|Annu. Rev. Astron. Astrophys. 2009. 47:
Copyright © 2009 by Annual Reviews. All rights reserved
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 "late-types" 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 ultraviolet (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 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 large-scale 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-infrared 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. 2003a, 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).
Throughout, we assume a standard cosmology of m = 0.3 and = 0.7, with H0 = 100 h km s-1 Mpc-1. All magnitudes are on the AB system unless otherwise specified.