEDs exposed to different radiation densities, G, (equivalent to U defined in Equation 5.4). These curves are normalized to the total bolometric luminosity (Zubko et al. 24) Spectral Energy Distribution: Central region of NGC 891 fit with the model described in Section 5.2. The yellow line represents the stellar contribution, the red line represents the dust contribution, the grey line is the total line of best fit and the blue line is the SED without extinction xiv

15 5.9 Spectral Energy Distribution: Disk region in southern half of NGC 891 fit with the model described in Section 5.2. The yellow line represents the stellar contribution, the red line represents the dust contribution, the grey line is the total line of best fit and the blue line is the SED without extinction Spectral Energy Distribution: Disk region in northern half of NGC 891 fit with the model described in Section 5.2. The yellow line represents the stellar contribution, the red line represents the dust contribution, the grey line is the total line of best fit and the blue line is the SED without extinction Spectral Energy Distribution: Halo region of NGC 891 fit with the model described in Section 5.2. The yellow line represents the stellar contribution, the red line represents the dust contribution, the grey line is the total line of best fit and the blue line is the SED without extinction xv

16 Chapter 1 Introduction The goal of this thesis is to provide detailed infrared observations of the nearby edge-on spiral galaxy, NGC 891 (Figure ). We will study the morphology of the disk and halo of this galaxy, and we will also determine some of its physical conditions by creating and modeling its infrared spectrum. This will be the first such study of this galaxy that brings together infrared data from a variety of telescopes. Before going into detail about what we have done, some motivation and background information are in order. Galaxies 2 can be observed at all electromagnetic wavelengths. Figure 1.2 shows NGC 891 s complete spectrum. Each wavelength can provide very specific information about the source. For instance, in radio waves we can observe neutral hydrogen gas because Hydrogen has a spectral line at λ = 21 cm (see under H I in Figure 1.2). In optical-uv waves at λ = nm 3, we can observe ionized Hydrogen gas (H II) (under UV in Figure 1.2). Therefore, by targeting a specific wavelength regime, we can extract specific information about the composition of a source. Cosmic dust emits mostly in the infrared (IR) and sub-millimeter wavelength range (between 0 and 3 on the x-axis of Figure 1.2). In this thesis we will focus on observations in this wavelength range (about λ = 1-10 µm). We will present IR data that provide 1 Note that throughout this thesis, coordinates on the sky are given in Right Ascension (RA) and Declination (DEC). The units for these are hours, minutes, seconds for RA, and degrees, arcminutes, arcseconds for DEC. 2 See next section. 3 The Hydrogen Balmer α line. The amount of energy that ionizes Hydrogen is quite close to the amount of energy that can create the Balmer line. Thus when the Balmer line is observed we can conclude that ionized Hydrogen is also present. 1

17 DECLINATION (J20) RIGHT ASCENSION (J20) Figure 1.1: NGC 891 Digitized Sky Survey (DSS) optical image from Skyview. Figure 1.2: The spectral energy distribution of NGC 891, obtained from the NASA Extragalactic Database (NED). 2

18 information on the physical properties of the cool stars, dust and molecules in this galaxy. Specifically, large, planar molecules called Polycyclic Aromatic Hydrocarbons, or PAHs, emit strongly in the mid-infrared (MIR) (about λ = 3-15 µm). PAHs are discussed further in Section 1.2. The next section will describe the components of a spiral galaxy. Following that Section 1.2 will go into more detail about PAH molecules and their importance for galactic observations. Then Section 1.3 will describe dust particles in galactic observations. Section 1.4 will inform the reader about what was previously known about NGC 891, focusing on both the disk and the halo of this galaxy. 1.1 Spiral Galaxies All spiral galaxies are composed of a central galactic bulge, a disk that contains the spiral arms, and a halo, which is a large, approximately spherical, region surrounding the disk and bulge (Carrol & Ostlie, 1996). Figure 1.3 illustrates a typical spiral galaxy, and the values given for sizes are approximations for the Milky Way (MW). The bulge is a dense region containing stars and possibly a super-massive black hole at its nucleus (Carroll & Ostlie, 1996). The disk also contains stars, but these are generally younger stars than those found in the bulge. There are also large numbers of molecular clouds and large amounts of dust in the disk region. The halo contains globular star clusters 4 and dark matter 5. Neutral hydrogen gas is often found in a much larger, thicker disk than the optical disk, and is thus said to extend into the halo of the galaxy. In this thesis the halo is considered to be anywhere 1 kpc above the plane of the galaxy. The halos of spiral galaxies are not commonly studied because emission from these regions is of low intensity. However, studying the halo is helpful for understanding the dynamics of the material in the galaxy. For instance, there are supernovae-driven outflows that can launch material from the interstellar medium (ISM) out of the disk and into the halo. 4 Globular clusters are spherical, dense, clusters of old stars that are found in the halos of galaxies. 5 Dark matter is matter that interacts gravitationally within the galaxy, but cannot be seen because it emits and absorbs no electromagnetic radiation. 3

19 Figure 1.3: Diagram of a spiral galaxy viewed from the edge-on (from Stephane Courteau s class notes, 26). Sizes given are typical approximations for the Milky Way. We will focus our attention on the halo of NGC 891 for part of this thesis. As this galaxy is seen edge-on, it is an ideal candidate for halo emission studies because it is much easier to view emission above and below the plane of the disk compared to galaxies seen face-on. It has been observed by many authors at many different wavelengths. We will be doing the most detailed study of this galaxy at IR wavelengths, and we will compare our results to previously published results in other wavelengths. 1.2 PAHs Unidentified infrared bands (UIBs) are broad emission features that dominate the MIR spectrum of many astronomical sources. For example, Figure 1.4 shows a typical MIR spectrum of a galaxy which exhibits the large emission bands known as UIBs. The most common model for the source of these bands is the vibrational modes of a family of molecules called polycyclic aromatic hydrocarbons, or PAHs (Figure 1.5). PAHs are large planar molecules composed of about 50 to 1 carbon (C) atoms. Polycyclic means that the carbon atoms are arranged in multiple loops. Aromatic refers to the kind of strong bonds 4

20 LW3 LW1 LW2 LW6 LW5 LW4 LW7 LW mu PAH LW9 7.7 mu PAH 11.3 mu PAH 6.2 mu PAH 8.6 mu PAH [ArII] Spectrum of M lambda (micron) Figure 1.4: Comparison MIR spectrum of the central position of the galaxy M 83 from the ISOCAM Circular Variable Filter observations of Vogler et al. (25). The major features are labeled and the bands used in the ISO observations of NGC 891 are indicated at the top. that exist between the carbon atoms (called π bonds 6 ). Hydrocarbons are molecules that contain hydrogen and carbon atoms. While the PAH model for UIBs is still not completely confirmed 7, most authors accept PAHs as the best model (Smith et al. 27, Draine & Li 27), and we will do the same. Thus, for the rest of this thesis, we assume that PAHs are the source of the UIBs. PAHs are about 10 angstroms in diameter (Peeters, 24a), and are the largest molecules found in space. They might also be the most abundant organic molecule in space. It is very possible that PAHs are pre-cursors to life. They can be accreted onto icy grains in dense clouds in space, which can then be incorporated into a meteorite or comet and be delivered to a planet. A theory called the PAH world hypothesis proposes that DNA and RNA molecules originally formed from PAH molecules in Earth s primordial sea (Ehrenfreund, 26). This theory is supported by the evidence that PAHs excited by UV photons can become soluble in water and organize themselves in stacks. When in this form, the PAH ring separation is the same size as that of RNA molecules. Thus, the study 6 The strength of a C - C π bond is between a single covalent bond and a double covalent bond. It is due to there being a delocalized electron which is free to cycle around the loop 7 other possible candidates include hydrogenated amorphous carbon, quenched carbon composites, coal and nanodiamonds (Peeters et al. 24) to name a few. 5

21 Figure 1.5: Drawing of typical PAH molecules. Green are carbon atoms, and yellow are hydrogen atoms, (NASA, 25). of these molecules is currently a very exciting field. PAHs are believed to be formed in the envelopes of stars that are in the late stages of evolution. These stars are rich in carbon, and inject their material into the ISM via a poorly-understood mechanism. This ejection may be related to the slow pulsations of the star and the formation of dust in the envelope of the star, which is pushed out by radiation pressure. Laboratory experiments have shown that PAHs may also be produced by bombardment of interstellar ices with galactic cosmic ray particles (Kaiser et al. 1998). PAH molecules are stochastically heated by single photon events. That is, each molecule gets excited when it absorbs a single optical-uv photon. It then de-excites by emitting photons at lower energies in the MIR. In Figure 1.4, we see the broad emission features typical of PAHs. The strongest PAH features are located at 6.2, 7.7, 8.6, 11.3 and 12.7 µm. In the same figure, at the top, we show the wavelength range that the Infrared Space Observatory (ISO) filters cover. The ISO narrow band filters at 6.0 (LW4 8 ), 7.7 (LW6), and 11.3 (LW8) µm isolate some of these PAH spectral features very well. The Spitzer Space Telescope also has coverage in this wavelength regime. Some of its filters are very similar to ISO s, whereas some are wider. We will discuss ISO and the other IR observatories further in Chapter 2 and we will see why ISO, in conjunction with Spitzer, is an ideal instrument for making galactic MIR observations. 8 LW stands for Long Wave, and is ISO s way of naming the various filters. 6

22 PAH excitation (and emission) can occur in two kinds of environments: circumstellar regions and interstellar regions. In circumstellar regions, the PAHs are excited by the optical and UV radiation from the nearby star(s). For example, PAHs have been detected in post-agb stars 9, planetary nebulae, and H II regions associated with massive star formation. While PAH molecules are relatively stable, they are destroyed by very intense UV photons and higher energy (E) photons (E 6.9 ev will de-hydrogenate a PAH molecule (Allamandola, 1989)). Thus, in starbursting regions 10, where high-energy photons are ubiquitous, PAHs are observed only where they are close enough to a star to be excited but far enough away to not be destroyed. For interstellar regions, PAHs are excited by the optical and UV photons that permeate the interstellar radiation field. Examples of PAH sources in the interstellar regions include reflection nebulae, and the diffuse ISM. When we observe extragalactic sources such as NGC 891, we are seeing both kinds of environments. For extragalactic sources, PAH emission features have been observed in elliptical and starbursting galaxies. They are usually associated with star forming regions (due to the abundance of UV radiation there). However, Irwin & Madden (26) found PAH emission in the halo of the spiral galaxy NGC 5907 (Figure 1.6). This was the first case of PAHs being found in the halo of an external galaxy, well above the disk, where there are no obvious sources of excitation. Engelbracht et al. (26) then found a PAH halo in the nearby starbursting galaxy M82, extending up to 6 kpc from the mid-plane. Furthermore, Irwin et al. (in preparation) found halo PAH emission in NGC These PAHs were observed up to 8.5 kpc from the mid-plane. PAHs have been observed in unlikely places before: Uchida et al. (1998) found MIR emission features in the reflection nebula vdb 133. This nebula is located in a stellar system that has a very low fraction of UV to total flux. Since it was believed that sufficient UV radiation is required to excite the PAHs and the associated continuum, observations like these have allowed for the refining of the models for MIR emission. In the Milky Way, PAH features are responsible for 20-30% of the Galactic IR radiation, and about 15% of cosmic carbon is actually locked up in PAH molecules (Peeters, 9 An AGB star is a star on the Asymptotic Giant Branch of the Hertzsprung-Russell diagram. 10 Starbursting regions are regions where there is an abnormally large amount of star formation occurring. 7

23 Figure 1.6: NGC Contours of 6.7 µm emission overlaid onto an optical image. (See Irwin & Madden 27 for contour levels.) Here 10 corresponds to 0.5 kpc. 24a). Because the PAH emission process is relatively complex, it is possible that the PAH spectrum could be a good indicator of the physical conditions in the environment in which the PAHs are located. Determining physical conditions is made possible from MIR spectral observations from many different kinds of astronomical sources and environments. Generally, most PAH features are similar no matter what the source. However, the PAH feature intensity ratios can vary depending on the astronomical source. The presence and relative strength of the PAH features are generally thought to trace star formation on a galactic scale. Studies of the 7.7 µm-to-continuum flux ratio show that it, along with the [OIV]/[NeII] flux ratio, can be used to distinguish between active galactic nuclei (AGN) and starburst galaxies. This can be done by comparing the ratios with ratios in already known AGN and starbursts (Peeters 24b). Additionally, the ratio of the 6.2, 7.7, and 8.6 µm bands to the 11.3 µm band varies significantly with the fraction of ionized PAHs (Figure 1.7) (Galliano, 26). Figure 1.7 illustrates how positive, singly ionized PAHs will have stronger emission in the emission 8

24 Figure 1.7: Modeled optical properties of the neutral and cationic PAHs by Draine & Li (27). a is the effective radius of the PAHs, and Q abs (a,λ) is their absorption efficiency. It is defined such that the cross-section of a PAH is πa 2 Q abs (a,λ). Ionized PAH absorption is in blue, and neutral PAH absorption is in red. bands between 6 and 9 µm. This is because the vibrational modes responsible for those emission bands are due to carbon-carbon (C-C) stretching. C-C stretching is intrinsically weaker for neutral PAHs and stronger for ionized PAHs, hence the difference in the spectrum between 6 and 9 µm. Conversely, the 3.3 and 11.3 µm emission features are due to carbonhydrogen (C-H) stretching and bending modes. These modes are not different for neutral and ionized PAHs. It is clear that PAH features can be used as a probe of the physical environment of the source, and their potential for being so used is only starting to be realized. With the recent and unexpected discovery of PAHs in the halo of NGC 5907, important questions are raised as to how these large molecules can be ejected from the disk and reach such high altitudes above the plane. Furthermore, the fact that NGC 5907 is a galaxy with a low star formation rate (SFR) and has only a radio continuum halo 11 exacerbates this problem, as there is otherwise little evidence for other gases in its halo. We 11 The primary form of emission in a radio continuum halo is synchrotron radiation. Thus, galaxies with a radio continuum halo must be surrounded with cosmic rays and magnetic fields (Duric et al. 1998). It seems unusual that NGC 5907 has a halo in PAHs and radio continuum, but no other materials. 9

25 have therefore chosen the very well-known galaxy NGC 891 to do a similar study but with more IR wavebands for imaging and constructing a spectrum. As will be further discussed in Section 1.4.2, there are many observations showing that NGC 891 has a halo with dust, H I and diffuse ionized gas. NGC 891 will provide a strong contrast in halo environment to NGC 5907 and will allow us to further investigate whether halo PAH emission is common in galaxies and/or whether it prefers low or high star formation environments. Also, by observing where PAH emission is located in the galaxy, we can see if extraplanar PAHs are correlated with the gas & dust distribution in the halo. This can answer questions about the processes that eject material from the disk and into the halo. These observations can also help to determine what physical conditions are required to excite PAHs. We will be looking for correlations between MIR emission from NGC 891 and its published Hα, H I, and dust maps - the results of which are presented in Section for the inner 14 kpc of NGC Dust Dust is one of the major components of the ISM. It can be described by its composition, morphology, size distribution, and the abundances of its elemental constituents. Dust grains are usually graphite (composed of carbon atoms) or silicate particles, but can also contain magnesium, nitrogen, oxygen, iron and other metals. We will be assuming a dust composition of bare graphite and silicate (MgFeSiO 4 ) grains in this thesis, as it is a simple model that works well for modeling the MW (Zubko et al. 24). Dust grains in galaxies affect emission in a few ways. Dust causes extinction of light as it absorbs optical, UV, and X-ray radiation and re-radiates it thermally at IR and submm wavelengths. It also causes reddening of the light emitted by the galaxy by scattering the light. The red (longer-wavelength) light is scattered preferentially towards the observer, while the radiation at shorter wavelengths is scattered to larger angles. Hence, the galaxy looks redder when there is a lot of dust. In spiral galaxies, dust is usually confined to a thin disk along the plane of the galaxy, 12 Only the central 14 kpc of the galaxy are presented because it is only that region which is common to all of the observations. ISOCAM images contained only this region. 10

26 called a dust lane (seen in Figure 1.1). However, dust structures vertical to the plane of the galaxy have also been observed in a few near-by galaxies, including NGC 891. This will be elaborated upon in Section 1.4. Often dust grains are distinguished by their size. There are very small grains (VSGs), and large grains or classical grains. VSGs are of the size 10 nm (Li, 24). They are excited in the same way as PAHs (by a single optical-uv photon), however they are not destroyed as easily as PAHs when there are high energy photons around. It is for this reason that VSGs are associated with star forming regions. Thus, when we observe VSGs, they usually have a higher temperature than large grains. The dust is responsible for the underlying continuum in Figure 1.4. We can see in this figure that the spectrum rises as it goes to longer wavelengths; this is the left wing of the dust continuum which is approximately a black body curve that peaks in the far-infrared. 1.4 NGC 891 The close proximity and nearly edge-on inclination of NGC 891 (Figure 1.1) make it ideal for extra-planar emission studies. This galaxy is believed to be similar to the MW in many respects; it is similar in B-band (λ = 442 nm) luminosity, rotational velocity and Hubble type (Sb). However, it is 2.5 times brighter in the IR (Scoville et al. 1993), which indicates that it has more dust, and therefore probably also has a higher SFR than the MW 13. NGC 891 is located at a distance of 9.6 Mpc (Strickland et al. 24), and it is a non-interacting major member of the NGC 1023 group. Table 1.1 summarizes the physical characteristics of this galaxy. NGC 891 is a very well-studied galaxy, and this section will describe its previous observations The Disk A study of the X-ray point sources in NGC 891 shows that it is probably a starbursting galaxy in a quiescent state. This means it has regular on-going star-formation like the MW, but it is slightly more active (Temple et al. 25). In comparisons with 16 other 13 SFR is often calculated from the IR luminosity because this is often light from stars that has been re-radiated by dust. 11

27 Table 1.1: Basic Galaxy Parameters of NGC 891 parameter value reference Hubble type Sb NED a RA(J20) 02h 22m 33.4s NED DEC(J20) NED redshift z=0.176 NED distance 9.6 Mpc Strickland et al. (24) optical major axis 13.5 NED optical minor axis 2.5 NED major axis angle b 22 Temple et al. (25) inclination angle 89 Sakamoto et al. (1997) rotational velocity 225 km/s Strickland et al. (24), Rupen et al. (1991) IR luminosity L Strickland et al. (24) K band luminosity L Strickland et al. (24) B band luminosity L Strickland et al. (24) M B (magnitude) Rupen et al. (1991) SFR 3.8 M /yr Popescu et al. (24) M c gas (HI & H 2 ) M Guelin at al. (1993), Dupac et al. (23) M dust M Alton et al. (20), Popescu et al. (20) M gas /M dust 240 Dupac et al. (23) a NED = NASA Extragalactic Database ( b This refers to the angle the plane of the galaxy makes with the northern direction in the counter clockwise direction. c M stands for mass unless otherwise indicated in parentheses. 12

28 spiral galaxies (some normal, some starbursting) Temple et al. (25) found that NGC 891 always falls near the median when comparing properties such as far-infrared (FIR) luminosity, and warm colour ratio 14. The warm molecular gas in this galaxy has been observed directly in H 2 at λ = 28.2 µm and 17.0 µm by Valentijn et al. (1999). There are very warm (T= K) molecular clouds scattered throughout the disk, as well as warm (T=80-90 K) massive clouds in the outer regions of the disk. The H 2 outweighs H I by a factor of 5-15 in the outer stellar disk. The cold molecular gas in the galaxy has a temperature range of K and is a few degrees warmer in the central region (Dupac, 23). The molecular gas observed in CO emission is mainly confined to a thin disk (4 pc thickness). The radial distribution of molecular gas is similar to the MW. There is a nuclear component within R < 225 pc (where R is the distance along the major axis from the centre of the galaxy), a molecular ring at 3 < R < 7 kpc and then a swift drop off of gas density extending out to 10 kpc (Scoville et al. 1993). The total SFR for NGC 891 is 3.8 M /yr (Popescu et al. 24) (Table 1.1). This is of the same order as that of the MW within the large uncertainties. However, as mentioned previously, NGC 891 s SFR is probably a few times higher than that for the MW. The cold dust mass (M dust in Table 1.1) was determined from ISOPHOT 15 observations at λ = 170 µm & 2 µm by Popescu et al. (24). However, Dupac et al. (23) determined a much lower dust mass using Sub-millimeter Common-User Bolometer Array (SCUBA) observations at λ = 450 & 850 µm. Hence, we show a range in Table 1.1 for dust mass which corresponds to the difference in published values. The diffuse dust emission accounts for 69% of the total FIR luminosity, with the remaining 31% from a clumpy dust component. There are faint dust sources that extend into the halo in both the southern and northern regions of the galaxy. These localized sources could be giant molecular cloud complexes associated with the spiral arms (Popescu et al. 24). The gas to dust ratio (M gas /M dust in Table 1.1) for the galaxy is quite close to the 14 The warm colour ratio is the IRAS (InfraRed Astronomical Satellite) 60µm/1µm ratio. 15 The Imaging Photo-Polarimeter on board ISO. 13

29 MW value (Dupac et al. 23) even though there is generally more gas and dust in NGC 891 compared to the MW. The dust, as observed by SCUBA, has a temperature in the range K (Israel, 1999). Dust emission at 850 µm and 450 µm in NGC 891 is very similar in spatial appearance to emission from 12 CO and emission at 1.3 mm (Israel, 1999). This implies that the dust and the molecular gas are correlated spatially in NGC 891. There is also good spatial coincidence between the cold dust continuum (observed at λ = 850 µm) and the 7.7 µm PAH feature. Also, both PAH and cold dust emission correlate with emission from very small grains (VSGs) (observed at 14.3 µm) - except in regions with starbursts, where there is an excess of VSG emission. This is expected since VSGs are heated similarly to PAHs - by single photon excitation. However they are not destroyed as easily as PAHs in high energy environments (see Section 1.3). Therefore VSGs are often observed near star forming regions, where PAHs are destroyed. Thus, PAHs may be preferentially related to regions dominated by cold dust and cold molecular clouds and are thus excited mainly by the interstellar radiation field (Haas et al. 22). Some ISO observations of NGC 891 have been presented already by Mattila et al. (1999) and LeCoupanec et al. (1999). Mattila et al. (1999) found that the absolute intensities at 6.2, 7.7, 8.6, & 11.3 µm are similar to the values observed for the diffuse emission in the MW. Emission from 5.9 to 11.7 µm accounts for about 9% of the total IR radiation. Le Coupanec et al. (1999) presented MIR maps of NGC 891 and concluded that the distribution of PAHs follows the molecular gas distribution and star forming regions. The 4.5 µm flux was shown to be a tracer of ionized matter, indicating that the 4.5 µm emission is probably a combination of warm dust emission as well as stellar continuum emission. They also found that the 11.3 µm emission is inhibited near the galactic centre, which could be an effect of absorption or PAH destruction. This thesis will be somewhat of a continuation of the studies of Le Coupanec et al. However we will use more IR wavebands for imaging 16, and we will model the infrared spectrum for numerous regions in the galaxy. NGC 891 is the host of SN 1986J, a recent type II supernova (SN) 17. This young SN 16 Le Coupanec et al. (1999) discussed results from nine ISOCAM observations of NGC 891. Our data also comes from these same ISOCAM bands, but we also use data from the Spitzer Space Telescope. 17 Located at RA 2h22m31.326s DEC +42d19m56.41s (NED), and of size 0.22 pc (Perez-Torres et al. 22). 14

30 is believed to have occurred in early VLBI 18 observations of SN 1986J have shown it to be extremely bright at 5 GHz (Perez-Torres, 22). This SN has also been observed in X-ray emission (which is quite rare for SNe) by Temple et al. (25). In our contour maps in Chapter 4.1 the location of the SN is denoted by a triangle The Halo NGC 891 is interesting because its halo emission has been well-studied. The H I halo extends up to at most 15 kpc above the galactic plane (Fraternali et al. 25). Also, the H I halo is asymmetric: It is thicker in the north than in the south of the galaxy (see Figure 1 of Swaters et al and Figure 1 of Fraternali et al. 25). The speed of this rotating halo gas is 25 to 1 km/s slower than the gas in the plane (Swaters, 1997). It has been determined that about 15% of total H I mass ( M ) is above 30 (1.4 kpc at a distance of 9.6 Mpc) from the plane. We have obtained Swaters (1997) data and present a minor axis surface brightness profile created from their H I image in Chapter This profile shows that the H I halo extends much further than any other halo emission. Cold dust has been detected in the H I halo at λ = 170 µm and 2 µm by Popescu et al. (23). A large number of grains (cold dust, with temperature, T 15 K) within the H I halo indicate that the H I halo is probably not primordial 19. As mentioned previously, the H I halo is asymmetric. The cold dust emission also follows that asymmetry, thus, the asymmetry is intrinsic and not due to the H I disk becoming ionized at smaller radii. Popescu et al. (23) suggest that macroturbulence could have been the mechanism for grain transport into the halo and the extended disk. They also suggest the possibility that this gas and dust could have been captured during a merger long ago. However, NGC 891 is currently non-interacting (meaning, NGC 891 does not show morphological effects from gravitational interactions with nearby companions). The dust has also been observed in absorption in optical observations by Howk & Savage (1997). The dust features (such as filaments) were found to extend out to 2 kpc 18 Very Long Baseline Interferometry. 19 That is, the H I halo probably did not form when the galaxy first formed. This is because dust grains are not a primordial substance, therefore, as the H I halo is filled with them, the H I halo is probably not primordial. 15

31 above the plane. The potential energy associated with this amount of dust ( M ) above the plane is equivalent to tens if not hundreds of SNe (Howk & Savage, 1997) (or ergs). The orientation of the dust features change from being perpendicular to the plane at low galactic latitudes (z), to being aligned parallel to the plane at high z (Rossa et al. 24). This suggests that magnetic fields play a role in the morphology of the dust in NGC 891. It also may suggest that there are soft (non-violent) transport mechanisms such as photolevitation 20 that bring some of the dust into the halo. In this thesis we will look at the dust emitting at 24, 70, 160, 450 and 850 µm (data from Bendo (personal communication) and Alton et al. 1998). We will present the halo emission in the form of minor axis surface brightness profiles from these data in Chapter to compare with the MIR profiles. A molecular gas halo was observed in CO emission by Garcia-Burillo et al. (1992). This molecular gas extends kpc into the halo, which is not typical for most spirals. However, Lee et al. (22) found a similar case in NGC NGC 5775 has CO emission associated with an H I supershell 21 that extends out to 5 kpc above the midplane of this edge-on galaxy. Out of 74 catalogued normal spiral galaxies, about 40% have diffuse ionized gas (DIG) in their halo (Rossa & Dettmar, 23). Usually the DIG is in the form of filaments and bubbles. However, a few of them have a widespread, smooth background of DIG. NGC 891 is one such galaxy that has widespread DIG in its halo. NGC 891 has one of the brightest ionized gas halos of all the nearby edge-on spirals (Alton et al. 20). This suggests that the energy and mass transfer from the disk to the halo is very pronounced in this galaxy. High resolution (0.1 ) images of the galaxy at Hα show that the DIG halo extends up to 5 kpc above the galactic plane (Rossa et al. 24). The diffuse emission is also asymmetric. This indicates that there is much more star formation occurring in the northern part of the disk than in the southern part. Radio continuum observations, which are not affected by dust, are also consistent with this. Therefore, extinction cannot be responsible for suppressed DIG emission in the southern part of the galaxy. There are 20 Radiation pressure on dust grains can raise small dusty clouds to high galactic latitudes (Franco et al. 1991). 21 A supershell is a large bubble of gas that is thought to be a result of a SN explosion. 16

32 also filamentary structures and other discrete structures in the halo in images observed at Hα. Some filaments reach up to 2.2 kpc above the galactic plane. In the mid-plane many bubbles, shells, and supershells have been observed. Comparing Hα results with X-ray observations obtained with the Chandra X-ray observatory shows good correlation of the warm (Hα) and hot (X-ray) ionized gas. The diffuse X-ray emission extends from the disk into the halo on the northwest side of NGC 891 (Temple et al. 25). We therefore see that there exists a variety of material in the halo of NGC 891. In this thesis we will employ our MIR observations to determine to what extent PAHs and warm dust contribute to the halo. 1.5 Organization of Thesis The remainder of the thesis is be organized as follows: Chapter 2 discusses the observations we used in this thesis. The instruments used and the details of the observations are outlined in this chapter. Our data reduction procedure is described in Chapter 3. Our results are presented in Chapter 4: in Section 4.1, the surface brightness contour maps in each waveband are presented. Section 4.2 shows the surface brightness profiles we created. Both of these show how the MIR emission is distributed in NGC 891. Our results continue in Chapter 5: Section 5.1 presents the spectrum of NGC 891, with measurements taken in nineteen different IR wavebands. Spectra were created for four different regions in the plane and halo of the galaxy. These have then been fit, in Section 5.2, with a common SED model to learn more about the physical conditions of these environments. In Chapter 6 there is a discussion of all of our results and their importance. Finally, Chapter 7 provides a summary and presents the conclusions of what we have found. 17

33 Chapter 2 Observations This thesis makes use of both archived and published data from 8 different instruments ranging in wavelength from optical to radio. The main results were obtained using 5 different IR detectors. These include the 3-Band Near-IR Camera used by the 2 Micron All Sky Survey (2MASS) 1, the Camera onboard the Infrared Space Observatory (ISOCAM) 2, the Infrared Array Camera (IRAC) and the Multiband Imaging Photometer (MIPS) on the Spitzer Space Telescope 2, and finally, the Sub Millimeter Common User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope 3. The ISOCAM raw data were taken from the ISO archive, and we did the complete data reduction ourselves. We will describe this data reduction in the following chapter. The data from all of the other instruments were already calibrated and reduced by various sources. The following sections will provide the details of all of these observations. 2.1 ISO s ISOCAM The Infrared Space Observatory (ISO) was launched by the European Space Agency (ESA) in late At the time, it took data of unprecedented sensitivity. With this instrument, a vast amount of IR data were made available. Results from ISO revealed that PAH features are widely present in space in a variety of objects. We will focus our discussion of ISO as 1 from the 1.3 m telescope at the Whipple Observatory on Mt Hopkins, Arizona. 2 space-based 3 15 m telescope located at Mauna Kea, Hawaii. 18

34 Figure 2.1: Diagram of ISOCAM (Blommaert et al. 23). it relates to our observations of NGC 891. For this thesis, we wanted general observations of NGC 891 with ISOCAM (Figure 2.1). The bands from about 3µm to 15µm are of interest for observing PAHs, therefore we selected two data sets from the ISO archive that spanned these wavelengths. The NGC 891 data presented in this thesis were obtained in August 1997 and March Using ISOCAM s longwave (LW) filter wheel (Figure 2.1), data were obtained in 9 different wavebands corresponding to the central wavelengths, λ ref, 4.5, 6.0, 6.7 4, 6.8, 7.7, 9.6, 11.3, , and 14.9 µm. Table 2.1 (on page 22) summarizes the observation and map parameters. Each waveband is denoted by the filter name: LW1, LW2, etc. Stepping through each row: The reference wavelength is the central wavelength for each of these filters. The wavelength range is the range of wavelengths that the filters transmits. ISOCAM has three main 4 The 6.7 & 14.3 µm bands had wider wavelength ranges than the others. Therefore those two bands are called wide bands, and the others are called narrow bands. See Table

35 observing modes: photometric, spectrophotometric, and polarimetric. CAM01 refers to the photometric general observations that include single pointing 5, celestial rasters 6, and micro-scanning (Blommaert et al. 23). As can be seen in Table 2.1, all of our ISOCAM observations used this mode. The observation number is a unique identifier in the ISO archive. As we have two data sets, the first row under observation number corresponds to the 1997 data set and the second row corresponds to the 1998 data set. Note that the 1998 data set only had data in the two wide bands. The pixel field of view specifies the angular area of the sky that each pixel sees. An example of a 1-D cut through a point spread function (PSF) is given in Figure 2.2. This is a graph of the intensity of a point source on the detector versus angular distance that the detector sees. Because of diffraction, we see a central (primary) maximum and the secondary maxima on either side. (We have only shown the PSF for the 11.3 µm band, but the PSFs of the other wavebands are similar in shape.) The spatial resolution of the images was determined by examining the full width at half the maximum (FWHM) of the PSFs corresponding to each data set, and these are listed in Table 2.1 beside PSF. The spatial resolution is larger for longer wavelengths as expected from diffraction 7. The dates of the observation are also given in Table 2.1. Again, there are two different dates for the two different data sets. The number of on-source pointings refers to the fact that during an entire observation, the telescope points to numerous positions along the galaxy (rastering). This is important, as the entire galaxy was too big to fit into one field of view. Therefore, the telescope pointed first to one side of the galaxy, then to the middle, then to the other side (see Figure 2.3). In addition, for some of the pointings the telescope was only directed at the background sky near the galaxy. Thus the number of on source pointings refers only to when the telescope had the galaxy in its field of view. As can be seen in Figure 2.3, there is a vertical line that is blanked (darkened) in all of the images (pointed out by the white arrow in Figure 2.3). This is because column 24 in the LW array was disconnected. The impact of this dead pixel column will be discussed 5 also called staring 6 stepping across the sky in order to cover areas larger than the field of view of the camera. 7 θ F W HM λ/d, where λ is the reference wavelength, D is the diameter of the aperture and θ F W HM is the maximum angular distance of the primary maximum. 20

36 um Point Spread Function 0.02 flux density (mjy/pix) arcseconds Figure 2.2: An example of the PSF that came with the ISO data. This graph is for the 11.3 µm band. The full width at half maximum (FWHM) of the graph gives the resolution. The PSF graphs for the other wavebands are similar, and thus not shown. further in Chapter 3. A frame refers to one picture or image taken by ISOCAM. The mean number of frames per pointing is the average number of pictures ISOCAM took at each pointing position. The integration time per frame is the amount of time the CCD (Charged Coupled Device) collects photons before being read out to create the image 8. Each frame was integrated for seconds before the CCD was read out. The mean number of frames per pointing is essentially the number of images ISOCAM took of the galaxy at each pointing. Thus the total on-source observing time is the integration time per frame times the number of frames per pointing times the number of on-source pointings. ISOCAM s field of view contained only portions of the galaxy at a given time per frame (called the sky coverage in Table 2.1). That is because the camera had a pixel array, with 6 6 pixel size. There were multiple pointings covering different parts of the galaxy that we had to mosaic 9 together to make the images we will see in Chapter 4. Note that even with the many pointings, the mosaicked images only cover the 8 similar to the amount of time a shutter stays open in a regular camera. 9 combining all of the different pointings into one complete image by using the astrometry. 21

37 Figure 2.3: Some examples of the different on-source pointings of ISOCAM on NGC 891. In this example, from 1 to 4, we see the pointing go from the southern end of the galaxy, to the centre regions of the galaxy, and finally to the northern end of the galaxy. Thus, the different pointings allow data from the entire galaxy to be collected and later mosaicked to make a complete picture of the whole galaxy. The white arrow points along the dead pixel column. 22

38 central 14 kpc of the galaxy. This represents about 2/5 of the galactic disk as observed in optical light 10. The procedure by which we create these images will be further elaborated in Chapter 3: Data Reduction. 10 fraction determined by the optical major axis in Table 1.1 at a distance of 9.6 Mpc. 23

39 24 Table 2.1: Observing and map parameters. See explanation in text, Section 2.1, for each parameter. wavebands parameter LW1 LW4 LW2(wide) LW5 LW6 LW7 LW8 LW3(wide) LW9 reference wavelength wavelength range(µm) observing mode CAM01 CAM01 CAM01 CAM01 CAM01 CAM01 CAM01 CAM01 CAM01 observation number pixel field of view ( ) PSF(FWHM)( ) date of observation 28 Aug Aug Aug Aug Aug Aug Aug Aug Aug 97 5 March 98 5 March 98 no. of on-source pointings mean # frames/pointing (16) integration time per frame total on source time (min) sky coverage (arcmin 2 )

40 The ISO filters each have a distinct spectral filter response 11. When measuring fluxes, a colour correction must be applied to the measurements because the transmission of the instrument optics and filters, as well as the quantum efficiencies of the detectors, exhibit non-flat spectral dependencies. Thus, two sources radiating the same power within a given wavelength range but with different spectral shapes produce two different signals. The spectral transmission, R(λ), is the product of the filter transmission and the detector quantum efficiency. R(λ) for each ISOCAM filter is given in Figure 2.4. To determine the actual flux density F actual (λ ref ), one must take the measured flux, F mea and divide by a colour correction factor, K(λ ref ), where λ ref is the reference wavelength defined in Table 2.1: F actual (λ ref ) = F mea K(λ ref ), (2.1) where K(λ ref ) is defined as: K(λ ref ) = F λ (λ) F λ (λ ref ) λ λ ref R(λ) dλ, (2.2) R(λ) dλ where F λ (λ ref ) is F actual (λ ref ) defined above. Here it is assumed that F λ (λ) λ 1 and the integral is over the full wavelength range for the filter (Blommaert et al. 23, Appendix A). We will apply this colour correction later when we analyze the spectrum in Chapter 5. Note that while we show ISOCAM s spectral response functions in Figures 2.4 and 2.5, these are typical for the IRAC, MIPS, SCUBA and 2MASS filters as well. So, while we do a unique colour correction to each waveband using the specific spectral response function for each, we only show those of ISOCAM in this thesis. 2.2 Spitzer s IRAC The Spitzer Space Telescope (SST) is currently in orbit and is taking data at unprecedented resolution and sensitivity. The IRAC instrument onboard the SST is the latest in spacebased MIR imaging. IRAC can image simultaneously in four MIR wavebands with central 11 That is, the filters allow different partial transmission of different wavelengths. 25

41 4.5um 6.0um λ λ 6.7um 6.8um λ λ 7.7um λ Figure 2.4: Spectral transmission response functions for the ISO wavebands. The x-axis is wavelength in µm, and the y-axis is the fraction of light that can pass through the filter, with 1 being complete transparency and 0 being completely opaque. 26

42 9.6um 11.3um λ λ 14.3um 14.9um λ λ Figure 2.5: Spectral transmission response functions for the ISO wavebands. The x-axis is wavelength in µm, and the y-axis is the fraction of light that can pass through the filter, with 1 being complete transparency and 0 being completely opaque. 27

43 wavelengths of 3.6, 4.5, 5.8 and 8.0 µm. Using the Spitzer software, Leopard, we have downloaded some of the public NGC 891 data. The data we downloaded were made public September 28th, 25. At the time this thesis was being written, these data were not yet published. There was a variety of data products available for download from the Spitzer archive. There are the raw data which are repackaged into FITS 12 files. These raw data files are in DN (data number) units, and also include instrument engineering, observatory pointing, calibration and housekeeping data 13. The archived raw data represent the original untouched data. There are also Basic Calibrated Data (BCD) available for download with Leopard. BCD are two-dimensional images in FITS format, and correspond to individual data collection events within an observation (ie: pointings). An image is flux calibrated, and surface brightness measurements are expressed in physical units (MJy/sr) 14. Note that a Jansky, written Jy, is a unit of flux: 1 Jy = W/m 2 Hz. In addition, flat-fielding and cosmetic restoration (e.g., cosmic-ray removal) algorithms have already been applied to the BCD. Spatial world coordinates are derived from observatory pointing information. The BCD represent the most reliable product achievable through automated processing ( however, they come in over 1 FITS files each of different pointings which must be mosaicked using the Spitzer software mosaic. Since we only planned to use the IRAC data to complement our ISOCAM observations, we have downloaded the Post-BCD. Post-BCD are an extended pipeline product which have had mosaicking done already. We used the IRAC data both to check our ISOCAM MIR images for consistency, and also to obtain additional flux data points to add to the IR spectrum. Thus, we have 12 Flexible Images Transport System. FITS files have a specific format which allows for convenient and consistent exchange of astronomical data. 13 Housekeeping data are data that record the voltages, currents and temperatures of the detector. They are used to reduce and interpret the data correctly. 14 To convert from MJy/sr (sr = steradian) to mjy/arcsec 2, we use the following equation: 1MJy/sr = 10 9 mjy MJy arcsec 2 sr (2.3) 28

44 Table 2.2: IRAC Observing Parameters reference wavelength (µm) wavelength range (µm) pixel field of view ( ) PSF (FWHM) ( ) date of observation 8 Sept 24 8 Sept 24 8 Sept 24 8 Sept 24 downloaded images that were created from observations of NGC 891 taken for the Brown Dwarf Galaxy Halos project on September 8, 24 (observation key ) 15. IRAC s wavebands are similar to, but broader than the ISO wavebands. Relevant observing details are listed in Table 2.2. Note that there are not as many observation details because these data were already calibrated and mosaicked. We must also use the filter response functions for the 4 IRAC bands to do a colour correction just as for the ISOCAM points. 2.3 Spitzer s MIPS MIPS is another instrument onboard the SST that performs photometry in three different wavebands corresponding to the central wavelengths 24, 70 and 160 µm. These are important wavebands, as they detect dust near the wavelength of its peak emission. We have been fortunate to obtain reduced and calibrated MIPS data for NGC 891 from an associate, George Bendo. At the time this thesis was being written, these data were not yet published. We will use these data to help create a complete IR spectrum of the galaxy. These data were created from observations taken for the Exploring Dust Content of Galactic Winds with MIPS project (program ID 20528) 16. Since archived MIPS Post-BCD are not as reliable as IRAC Post-BCD, (especially for the 70 and 160 µm wavebands), it has been very important that we obtained reliable data that were calibrated and reduced by George Bendo. Relevant MIPS observing details are given in Table 2.3. Note that the 160 µm data have a broad PSF. This becomes problematic later on (see Chapter 4, Section and Chapter 5, Section 5.1). 15 These observations were proposed by Fazio Giovanni. 16 These observations were proposed by Crystal Martin. 29

45 Table 2.3: MIPS Observing Parameters reference wavelength (µm) central wavelength (µm) wavelength range (µm) pixel field of view (arcsec) PSF (FWHM) (arcsec) date of observation 4 Sept 25 4 Sept 25 4 Sept 25 mjy/arcsec 2. Again, MIPS data are in units of MJy/sr, and we use Equation 2.3 to convert to 2.4 Published Observations We will also be using published observations in additional wavebands. For our study of NGC 891 s morphology, we will look at a beautiful observation in neutral hydrogen by Swaters et al. (1997) taken with the Westerbork Synthesis Radio Telescope (WSRT) 17. From NED, we have a Very Large Array (VLA) radio continuum image (Condon et al. 1996) and an Hα image (Rossa, 23) which was taken with the Calar Alto Faint Object Spectrograph (CAFOS). We will use these three observations to look for extensions in the galactic halo which correlate to the extensions seen in the MIR (see Chapter 4, Section and Chapter 6, Section 6.1.7) SCUBA We also have SCUBA 450 & 850 µm images from Alton et al. (1998) 18. These we will examine for the same purpose as above (looking for correlations), but we will also add the SCUBA fluxes to the spectra we will construct. The SCUBA images have spatial resolution of 7.5 and 14 for the 450 & 850 µm wavebands respectively, and have surface brightness units given in Jy/beam. The conversion from these units to mjy/arcsec 2 is complex because the beam is a circular Gaussian, not a square. Therefore, we have kept the SCUBA images in their original units, and used the AIPS 19 task imean to calculate the flux density when 17 Note that newer, more sensitive observations from WSRT have since been published by Fraternali et al. (25), however, we were unable to obtain these data. 18 These SCUBA observations were taken in October, 1997 using the JCMT. The data were given to us by E. Xilouris. 19 Astronomical Image Processing System. 30

46 we need to measure it (in Chapter 5) MASS In order to obtain more information for our study of NGC 891 s IR spectrum, we require Near Infrared (NIR) data. NIR data were obtained from the Skyview database (NASA Goddard Space Flight Centre, 2MASS images, in the J (λ = 1.25 µm), H (λ = 1.65 µm), and K (λ = 2.17 µm) wavebands. The dominant source of emission in these wavebands is the old stellar population. Thus, these data will also be important for modeling the spectrum, as they will constrain the stellar continuum at shorter wavelengths (refer to Chapter 5). These images each have a spatial resolution of about 1 ( about2mass.html). For 2MASS, the pixel values are in DN units. To convert to a calibrated magnitude 20 (mag), the zero point mag given in the image header is needed. The keyword MAGZP in the image header (and in Table 2.4) is used to compute the calibrated mag using: mag = MAGZP 2.5 log(f (DN)), (2.4) where the F(DN) is the integrated flux in DN units. Once we have 2MASS magnitudes, we need to convert to Jy to be consistent with the other wavebands. This is done using F ν (0) in Table 2.4: F (Jy) = F ν (0)10 0.4mag, (2.5) where F ν (0) is the flux for a 0 mag source. The net result of Equations 2.4 & 2.5 is that the flux in DN can be converted to a flux in mjy by multiplying it by a constant. For the J, H and K 2MASS bands, the 20 A magnitude is a measure of the brightness. mag = -2.5log(F), where F is the flux. 31

47 Table 2.4: 2MASS Isophotal Bandpasses and Fluxes for 0 Magnitude (Cohen et al. 23) parameter J H K central wavelength (µm) ± ± ± wavelength range (µm) ± ± ± 0.2 F ν (0) (Jy) 1594 ± ± ± 13 MAGZP conversion is as follows: F J (mjy) = F J (DN) (2.6) F H (mjy) = F H (DN) (2.7) F K (mjy) = F K (DN) (2.8) 2.5 Summary of Observations To summarize, this thesis presents our results of the central 14 kpc of NGC 891. The results can be grouped into those that reflect on the morphology of the galaxy, and those that define the spectral energy distribution of the galaxy. In our study of the galaxy s morphology, we focus mainly on the MIR data (ISOCAM and IRAC observations), with the other wavebands only used for comparisons. This is because the galaxy s morphology has already been well-studied at other wavelengths (see Chapter 1.4), and because we are interested in the location and structures of PAH emission. For this part of our study we will use observations/images from all of our sources discussed in this Chapter. Contour maps for almost all of these wavebands will be presented in Section 4.1. In our study of the galaxy s SED, we focus on all of the IR observations (from 2MASS, ISOCAM, IRAC, MIPS, SCUBA). These observations cover the spectrum from about 1 to 10 µm. A total of 19 wavebands are used in our SED analysis. In this way we can do a detailed analysis of NGC 891 for the entire IR spectrum. But first we will describe the data reduction procedure that was necessary for the ISOCAM data, as well as the archived Spitzer data. 32

48 Chapter 3 Data Reduction Astronomical data collected by telescopes are subject to many sources of error because no instrument is ideal. Correcting for these non-ideal behaviours, as well as calibrating and doing other modifications to the data is called data reduction. The sources of error for ISOCAM and how we correct for them will be discussed in this Chapter. These non-ideal instrumental behaviours include: long stabilization times after flux changes, memory effects of the detectors, the response variation after glitches 1, astrometric uncertainties, and the field distortion (Blommaert et al. 23). ISO data reduction algorithms have been assembled by ISO into the CAM Interactive Analysis (CIA) package. Gastaud & Chanial (20) and Chanial (23) improved this package and created the reduction package CAM Interactive Reduction (CIR), which runs in the Interactive Data Language (IDL) environment. Later, additional improvements were made. Thus the program we have used to reduce our ISO data is called new CIR. 3.1 ISOCAM Data Reduction Procedure Once the data were downloaded from the archive, the raw data (called cisp files) were read into newcir. The raw data were stored in the variable xcisp, which is an IDL structure. The raw ISO data are in the form of a cube. That is, there are two spatial dimensions corresponding to the CCD camera s pixel configuration as it sees the sky, and a third 1 Glitches are spikes of intensity seen at the detector caused by cosmic rays. This will be discussed further later in the chapter. 33

49 Figure 3.1: An illustration of an ISO data cube. dimension being the frames (see Figure 3.1). The signal is recorded in Analogue to Digital Units (ADU), (Galliano, 24). The first step in data reduction is to do the dark correction. The term dark current refers to the level of signal detected when there is no light source incident on the detector (ie: the detector is in darkness). The dark level in the LW array was caused by two effects: a leakage of charges during reset, and a thermal charge generation in the photoconductors 2. The first effect was dominant for integration times up to 10 s (as is the case for our observations). The second effect dominates for longer integration times. An example of a dark frame for the LW array is shown in Figure 3.2. It shows that there is a difference in dark levels for the even and odd rows (Blommaert et al. 23). The dark correction also corrects for the difference in levels of the even and odd rows in the CCD pixels. In the new CIR package, the command, correct_dark_vilspa,xcisp does four things: it applies a dark correction as described above following the characterization of Biviano (Biviano et al. 1998b) and Sauvage, it applies a second-order correction depending on the detector temperature and the time since activation, and it applies the short-drift correction 3. The main end result is that the horizontal lines in the raw data are removed. 2 When the detector has a temperature greater than 0 K, there is thermal emission from the instruments themselves. 3 The short term drift is the modification of the dark current within a single revolution about the earth of the satellite, from the beginning of the revolution to the end. It is different for different pixels and for different integration times (Blommaert et al. 23). 34

50 Figure 3.2: An example of the LW dark frame, after a 2.1 s integration time. Note the evident line pattern (Blommaert et al. 23). This step also converts the units from ADU to ADU/G/s (ADU/Gain/second), (Galliano, 24). Note that for the remainder of the data reduction discussion, we will include the newcir commands in parentheses after the description of each step. The next step is to do the automatic deglitching which attempts to mask the transient cosmic rays 4. The glitches are masked using multi resolution deglitching (Starck et al. 1999) (correct_glitch_mr,xcisp). The glitches are usually short in time, but can have a lasting memory effect on the detector. The automatic deglitching only works to mask the glitches that are short in duration and have a signal to noise ratio (S/N) above 8σ 5. Therefore the memory effects remain and must be removed in the two next steps. We then correct for transient/memory effects that the pixels of the camera experience. This is due to the fact that the detectors have very low temperatures. Thus, their response after a variation in flux is not instantaneous 6. It depends strongly on the history of the pixel [stab_img=median_array(xcisp[0:20].data,xcisp[0:20].mask,dim=3) and correct_transient_fs,xcisp,stab_img=stab_img]. The program corrects the transients with the Fouks-Schubert (1999) method 7 (Blommaert et al. 23). We then examined the data (xcube). At this point not all of the deglitching is done, 4 Cosmic rays are charged particles from space that hit the detector and cause a glitch, which is a spike in intensity at a pixel or a range of pixels. 5 Where σ is the level of the root mean square (RMS) noise of the images. 6 The temperature is raised and does not cool off instantaneously. 7 The Fouks-Schubert method is a physical model for the pixel response to a change in flux. It is nonlinear, and non-symmetrical and describes the detector behaviour to a step up or down in flux very well. It is based upon the detector construction and properties (for more on the Fouks-Schubert model, refer to Fouks & Schubert, 1995). 35