University of Groningen Understanding disk galaxies with the Tully-Fisher relation Ponomareva, Anastasia IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2017 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Ponomareva, A. (2017). Understanding disk galaxies with the Tully-Fisher relation [Groningen]: Rijksuniversiteit Groningen Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 22-03-2018
Chapter 1 Introduction
2 Chapter 1. Introduction Abstract The research presented in this Ph.D thesis is mainly focused on the Tully Fisher relation as a tool, leading towards understanding the intrinsic properties of spiral galaxies such as their multi wavelength luminosities, rotational velocities and masses of various baryonic components. The main focus of this work is to investigate the statistical properties of the multi wavelength and baryonic Tully Fisher relations, while taking advantage of spatially resolved Hi kinematics. This study performs fair comparisons and answers various open questions regarding the nature of this relation. In this introductory chapter, I discuss the importance of the Tully Fisher relation in different areas of astrophysics, from measuring distances to galaxies to using it as a tool to test theories of galaxy formation and evolution. The following open questions regarding the Tully Fisher relation are discussed in detail: i) in which photometrical band the Tully Fisher relation demonstrates the tightest correlation and the steepest slope; ii) which rotational velocity measure helps to reduce the scatter in the observed Tully Fisher relation; iii) does the baryonic Tully Fisher relation demonstrate a tighter correlation than the luminosity based relation? Finally, I outline how the research presented in this thesis helps to answer these open questions by using a unique sample of spiral galaxies and detailed analysis of multi wavelength data.
1.1. The Tully Fisher relation 3 1.1 The Tully Fisher relation The Tully Fisher relation (TFr) describes the empirical correlation between the intrinsic luminosity of a spiral galaxy and its rotational velocity (Tully & Fisher 1977). It is the most widely studied and least understood scaling relation of spiral galaxies. Moreover, it serves different astrophysical purposes. The TFr was first established as a powerful, redshift independent tool to measure distances to spiral galaxies. Indeed, while estimating the rotational velocity from the width of its neutral hydrogen (Hi) line profile, one can infer the intrinsic luminosity from a calibrated relation and then determine the distance to the galaxy by calculating the distance modulus as a difference between the inferrred absolute magnitude and the observed apparent magnitude (see Figure 1.1). Therefore, it is very important to establish a representative calibrator sample for the TFr with independently measured distances, as the distance uncertainty is one of the main contributors to the observed scatter of the TFr. In the recent past, a number of studies were carried out, aiming to measure the distances to as many galaxies as possible. The reason is that knowing the distance to a galaxy and the Hubble constant one can estimate its peculiar velocity the difference of a galaxy motion from the homogeneous cosmic expansion. This knowledge provides information on how the matter is distributed in the Universe and helps to identify large scale structures, such as voids, walls and dense regions of galaxy clusters. The most successful program of measuring the peculiar velocities up to date is the Cosmic Flows program (Courtois et al. 2011; Courtois & Tully 2012; Tully et al. 2016) which built a unique catalogue of galaxy peculiar velocities using distances to galaxies inferred from the calibrated TFr. Consequently, decades of these studies resulted in an outstanding discovery of our home supercluster, Laniakea (Tully et al. 2014, Figure 1.2). 1.1.1 From galaxy distances to theoretical constraints Being originally used as a powerful tool to measure distances to disk galaxies, the TFr established itself very fast as the most widely used relation in different areas of extragalactic astronomy. As an empirical correlation between fundamental properties of spiral galaxies it was used to estimate the unknown mass of any baryonic component of a galaxy (McGaugh &
4 Chapter 1. Introduction Measure$V rot$ M abs$ $115$$$$$$$$$$$$$$118$$$$$$$$$$$$$121$$$$$$$$$$$$$$123 $ Retrieve$M abs$ place$$v rot$ on$x1axis$ $ $1.5$$$$$$$$$$$$$$$$$$$$$$$$$2$$$$$$$$$$$$$$$$$$$$$$$$$2.5$$$$$$$$$$$$$$$$$$$$$$$$$3 Log$2V $ rot$ Figure 1.1 A schematic view on inferring the distance to a disk galaxy using the Tully Fisher relation. Schombert 2015; Sorce 2015), to study the nature of tidal dwarf galaxies by examining their position in the TFr (Lelli et al. 2015), to compare the behaviour and nature of galaxies of different morphological types, surface brightnesses and sizes by studying whether they share the same TFr (Courteau et al. 2003; Russell 2004; Zwaan et al. 1995; Courtois et al. 2015). It was proven to hold over a broad wavelength range (Verheijen 2001) and in different galaxy environments (Willick 1999). Outlying galaxies on the TFr were even studied as potential hosts of extraterrestrial civilisations (Zackrisson et al. 2015). Even though the nature and origin of the TFr remains unclear, it obviously links the baryonic content of a galaxy, characterised by its luminosity, to its total dynamical mass, characterised by the rotational velocity. Therefore, it has become one of the most powerful relations in extragalactic astronomy, providing important constraints for semi analytical models and numerical simulations of galaxy formation and
1.1. The Tully Fisher relation 5 Fig. 1. Two views of the Laniakea Supercluster. The outer surface demarcates the limits of Figure 1.2 The different views at the Laniakea Supercluster from Tully et al. (2014). Coloured dots indicated known structures such as the Great Attractor (orange) and the Pavo Indus filament (purple). evolution. The success of any particular theory of galaxy formation and evolution depends on how well it can simultaneously reproduce the observed slope, scatter and zero point of this relation. In the past, it was a huge challenge for theoretical studies to reproduce the statistical properties of the TFr (Navarro & Steinmetz 2000). However, recent cosmological simulations of galaxy formation and evolution have reached sufficient advancement to produce realistic galaxies which do follow the observed TFr ( Vogelsberger et al. 2014; Schaye et al. 2015). Yet, the literature contains many observational results on the TFr which are often inconsistent due to different corrections applied to the observables, e.g. for extinction or inclination, due to different photometric systems, due to different observing techniques or due to different samples. Consequently, theoretical efforts to understand galaxy formation can pick and choose from a plethora of observed TFrs from the literature to match their results.
6 Chapter 1. Introduction 3.6μm Figure 1.3 The Tully Fisher relation as studied at different wavelengths (from optical to infrared). The B-, I- and K -bands TFrs are from Verheijen (2001) and the 3.6 µm band TFr is from Sorce et al. (2012). The goal of this thesis is to establish the definitive Tully Fisher relation and to study its statistical properties (scatter, slope and tightness), taking advantage of a representative galaxy sample, the best available panchromatic photometry and high quality, spatially resolved Hi kinematics. 1.2 Multi wavelength photometry Over the past decades the Tully Fisher relation appeared to hold over a broad wavelength range (Figure 1.3). However, it has been suggested for decades that the scatter in the TFr can be tightened by using luminosities in the NIR bands where the spectrum of the old stellar population peaks and thus the NIR photometry provides a good proxy for the stellar mass of galaxies (Peletier & Willner 1991). Indeed, the observed scatter in the TFr decreased significantly due to more accurate photometric measures, from photographic magnitudes in the 1970 s (Tully & Fisher 1977) to space-based infrared photometry with the Spitzer Space Telescope (Werner et al. 2004) in 2010 s. However, despite the obvious advantages of deep NIR photometry, it is still not clear at which wavelength the smallest scatter in the TFr can be achieved. For instance, Bernstein et al. (1994) showed that the H- band TFr has no less scatter than the I-band relation. Verheijen (2001) found the smallest vertical scatter in the R-band for galaxies in the Ursa
1.3. The importance of Hi rotation curves 7 V max V flat V max = V flat V max > V flat Figure 1.4 The rotation curves of three spiral galaxies from Verheijen (2001). Left panel: NGC 4389 with the rising rotation curve (V max < V flat ). Middle panel: NGC 3917 with the flat rotation curve (V max = V flat ). Right panel: NGC 3992 with the declining rotation curve (V max > V flat ). Major cluster using the corrected width of the global Hi profilex. Sorce et al. (2012) demonstrated that the 3.6 µm Tully-Fisher relation has an even larger scatter than the I-band relation. Once again, it is extremely difficult to make a fair comparison between these inconsistent observational studies. Therefore, to properly study the wavelength-dependence of the TFr, a systematic approach is required. The strength of the current study is that for the first time a well defined sample of nearby galaxies with known distances and uniform photometry ranging from the ultraviolet to the infrared, including the optical wavelength range, with consistent corrections applied, is considered. Hence, with our study we can certainly answer one of the most important questions regarding the TFr: In which band is the relation the tightest and how does the slope of the TFr change with wavelength? 1.3 The importance of Hi rotation curves With the improved photometric measurements of the luminosities of spiral galaxies, the measurement errors on the absolute magnitudes have been reduced to a point where they no longer contribute significantly to the observed scatter in the TFr. Hence, other measurement errors, such as uncertainties on a galaxy s inclination and/or rotational velocity, combined with a certain intrinsic scatter, are responsible for the total observed scatter.
8 Chapter 1. Introduction So far, nearly all observational studies of the TFr have been based on the corrected width of the global Hi profile while very little attention has been given to improving measurements of the rotation velocity. The width and shape of the global HI profile, however, are determined by the detailed distribution of the Hi gas in the disk, the shape of a galaxy s rotation curve and the presence of non circular streaming motions and/or a warp. Under certain conditions, the rotational velocity of a galaxy is reasonably well defined and can be estimated from the corrected width of the global Hi profile. In particular, a galaxy needs to be axisymmetric, have no warps, and have a monotonically rising rotation curve that reaches an outer flat part. Unfortunately, galaxies are rarely so well-behaved and the column density distribution and kinematic structure of their gas disks may significantly affect the shape and width of the global Hi profiles, introducing errors on the derived rotational velocity that cannot be corrected for. It was shown by Verheijen (2001), with a study of spiral galaxies in the Ursa Major cluster, that the scatter in the TFr can be reduced when extended Hi rotation curves are available to substitute the corrected width of the global Hi profile as a kinematic measure. The 2-dimensional velocity fields allow for the identification of warps, streaming motions, lopsidedness and possibly declining rotation curves. The declining rotation curves are usually associated with massive spiral galaxies like, for example, M31. In these cases V flat (the velocity of the outer flat part of the rotation curve, often associated with the dark matter (DM) halo) as derived from the rotation curve is systematically lower than the rotational velocity derived from the corrected width of the global Hi profile. Moreover, rotation curves of low surface brightness and dwarf galaxies are typically still rising at the outer most measured point. Therefore, their observed maximum rotational velocity provides only a lower limit to the actual maximum rotational velocity defined by the DM halo potential. Figure 1.4 demonstrates the three types of rotation curves for three different galaxies from the Ursa Major sample (Verheijen 2001). Following the uncertainties induced by the use of the width of the global Hi profile as a rotational velocity measure, Hα rotation curves obtained from long slit spectroscopy were used for TFr studies (Rubin et al. 1980; Mathewson et al. 1992; Simon et al. 2003). However, these rotation curves do not extend far enough and do not allow for accurate measurements of the velocity associated with the DM halo potential.
1.4. Measurements of the baryonic mass of spiral galaxies 9 Hence, the availability of extended Hi rotation curves is the other strength of the current study. 1.4 Measurements of the baryonic mass of spiral galaxies Due to the fact that the TFr links the baryonic content of a galaxy through its luminosity to the total dynamical mass (rotational velocity), the physical nature of the TFr is considered to be a relation between the baryonic mass of a galaxy and the mass of its host dark matter halo. Therefore, recently, the relation between the total baryonic mass of a galaxy and its rotational velocity, the so called baryonic TFr (BTFr), became widely studied (e.g. Papastergis et al. 2016; Lelli et al. 2016; Zaritsky et al. 2014). However, a study of the BTFr is associated with even more uncertainties and observational limitations because, unlike the light, the baryonic mass of a galaxy is very difficult to infer. The baryonic mass of a galaxy is usually measured as a sum of the stellar and gaseous components. Yet, the atomic gas mass is the only component which can be estimated directly from the measured Hi flux. Furthermore, even though the molecular gas mass contribution to the baryonic mass of a spiral galaxy is often negligible, there are various cases where the amount of molecular hydrogen (H 2 ) is the same or even exceeds the atomic gas mass (Leroy et al. 2009; Saintonge et al. 2011; Martinsson et al. 2013). As the distribution of the H 2 in galaxies can not be directly observed, there are various tracers associated with the H 2 emission, such as the CO emission line or the mid infrared luminosity from which the amount of H 2 can be estimated, using conventional conversion factors. However, both of these methods have very large uncertainties which contribute significantly to the uncertainty in the BTFr. Nonetheless, the greatest unresolved issue to date is the measurement of the stellar mass of spiral galaxies. Indeed, stellar mass, unlike the light, cannot be measured directly. Therefore, the natural and most commonly used method to estimate the stellar mass of spiral galaxies is to convert measured light into mass by assigning a certain conversion factor. The value of this factor defines the fractional contribution of all components to the baryonic mass budget of a galaxy. Thus, for a galaxy of a low stellar mass to light ratio, more significance is given to the gas contribution to
10 Chapter 1. Introduction the baryonic mass of a galaxy. The stellar mass to light estimation is not straightforward and every method of estimating the mass to light ratio has its uncertainties and limitations. It can be derived either from stellar population synthesis models or by measuring the dynamical mass (surface) density of a galaxy. In this study, I abandon the use of a single method to estimate the stellar mass and consider instead various methods of the stellar mass to light ratio estimations. Thus, I study not only the statistical properties of the Baryonic Tully Fisher relation, but also the effect of various stellar mass to light ratios on the statistical properties of the BTFr. 1.5 This Ph.D. Thesis research In this Ph.D. thesis I investigate the statistical properties (slope, scatter and tightness) of the multi wavelength and baryonic Tully-Fisher relations for a well defined sample of 32 nearby spiral galaxies, taking advantage of spatially resolved Hi kinematics. The sample of spiral galaxies used for this research has various advantages over previous studies: 1. All galaxies have independent distance measures based on the Cepheid Period Luminosity relation (Freedman et al. 2001) and/or based on the Tip of the Red Giant Branch (Rizzi et al. 2007). Independently measured distances reduce the error on the absolute magnitude of a galaxy and therefore reduce the impact of distance uncertainties on the observed scatter of the TFr. The distance uncertainties in our case contribute only σ dist = 0.07 mag to the total observed scatter of the TFr, which is much lower in comparison with σ dist = 0.41 mag if the Hubble flow distances were adopted. 2. All galaxies are selected for their regular morphology and a large Hi content. The latter allows us to study the global Hi properties of the sample galaxies (Chapter 2). 3. Panchromatic photometry is available for all galaxies, which gives us an opportunity to compare the statistical properties of the TFr at various wavelengths (from FUV to NIR) consistently (Chapter 3). 4. The availability of extended Hi rotation curves provides an opportunity to compare the statistical properties of the TFrs based on
1.5. This Ph.D. Thesis research 11 different rotational velocity measures (W 50 from the global Hi profile, V max and V flat : Chapter 3 and Chapter 4). 5. The availability of multi wavelength photometry combined with 21 cm synthesis imaging allows us to measure the baryonic content of the sample galaxies with various methods and, finally, to estimate the effect of the stellar mass to light ratio on the statistical properties of the BTFr (Chapter 4). Overall, by understanding spiral galaxies as physical objects, namely what are they composed of and how they rotate, this Ph.D. research helps to answer some open questions about the statistical properties of the TFr. Which relation shows the smallest vertical scatter, the steepest slope and is intrinsically tighter? Which relation, luminosity based or baryonic, is intrinsically tighter and how does the choice of the mass to light ratio affect its statistical properties? Finally, this research determines how the choice of the rotational velocity measure affects the statistical properties of the TFr, and which rotational velocity corresponds to the steepest slope and the smallest intrinsic scatter of the TFr. Moreover, this detailed study makes it possible to compare consistent observational results with theoretical predictions, making our measurement of the TFr a reliable tool to observationally constrain theories of galaxy formation and evolution.
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