Physical Properties and Environments of Nearby Galaxies

We review the physical properties of nearby, relatively luminous galaxies, using results from newly available massive data sets together with more detailed observations. First, we present the global distribution of properties, including the optical and ultraviolet (UV) luminosity, stellar mass, and atomic gas mass functions. Second, we describe the shift of the galaxy population from late galaxy types in underdense regions to early galaxy types in overdense regions. We emphasize that the scaling relations followed by each galaxy type change very little with environment, with the exception of some minor but detectable effects. The shift in the population is apparent even at the densities of small groups and therefore cannot be exclusively due to physical processes operating in rich clusters. Third, we divide galaxies into four crude types—spiral, lenticular, elliptical, and merging systems—and describe some of their more detailed properties. We attempt to put these detailed properties into the global context provided by large surveys. 159 A nn u. R ev . A st ro . A st ro ph ys . 2 00 9. 47 :1 59 -2 10 . D ow nl oa de d fr om w w w .a nn ua lr ev ie w s. or g by U ni ve rs ity o f M ar yl an d C ol le ge P ar k on 0 9/ 16 /1 2. F or p er so na l u se o nl y. ANRV385-AA47-05 ARI 22 July 2009 3:57 1. NEW WINDOWS ON THE NEARBY UNIVERSE As early as the work of Hubble (1936), astronomers recognized the existence of distinct galaxy types—smooth early-types preferentially found in groups and clusters and complex-looking latetypes preferentially found in less dense regions of the sky. Despite this long history, we have not yet determined with certainty the physical mechanisms that differentiate galaxies into classes. The past decade of astronomers’ effort has yielded both massive new surveys of the nearby Universe and more detailed observations of individual objects. Although many of these observations have yet to be digested and fully understood, already we have a clearer view of the detailed physical properties than we did only a decade ago. Focusing on nearby galaxies comparable in mass to the Milky Way, we review some of the latest results on the census of the galaxy population. Recently developed observational tools for understanding galaxy formation fall into two general types: wide-field surveys and targeted (but more detailed) observations. The wide-field surveys we focus on are the Galaxy Evolution Explorer in the UV (GALEX; Martin et al. 2005), the Sloan Digital Sky Survey in the optical (SDSS; York et al. 2000), the Two-Micron All Sky Survey in the near-IR (infrared) (2MASS; Skrutskie et al. 2006), as well as 21-cm radio surveys such as the Hi Parkes All Sky Survey (HIPASS; Meyer et al. 2004) and ALFALFA on Arecibo (Giovanelli et al. 2007). Each of these surveys covers a substantial fraction of the sky with imaging; redshifts from the SDSS and the 21-cm surveys then provide a third dimension. Along with the Two-degree Field Galaxy Redshift Survey (2dFGRS; Colless et al. 2001), the Six-degree Field Galaxy Redshift Survey (6dFGRS; Jones et al. 2004) and the 2MASS Redshift Survey (Crook et al. 2007), these massive surveys supply a detailed map of the galaxy density field and the framework of largescale structure within which galaxies evolve. In addition, they supply a host of spectroscopic and photometric measurements for each galaxy: luminosities, sizes, colors, star-formation histories, stellar masses, velocity dispersions, and emission line properties. The second type of tool consists of targeted but more detailed programs that are too expensive to conduct on massive scales right now, but for which even small numbers of galaxies can be revealing. Such programs include the Spectroscopic Areal Unit for Research on Optical Nebulae (SAURON; Bacon et al. 2001), the Spitzer Infrared Nearby Galaxy Survey (SINGS; Kennicutt et al. 2003), The Hi Nearby Galaxy Survey (THINGS; Walter et al. 2008), new large Hubble Space Telescope (HST) programs, and large compilations of individual efforts such as detailed radio observations, long-slit spectroscopy, and deep optical and near-IR imaging. Using the results of these recent efforts, we begin by exploring the global demographics of galaxies and their dependence on environment. Then, dividing galaxies into classes (spirals, lenticulars, ellipticals, and mergers), we review recent results concerning the scaling relations, star-formation histories, and other properties of each class. We cannot hope to be exhaustive, and instead focus on recent results rather than historical ones. We omit discussion of dwarf systems, which are rather different than their more massive counterparts, being generally more gas-rich, disk-dominated, and usually lacking in spiral structure (a topic ripe for review; meanwhile, see Geha et al. 2006). Because of our focus on global properties, we also deemphasize central black holes (Kormendy 2004) and active galactic nuclei (AGN; Ho 2008), which might have an important influence on galaxy evolution as a whole (Kauffmann et al. 2003b, Best et al. 2006, Khalatyan et al. 2008). For elliptical galaxies, we refer the reader to multiple other reviews examining their more detailed properties, including their stellar populations (Renzini 2006), their structure and classification (Kormendy et al. 2009), and their hot gas content (Mathews & Brighenti 2003). A final warning is that we adopt some of the language of morphology (elliptical or E, lenticular or S0, and spiral galaxies) without fully addressing the problem of classification (Sandage 2005). 160 Blanton · Moustakas A nn u. R ev . A st ro . A st ro ph ys . 2 00 9. 47 :1 59 -2 10 . D ow nl oa de d fr om w w w .a nn ua lr ev ie w s. or g by U ni ve rs ity o f M ar yl an d C ol le ge P ar k on 0 9/ 16 /1 2. F or p er so na l u se o nl y. ANRV385-AA47-05 ARI 22 July 2009 3:57 Throughout, we assume a standard cosmology of m = 0.3 and = 0.7, with H0 = 100h km s−1 Mpc−1. All magnitudes are on the AB system unless otherwise specified. 2. A GLOBAL VIEW OF GALAXY PROPERTIES A breakthrough of recent surveys has been the ability to explore many dimensions of galaxy properties simultaneously and homogeneously, in order to put galaxy scaling relationships in context with respect to one another. In this section, we describe these general properties, their distribution, and their dependence on environment. We concentrate in this section primarily but not exclusively on SDSS results, which currently yield the most homogeneous and consistent measurements for the broadest range of galaxy varieties. In particular, in the subsections below we make use of 77,153 galaxies with z < 0.05 in the SDSS Data Release 6 (DR6; Adelman-McCarthy et al. 2008), an update of the low-redshift sample of Blanton et al. (2005c). 2.1. Optical Broad-Band Measurements Figure 1 shows the simplest measurable properties of galaxies from the SDSS sample: the absolute magnitude Mr , the g − r color, the Sérsic (1968) index n in the r-band, and the physical half-light radius r50 (sometimes called the effective radius). These properties reveal a variety of correlations, most known for many years, but now quantified much more precisely. The absolute magnitude Mr is a critical quantity, correlating well with stellar mass as well as with dynamical mass [see the discussion of the Tully-Fisher (TF) relation in Section 3.9 and the fundamental plane in Section 5.4]. Clearly, the other properties are a strong function of overall mass. Although Figure 1 shows the raw distribution for the flux-limited SDSS sample, in Section 2.3 we correct for selection effects and calculate the luminosity and stellar mass functions. In many of these plots, particularly those involving g − r color, there is a bimodal distribution— galaxies can be divided very roughly into red and blue sequences (Strateva et al. 2001, Blanton et al. 2003, Baldry et al. 2004). The red and blue classification is not always related in a simple way to classical morphology—though, of course, there is some relationship (Roberts & Haynes 1994). In particular, galaxies in the blue sequence are very reliably classifiable as spiral galaxies with ongoing star formation (Section 3). However, the red sequence contains a mix of types. The lower luminosity end consists of compact ellipticals (cEs) and dwarf ellipticals (dEs; sometimes known as spheroidals, Sph). Around Mr −5 log10 h ∼ −20, the red sequence is a mix of early-type spirals, dust-reddened spirals (Section 3.6), lenticulars (S0s; Section 4), and giant ellipticals (Es; Section 5). At the highest luminosities, it consists of cD galaxies (Section 5.5). For a rough quantification of how these types populate the red sequence, we use the classifications of our sample galaxies stored in the NASA Extragalactic Database (NED). In practice, most of these classifications come from The Third Reference Catalog of Bright Galaxies (RC3; de Vaucouleurs et al. 1991). For our purposes, we select galaxies within ( g − r) ∼ 0.03 of the red sequence, with no detected lines associated with star formation (see Section 2.2 and Figure 2). For any luminosity Mr − 5 log10 h < −17, only about 40% of these galaxies are Es. About 25–50% of them are S0s, with the lowest fractions at around Mr − 5 log10 h ∼ −20, increasing to both higher and lower luminosities. We suspect these fractions are, in practice, overestimates, because spiral systems are far more commonly misclassified as E/S0s than vice-versa. It remains clear, however, that restricting to red sequence galaxies does not suffice to guarantee an E/S0 sample—at Mr − 5 log10 h ∼ −20 at least one-third of the red sequence population is Sa or later. Both the red and blue sequences have mean colors that are a function of absolute magnitude. Blue galaxies have recent star formation, and their color is strongly related to the recent www.annualreviews.org • Nearby Galaxies 161 A nn u. R ev . A st ro . A st ro ph ys . 2 00 9. 47 :1 59 -2 10 . D ow nl oa de d fr om w w w .a nn ua lr ev ie w s. or g by U ni ve rs ity o f M ar yl an d C ol le ge P ar k on 0 9/ 16 /1 2. F or p er so na l u se o nl y. ANRV385-AA47-05 ARI 22 July 2009 3:57