514 research outputs found
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
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
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
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
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
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