Annu. Rev. Astron. Astrophys. 2009. 47: 159-210 Copyright © 2009 by Annual Reviews. All rights reserved

7. DISCUSSION

Over the past decade, astronomers have gathered a huge amount of information about nearby galaxies. In some cases these data have confirmed and made more precise previously known correlations - such as the fundamental plane. In others, they have broken newer ground - such as the refinement of infrared indicators of star-formation and full dynamical modeling of galaxy centers. An overarching theme has been that the size of the new data sets has allowed us to ask more sophisticated statistical questions of the galaxy population. In particular, our understanding of the effect of galaxy environment on galaxy properties is now highly refined.

One clear result from these studies is that while a galaxy's surroundings affect its probability of being a "late type" or "early type" galaxy, they only secondarily affect the scaling relations of those types. That is, whereas elliptical galaxies are more common in groups and clusters than in the field, the processes that produce them are likely to have been similar no matter where they are found today. Galaxy formation theorists ought to be trying to explain this general rule.

Nevertheless, there are several notable exceptions to the rule. In particular, the central galaxies in groups and clusters appear to be a special class (Section 5.5). Very close neighbors (< 50 kpc) seem to affect each other substantially (Section 2.5, Section 6). Galaxies in dense regions are at least 0.02 dex more metal rich than those in the field (Section 3.7). Elliptical galaxies in dense regions may be slightly older (Section 5.7, Section 5.8), and have a slightly different fundamental plane relationship (Section 5.4). Ellipticals in the field may also have had a more active recent merger history (Section 5.6). In addition, while the fundamental plane and Tully-Fisher relations, and the relationship between luminosity and Sérsic index, might be constant with environment, each of those relationships mask more complex structures - be they kinematic complexities, bars, or other features. It may yet be that these more detailed properties depend on environment in some revealing fashion, even while maintaining the gross relations. While the theoretical galaxy formation community has yet to come to agreement on how the broad trends come to be, the observers ought still to work on teasing out these intriguing deviations.

Another trend that has recently come into focus is the dependence of mass-to-light ratio on mass. For galaxies around L_∗, there is generally only a weak relationship; the Tully-Fisher and fundamental plane relationships are (approximately) constant mass-to-light ratio. But clearly the fundamental plane relationship shows a slow increase to high masses, which for the BCGs becomes even more extreme (Section 5.5). Meanwhile, at low masses the dwarf disk galaxies deviate from the Tully-Fisher relationship, again showing high mass-to-light ratios; in this case, because they appear to not have converted much of their cold gas into stars (Section 3.4). Thus, the halos that seem to produce the most stellar mass per unit dark matter mass are somewhere around L_∗. Interestingly, such a trend is exactly what is required for the CDM mass function, with its slow cutoff at high mass and steep faint-end slope, to successfully produce the observed luminosity function (Section 2.3; Tasitsiomi et al. 2004, Seljak et al. 2005). The best quantification of this trend is that of Zaritsky, Zabludoff & Gonzalez (2008), who attempt to construct a dynamical relationship that applies to all galaxy classes.

What is even more clear, however, is that even with the vast number of papers written, the available information in the new data sets is nowhere near being exhausted. Progress is possible because much (though not all) of the new data is amenable to the sophisticated techniques that have been used on smaller, less homogeneous surveys. In particular, a few interesting questions have barely been touched with the modern data sets and bear further discussion:

1. The lenticular population has received far too little attention, either being tossed in along with the elliptical galaxies or ignored altogether. The E/S0 separation is not an arbitrary one - the edges, rings and bars are clear indications of type - we need well-defined and consistent methods of identifying S0s in the new massive surveys.
2. The nature and existence of breaks in the stellar profile of disks, and the incidence of extended UV disks, seem topics ripe for study in the new massive surveys. While there is now a real census of these galactic features, there has been no study of how they vary with environment or with other galaxy properties. Similarly, there are now reasonably large data sets of bulges, pseudobulges, and bars, as well as other detailed properties, whose relationship with close neighbors and the larger scale environment are largely unknown.
3. The cores of elliptical galaxies are complex and interesting places. Evaluating the recently proposed role of AGN in these cores to the shut-off of star-formation in massive galaxies (e.g. Croton et al. 2006) may require understanding these cores much better. Photometric information alone does not disentangle the effects of dust and stellar populations, and the stellar population and dynamical studies available with larger sets of IFU observations may yield a less ambiguous set of models for elliptical cores.
4. More generally, while some crude measures of galaxy structural properties exist, there are few efforts to quantify classical morphological features like spiral arm organization and pitch angle. Modern techniques in inference applied to modern data sets should allow such a quantification. Classical morphology clearly is important and relevant - witness its continued popularity among astronomers studying the detailed properies of galaxies even as "large data set" astronomers eschew it. We can and ought to measure quantitatively the properties it is putatively based upon.
5. New surveys allow us to connect the radio properties of galaxies (such as neutral and molecular gas content) with their optical properties (such as stellar mass and age) with large statistical samples. Although many interesting findings have resulted from such comparisons, a comprehensive analysis has not yet been completed. The stellar populations are intimately tied to the interstellar gas, without which our understanding is at best incomplete. With upcoming blind H i surveys such as ALFALFA, hopefully this gap will be filled with respect to atomic gas at least.

During the course of writing this review, we benefited from conversations with Adam Bolton, Marla C. Geha, David W. Hogg, Dusan Keres, Michael Hudson, Jill Knapp, and David Schiminovich. We would like to thank Christina Ignarra and Alejandro Quintero for providing some of the data necessary for the Figures 16 and 17. Finally, we thank our scientific editor John Kormendy for his extremely useful and challenging comments.

This research has made use of NASA's Astrophysics Data System and of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Funding for the creation and distribution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS Web site is http://www.sdss.org/.

The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, the Korean Scientist Group, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.

The Galaxy Evolution Explorer (GALEX) is a NASA Small Explorer. The mission was developed in cooperation with the Centre National d'Etudes Spatiales of France and the Korean Ministry of Science and Technology.