ENERGY SOURCES OF THE FAR-INFRARED EMISSION OF M33

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1 The Astrophysical Journal Supplement Series, 154: , 2004 September # The American Astronomical Society. All rights reserved. Printed in U.S.A. ENERGY SOURCES OF THE FAR-INFRARED EMISSION OF M33 J. L. Hinz, G. H. Rieke, K. D. Gordon, P. G. Pérez-González, C. W. Engelbracht, A. Alonso-Herrero, J. E. Morrison, K. Misselt, and D. C. Hines 1 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721; jhinz@as.arizona.edu, grieke@as.arizona.edu, kgordon@as.arizona.edu, pgperez@as.arizona.edu, cengelbracht@as.arizona.edu, aalonso@as.arizona.edu, jmorrison@as.arizona.edu, kmisselt@as.arizona.edu, dhines@as.arizona.edu R. D. Gehrz, E. Polomski, C. E. Woodward, and R. M. Humphreys Department of Astronomy, University of Minnesota, Minneapolis, MN 55455; gehrz@astro.umn.edu, elwood@astro.umn.edu, chelsea@astro.umn.edu, roberta@astro.umn.edu M. W. Regan Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; mregan@stsci.edu J. Rho Spitzer Science Center, California Institute of Technology, MC 220-6, Pasadena, CA 91125; rho@ipac.caltech.edu and J. W. Beeman and E. E. Haller Lawrence Berkeley National Laboratories, 1 Cyclotron Road, Berkeley, CA Received 2004 April 7; accepted 2004 May 12 ABSTRACT We present observations of the spiral galaxy M33 with Spitzer at 24, 70, and 160 m. The excellent resolution and mapping capabilities of Spitzer combined with the proximity of M33 result in observations that enable a detailed study of the distribution of star formation (SF) and dust in the galaxy. We compare the morphology of M33 at far-infrared wavelengths with other standard SF indicators such as H and radio continuum using a Fourier filtering technique to separate the diffuse emission components from compact sources. We find that the infraredemissionat24and70m follows closely the structure of the ionized gas, indicating that it is heated largely by hot, ionizing stars. At 160 m a diffuse cold dust component matches only approximately the structure of the old red stellar population or the distribution of blue light. It is, however, very similar to the structure of the diffuse nonthermal radio emission. Subject headings: galaxies: individual (M33) galaxies: spiral galaxies: structure infrared: galaxies 1. INTRODUCTION Far-infrared (FIR) fluxes from normal galaxies are commonly taken to indicate the rate of recent star formation. However, placing this interpretation on firm footing has been frustrated by uncertainties in the basic issue of what component of the galaxy provides the energy that is absorbed by the dust and reradiated (Kennicutt 1998). For example, Devereux and coworkers (Devereux et al. 1994, 1996, 1997; Devereux & Scowen 1994) and Jones et al. (2002) argue from the close correspondence between hydrogen recombination line emission and FIR morphologies that the FIR is powered predominantly by H ii regions. Deul (1989), Walterbos & Greenawalt (1996), and Hirashita et al. (2003) argue that about half of the FIR emission, perhaps more, is due to dust heated by a diffuse interstellar radiation field, defined as an average radiation field that is not dominated by any particular star or star cluster and that is of similar color to that of the solar neighborhood. Sauvage & Thuan (1992) have suggested that the relative role of young stars compared with the diffuse interstellar radiation field increases with later galaxy type. M33 (NGC 598), also known as the Triangulum galaxy, is ideal for studying this question. It is a well-known late-type 1 Current address: Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO spiral (SA(s)cd) galaxy at a distance of 840 kpc (Freedman et al. 1991). Its low metallicity (e.g., Cohen et al. 1984), bright nucleus with a small bulge, and lack of a supermassive black hole (Merritt et al. 2001; Gebhardt et al. 2001) set it apart from our own and other nearby galaxies. For the current study, it is sufficiently close for good physical resolution in the FIR (1 00 4pc)andnearlyface-on(i ¼ 56 ; Regan & Vogel 1994), making it virtually unique in the quality of comparisons possible in the morphologies of different components of the galaxy. As a result, M33 has been the subject of a number of previous studies, but they have not achieved a consensus on the physical processes causing the infrared emission. For example, Deul (1989) concludes from the morphology of the Infrared Astronomical Satellite (IRAS ) images that the FIR output is divided approximately equally between the H ii regions and the diffuse interstellar radiation field. Walterbos & Greenawalt (1996) model the IRAS radial distributions and find that nearly all of the output can arise from the diffuse interstellar field, but Devereux et al. (1997) find that most of the energy is produced in the H ii regions. The IRAS beam of ; 4 0 (at 60 m) translates into about 400 ; 1000 pc at the distance of M33, large enough to make the interpretation of the morphology ambiguous. Recently, Hippelein et al. (2003) reported imaging with ISOPHOT using a60 00 beam (translating to a diameter of about 250 pc), a substantial improvement, i.e., a reduction in beam area of

2 260 HINZ ET AL. Vol. 154 nearly an order of magnitude. They report two main dust components: a warm (45 K) component that is present in the spiral arms and star-forming regions, and a cold (16 K) component that is distributed across the disk of the galaxy and is presumably heated by diffuse interstellar radiation, as well as localized cold dust associated with molecular clouds. With the Multiband Imaging Photometer Spitzer for (MIPS), the point-spread function is critically sampled, allowing another order-of-magnitude reduction in beam area. We report new images of M33 at resolutions of 6 00 (25 pc) at 24 m, (75 pc) at 70 m, and (165 pc) at 160 m. These images are compared with each other and with images of potential energy sources for the FIR emission to identify how the dust seen at these wavelengths is heated. 2. OBSERVATIONS AND DATA REDUCTION Multiple overlapping medium-rate scans across the face of the galaxy with MIPS produced maps at 24, 70, and 160 m (see Engelbracht et al in this volume for details on MIPS large-galaxy observations). The final mosaics are shown in Figure 1 (Plate 1). Data reduction was performed using the MIPS instrument team Data Analysis Tool (Gordon et al. 2004a). The 70 and 160 m images exhibit faint linear stripes along the scan direction that are residual instrumental artifacts due to the time-dependent responsivity of the Ge:Ga detectors. The integrated flux in our 160 m image is Jy, in good agreement with the value of 2200 Jy at 175 m reported by Hippelein et al. (2003). Both measurements are significantly brighter than the whole-galaxy measurement from COBE reported by Odenwald et al. (1998). Hippelein et al. (2003) suggest that the difference arises because of the structure of the Galactic cirrus around the galaxy. We have recomputed the DIRBE flux densities using an online photometry tool, 2 finding much better agreement with our data. Figure 2 shows a spectral energy distribution (SED) plot for M33 using data from MIPS, IRAS, and DIRBE. The total emission in the MIPS images was measured using IRAF s polyphot task, while the IRAS measurements were taken from Rice et al. (1988). A variety of ancillary data have been collected to compare the FIR structure of M33 with other images at wavelengths indicative of the potential energy sources for heating the dust. To locate the ionizing stars, we used H and 6 cm radio continuum images from Devereux et al. (1997). The H observations were obtained at the Case Western Burrell Schmidt Telescope at Kitt Peak, providing a 1 field of view with 2 00 pixels. The 6 cm observations were taken with the Very Large Array (VLA) and the Westerbork Synthesis Radio Telescope (WSRT). The typical sensitivity is 30 Jy. Their clean radio mosaic was made by smoothing to a common resolution of 10 00, and all nonthermal sources were subtracted as detailed in Devereux et al. (1997). The near-infrared K-band image of M33 is taken from the Two Micron All Sky Survey (2MASS) Large Galaxy Atlas (Jarrett et al. 2003). The B-band images were taken at the Bok 2.3 m telescope on Kitt Peak with the 90-Prime instrument (Williams et al. 2001, and 2004, in preparation) on 2003 December 29 during nonphotometric conditions. Six pointings were taken, with each pointing consisting of at least four 1 minute exposures. The mean sky level was subtracted from each of the six images before combining into a single mosaic. 2 Available at Fig. 2. Infrared SED of M33. For the purposes of comparison, the H and 24 m M33 images were convolved to the resolution of the MIPS 70 m map, and all were cropped to a common field of view. The convolution technique is described in detail by Engelbracht et al. (2004). The convolved H and 24 m images,theband K-band images, and the 70 and 160 mmapsareshownin Figure 3 (Plate 2). 3. RESULTS 3.1. General Description of the Images The near-ir image traces the smooth distribution of the older stellar population in two broad and indistinct arms and a small bulge, with diffuse emission across the disk of the galaxy. The blue image is dominated by the spiral arms. The H, 24,and70m and radio images also show distinct spiral arms, studded with many individual H ii regions, and a near absence of a bulge. These spiral arms appear more subdued at 160 m and with smoother structure, indicating that the individual H ii regions contribute less to the overall emission. The H traces the individual sources well, dominated by emission from H ii regions with a total luminosity of 7:06 1:40 ; 10 6 L (Devereux et al. 1997). As shown by the smoother spiral arms in the 24 mimage,theh emission suffers from slight attenuation by dust in the star-forming regions. For instance, in the northeastern spiral arm, just south of the bright H ii region NGC 604, the 24 m image reveals a stream of connected emission, while H shows only individual sources Comparison of Infrared and Compact Free-Free Emission Regions Figure 4 shows the 6 cm radio map and a subtraction of the 6 cm map from the convolved 24 m image, performed using an IRAF script originally designed to identify supernovae in nearby galaxies (Van Dyk et al. 2000). Using input coordinates of identical objects in the two frames, the script fits Gaussian profiles to those objects to get more accurate centers, aligns the images, calculates the FWHM difference between the two images (if any) and transforms one to another, calculates a scaling factor determined by the number of counts in a user defined aperture around the chosen objects, and finally subtracts the two images. Five bright H ii regions were used to align the 24 m and 6 cm images. The resultant image reveals almost no diffuse residual, except for false faint structure near

3 No. 1, 2004 ENERGY SOURCES OF FIR EMISSION OF M Fig. 4. The 6 cm radio map from Devereux et al. (1997) is on the left. Subtraction of the 6 cm radio data from the 24 m map produces the image on the right. North is up, and east is to the left. Both the 24 m and 6 cm data were convolved to the 70 m resolutionof18 00 before subtraction. the center of the galaxy caused by the dark noise patches in the radio image. We conclude that the compact structures at 24 m are heated almost entirely within the H ii regions. Because of the close similarity in structure of the galaxy at 70 m, the compact emission at the latter wavelength probably has the same origin. At 160 m, the compact structure is subdued; some of the emission must originate in the H ii regions, but it appears that much of it may be more extended. We see no evidence for sources that appear to be relatively brighter at 70 than at 24 m as seen in other galaxies observed by Spitzer (e.g., M81; Gordon et al. 2004b) or for increased extended emission at the outskirts of the galaxy at the longer wavelengths Diffuse Emission The images of M33 are dominated by the compact structures, which must be suppressed to study the diffuse emission. Figure 5 shows images on which a Fourier transform has been performed to filter out the high-frequency information. These were created using the Perl Data Language fast Fourier transform package where the boundary for the high-frequency filter was chosen such that all point sources and structures were suppressed, leaving only the diffuse emission. This corresponded to suppressing structures with frequencies smaller than five 160 m pixels(300 pc). In this comparison, we see that the H, 24,and70m emission still look very similar, with the center of the galaxy and NGC 604 being the prominent features, surrounded by diffuse emission of virtually identical structure. Thus, even the extended emission at these wavelengths appears to be powered largely by ionizing stars. The 160 m filtered map, on the other hand, shows smooth contours of emission originating at the center of M33 and a barlike structure elongated in the northeast-southwest direction. This is roughly consistent with the results of Hippelein et al. (2003), who observed diffuse cold dust emission with a disklike, nonspiral structure in a scaled difference map of 60 and 170 m images. It has often been suggested that diffuse interstellar radiation field heats the cold, diffuse dust. To test this hypothesis, we have applied the same Fourier filtering technique to images at K and B bands to test whether the cool stars, or hot but nonionizing ones, have a similar distribution to the 160 m diffuse image. As shown in Figure 5, the match to the B band is poor while there is some emission both in the B band and at 160 m in the innermost regions of M33, the outer structures in the B band do not resemble the simple bar at 160 m. The K-band image is largely centrally concentrated but does seem to mirror the northeast-southwest elongation of the 160 m emission. Thus, it appears that the cool and hot nonionizing stars could contribute to the diffuse cold dust heating, particularly in the center of the galaxy. However, because the K-band and 160 m smoothed images are not identical, it is possible that some other component plays a role in heating the dust. Simple inspection of soft X-ray images of M33 (Haberl & Pietsch 2001) shows that the diffuse emission is not matched to the 160 m structure, with no elongation in the northeastsouthwest direction, implying that the hot thermal plasma is not a source of cold dust heating. An elongated structure has been seen in radio continuum measurements at 17.4 cm by Buczilowski (1988). Figure 6 shows contour maps of the distribution of the total intensity at 17.4 cm after subtraction of 24 unrelated sources (Buczilowski 1988). The thermal emission at 17.4 cm is estimated to be 15% (Buczilowski 1988) of the total radio continuum emission. The compact structures that dominate most radio maps stand out because the low spatial frequencies are suppressed by interferometer-type telescopes; for example, Viallefond et al. (1986) show that the WSRT map at 1.4 GHz accounts for only 16% of the total emission. Thus, the filled aperture image in Figure 6 represents almost entirely the diffuse nonthermal emission. Figure 6 also shows the 160 m Fourier-filtered image observed by MIPS. The image is very similar to the diffuse nonthermal image. We conclude that the diffuse, cold dust is heated not only by the old stellar population, but also by a mechanism closely related to the nonthermal emission, either directly by cosmic-ray heating of the interstellar grains or

4 Fig. 5. Images shown in Fig. 3 have now been Fourier-filtered and are presented here at (a) H, (b) B band, (c) K band, (d) 24m, (e) 70m, and ( f )160m. North is up, and east is to the left. The field of view is approximately 38 0 ; Fig. 6. Contour maps of the 17.4 cm data taken from (a) Fig. 1 of Buczilowski (1988) and the (b) 160 m Fourier-filtered image scaled to the same size.

5 ENERGY SOURCES OF FIR EMISSION OF M indirectly by some mechanism, for example, related to the acceleration of the radio-emitting electrons that in parallel heats the dust. A correlation between the nonthermal radio emission and FIR emission has also been seen in M31 (Hoernes et al. 1998), where it is proposed that the correlation arises due to a coupling of the magnetic field to the gas which is mixed with the cool dust. 4. CONCLUSIONS New MIPS images of M33 offer an unprecedented look at the mechanisms for heating the dust that accounts for the FIR emission of this well-studied late-type spiral. A morphological comparison of the FIR data with ground-based H, near-ir, and high-frequency radio continuum observations suggests that the dust heating at 24 and 70 m is dominated by ionizing stars. However, at 160 m, there is a diffuse, cold dust component with a dramatically different spatial distribution that most closely matches the distribution of diffuse nonthermal radio emission. Future work should address whether this cold dust is heated directly by cosmic rays or whether it is heated in parallel to cosmic-ray acceleration. J. L. H. thanks Nick Devereux for kindly providing the radio and H data sets. J. L. H. also thanks Grant Williams, Ed Olszewski, and the 90-Prime Camera Team for allowing us to use their B-band images of M33. This work is based on observations made with the Spitzer Space Telescope, whichis operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract Support for this work was provided by NASA through contract , issued by JPL/Caltech. R. D. G., E. P., C. E. W., and R. M. H. are supported by NASA through the Spitzer GTO Program. Buczilowski, U. R. 1988, A&A, 205, 29 Cohen, J. G., Persson, S. E., & Searle, L. 1984, ApJ, 281, 141 Deul, E. R. 1989, A&A, 218, 78 Devereux, N., Duric, N., & Scowen, P. A. 1997, AJ, 113, 236 Devereux, N., Jacoby, G., & Ciadullo, R. 1996, AJ, 111, 2115 Devereux, N., Price, R., Wells, L. A., & Duric, N. 1994, AJ, 108, 1667 Devereux, N., & Scowen, P. A. 1994, AJ, 108, 1244 Engelbracht, C., et al. 2004, ApJS, 154, 248 Freedman, W. L., Wilson, C. D., & Madore, B. F. 1991, ApJ, 372, 455 Gebhardt, K., et al. 2001, AJ, 122, 2469 Gordon, K., et al. 2004a, PASP, submitted. 2004b, ApJS, 154, 215 Haberl, F., & Pietsch, W. 2001, A&A, 373, 438 Hippelein, H., Haas, M., Tuffs, R. J., Lemke, D., Stickel, M., Klaas, U., & Völk, H. J. 2003, A&A, 407, 137 Hirashita, H., Buat, V., & Inoue, A. K. 2003, A&A, 410, 83 Hoernes, P., Berkhuijsen, E. M., & Xu, C. 1998, A&A, 334, 57 REFERENCES Jarrett, T. H., Chester, T., Cutri, R., Schneider, S. E., & Huchra, J. P. 2003, AJ, 125, 525 Jones, L. V., Elston, R., & Hunter, D. 2002, AJ, 124, 2548 Kennicutt, R. C. 1998, ARA&A, 36, 189 Odenwald, S., Newmark, J., & Smoot, G. 1998, ApJ, 500, 554 Merritt, D., Ferrarese, L., & Joseph, C. L. 2001, Science, 293, 1116 Regan, M. W., & Vogel, S. N. 1994, ApJ, 434, 536 Rice, W., Lonsdale, C. J., Soifer, B. T., Neugebauer, G., Koplan, E. L., Lloyd, L. A., de Jong, T., & Habing, H. J. 1988, ApJS, 68, 91 Sauvage, M., & Thuan, T. X. 1992, ApJ, 396, L69 Van Dyk, S. D., Peng, C. Y., King, J. Y., Filippenko, A. V., Treffers, R. R., Li, W., & Richmond, M. W. 2000, PASP, 112, 1532 Viallefond, F., Goss, W. M., van der Hulst, J. M., & Crane, P. C. 1986, A&AS, 64, 237 Walterbos, R. A. M., & Greenawalt, B. 1996, ApJ, 460, 696 Williams, G. G., et al. 2001, BAAS, 33, 791

6 Fig. 1. MIPS images of M33 at (a) 24,(b) 70m, and (c) 160m, for which the resolutions are 6 00,18 00, and 40 00, respectively. The field of view is approximately 38 0 ; North is up, and east is to the left. The coordinate system is J Plate 1

7 Fig. 3. Images of M33 at (a) H, (b) B band, (c) K band, (d) 24m, (e) 70m, and ( f )160m. The H and 24 m data have been convolved to the 70 m resolution of The field of view is approximately 38 0 ; North is up, and east is to the left. Plate 2