Constraining Galaxy Formation Models with Dwarf Ellipticals in Clusters

Recent observations demonstrate that dwarf elliptical (dE) galaxies in clusters, despite their faintness, are likely a critical galaxy type for understanding the processes behind galaxy formation. Dwarf ellipticals are the most common galaxy type, and are particularly abundant in rich galaxy clusters. The dwarf to giant ratio is in fact highest in rich clusters of galaxies, suggesting that cluster dEs do not form in groups that later merge to form clusters. Dwarf ellipticals are potentially the only galaxy type whose formation is sensitive to global, rather than local, environment. The dominant idea for explaining the formation of these systems, through Cold Dark Matter models, is that dEs form early and within their present environments. Recent results suggest that some dwarfs appear in clusters after the bulk of massive galaxies form, a scenario not predicted in standard hierarchical structure formation models. Many dEs have younger and more metal rich stellar populations than dwarfs in lower density environments, suggesting processes induced by rich clusters play an important role in dE formation. Several general galaxy cluster observations, including steep luminosity functions, and the origin of intracluster light, are natural outcomes of this delayed formation.Comment: 8 page

Unveiling the Formation of Massive Galaxies

Massive galaxies, such as nearby ellipticals, have relatively low number densities, yet they host the majority of the stellar mass in the universe. Understanding their origin is a central problem of galaxy formation. Age dating of stellar populations found in modern ellipticals, and observations of star formation in high redshift galaxies, allow us to determine roughly when these systems formed. These age diagnostics however do not tell us what triggered star formation, or how galaxies form as opposed to simply when. Recent analyses of the structures of z > 2 ultraviolet selected galaxies reveal that major galaxy mergers are a likely method for forming some massive galaxies. There are however galaxy populations at high redshift (z > 2), namely infrared and sub-millimeter bright systems, whose evolutionary relationship to modern ellipticals is still uncertain. An improved characterization of these and other high redshift galaxy populations is achievable with large infrared imaging and spectroscopic surveys.Comment: Science Magazine (April 16, 2004) invited perspectiv

Observing Massive Galaxy Formation

A major goal of contemporary astrophysics is understanding the origin of the most massive galaxies in the universe, particularly nearby ellipticals and spirals. Theoretical models of galaxy formation have existed for many decades, although low and high redshift observations are only beginning to put constraints on different ideas. We briefly describe these observations and how they are revealing the methods by which galaxies form by contrasting and comparing fiducial rapid collapse and hierarchical formation model predictions. The available data show that cluster ellipticals must have rapidly formed at z > 2, and that up to 50% of all massive galaxies at z ~ 2.5 are involved in major mergers. While the former is consistent with the monolithic collapse picture, we argue that hierarchal formation is the only model that can reproduce all the available observations.Comment: Invited Review, 10 pages, to appear in "Galactic Dynamics", JENAM 200

How do galaxies get their baryons?

Understanding how galaxies obtain baryons, their stars and gas, over cosmic time is traditionally approached in two different ways - theoretically and observationally. In general, observational approaches to galaxy formation include measuring basic galaxy properties, such as luminosities, stellar masses, rotation speeds, star formation rates and how these features evolve through time. Theoretically, cosmologically based models collate the physical effects driving galaxy assembly - mergers of galaxies, accretion of gas, star formation, and feedback, amongst others, to form predictions which are matched to galaxy observables. An alternative approach is to examine directly, in an observational way, the processes driving galaxy assembly, including the effects of feedback. This is a new `third way' towards understanding how galaxies are forming from gas accretion and mergers, and directly probes these effects instead of relying on simulations designed to reproduce observations. This empirical approach towards understanding galaxy formation, including the acquisition history of baryons, displays some significant differences with the latest galaxy formation models, in addition to directly demonstrating the mechanisms by which galaxies form most of their baryonic mass.Comment: Review for proceedings of "Tracing the Ancestry of Galaxies on the Land of our Ancestors", Eds Carignan, Freeman & Combe

Properties of Spiral and Elliptical Galaxy Progenitors at z > 1

We present the results of a Hubble Space Telescope and ground-based optical and near-infrared study to identify progenitors of spirals and ellipticals at z > 1. We identify these systems through photometric and spectroscopic redshifts, deep K-band imaging, stellar mass measurements, and high resolution imaging. The major modes of galaxy formation, including major mergers, minor mergers, and accretion of intergalactic gas, and their relative contributions towards building up the stellar masses of galaxies, can now be directly measured using these data.Comment: Proceedings of the ESO/USM/MPE Workshop on "Multiwavelength Mapping of Galaxy Formation and Evolution", eds. R. Bender and A. Renzini, 6 page