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"Conselice, Christopher J"
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Galactic encounters : our majestic and evolving star-system, from the Big Bang to time's end
This book tells the story of how astronomers have pieced together what is known about the vast and complicated systems of stars and dust known as galaxies. The first galaxies appeared as violently disturbed exotic objects when the Universe was only a few 100 million years old.
Book
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.
Journal Article
Breaking down the link between luminous and dark matter in massive galaxies
We present a study on the clustering of a stellar mass selected sample of galaxies with stellar masses M* > 1010M⊙ at redshifts 0.4 < z < 2.0, taken from the Palomar Observatory Wide-field Infrared Survey. We examine the clustering properties of these stellar mass selected samples as a function of redshift and stellar mass, and find that galaxies with high stellar masses have a progressively higher clustering strength than galaxies with lower stellar masses. We also find that galaxies within a fixed stellar mass range have a higher clustering strength at higher redshifts. We further estimate the average total masses of the dark matter haloes hosting these stellar-mass selected galaxies. For all galaxies in our sample the stellar-mass-to-total-mass ratio is always lower than the universal baryonic mass fraction and the stellar-mass-to-total-mass ratio is strongly correlated with the halo masses for central galaxies, such that more massive haloes contain a lower fraction of their mass in the form of stars. The remaining baryonic mass is included partially in stars within satellite galaxies in these haloes, and as diffuse hot and warm gas. We also find that, at a fixed stellar mass, the stellar-to-total-mass ratio increases at lower redshifts. This suggests that galaxies at a fixed stellar mass form later in lower mass dark matter haloes, and earlier in massive haloes. We interpret this as a ‘halo downsizing’ effect.
Journal Article
Galaxy Mergers and Interactions at High Redshift
In this review we discuss the evidence for galaxy interactions and mergers in the distant universe and the role of mergers in forming galaxies. Observations show that the fraction of massive (M>M*) galaxies involved in major mergers is roughly 5–10% at z ~ 1. The merger fraction however increases steeply for the most massive galaxies up to z ~ 3, where the merger fraction is 50± 20%. Using N-body models of the galaxy merger process at a variety of merger conditions, merger mass ratios, and viewing angles this merger fraction can be converted into a merger rate, and mass accretion rate due to mergers. A simple integration of the merger rate shows that a typical massive galaxy at z ~ 3 will undergo 4–5 major mergers between z ~ 3 and z ~ 0, with most of this activity, and resulting mass assembly, occurring at z>1.5.
Journal Article
Unveiling the Formation of Massive Galaxies
Conselice discusses the predictions of astronomers for how massive galaxies should form. It is difficult to determine the ages of massive galaxies with the use of modern methods for dating stellar populations and the solution is to study galaxies forming in the early universe by examining galaxies whose light is highly redshifted and thus emerging from the most distant parts of the universe.
Journal Article
Galaxy bulges at mid- and high-redshift
Bulges are a major galaxy component in the nearby universe, and are one of the primary features that differentiates and defines galaxies. The origin of bulges can be directly probed in part by examining distant galaxies to search for high redshift bulges, and to study the properties of bulges in formation. We review the evidence for bulges at high redshift in this article, and how by studying bulges through a variety of approaches, including through morphological, colour, and stellar mass selection, we can determine when and how these systems assembled. We argue that the majority of the most massive ‘classical’ bulges are in place by z ~ 1.5 − 2, and likely formed very early through major mergers. Other, likely lower mass, bulges form through a secular process along with their disks. Direct observations suggest that these two formation processes are occurring, as spheroids are commonly seen at z < 1, as are disks and spiral galaxies in the form of luminous diffuse objects, clump-clusters, and chain galaxies. However, bulge+disk systems are relatively rare until z ~ 1, suggesting that this structural assembly occurred relatively late.
Journal Article
Evolution of Massive Galaxy Structural Properties and Sizes via Star Formation
We investigate the resolved star formation properties of a sample of 45 massive galaxies (M* > 1011 M⊙) within a redshift range of 1.5 ⩽ z ⩽ 3 detected in the GOODS NICMOS Survey (Conselice et al. 2011), a HST H160-band imaging program. We derive the star formation rate as a function of radius using rest frame UV data from deep z850 ACS imaging. The star formation present at high redshift is then extrapolated to z = 0, and we examine the stellar mass produced in individual regions within each galaxy. We also construct new stellar mass profiles of the in situ stellar mass at high redshift from Sérsic fits to rest-frame optical, H160-band, data. We combine the two stellar mass profiles to produce an evolved stellar mass profile. We then fit a new Sérsic profile to the evolved profile, from which we examine what effect the resulting stellar mass distribution added via star formation has on the structure and size of each individual galaxy.
Journal Article
Constraining galaxy formation models with dwarf ellipticals in clusters
Recent observations demonstrate that dwarf galaxies in clusters, despite their faintness, may be a critical galaxy type for understanding the physical processes behind galaxy formation. Dwarfs are the most common type of galaxy and are particularly abundant in rich galaxy clusters. The dominate model for explaining the formation of these systems thorough Cold Dark Matter models is that the bulk of their stellar mass formed early, and within their present environments. Recent results however suggest that some dwarfs appear in clusters after the bulk of its members form, a scenario not predicted in standard hierarchical structure formation models. Many of these systems appear to be younger and more metal rich than dwarfs in lower density areas, suggesting they are possibly created by a tidal process induced by the cluster. Several general galaxy cluster observations, including steep luminosity functions and the origin of intracluster light, are natural outcomes of these processes.
Journal Article
Truncated star formation in dwarf spheroidal galaxies and photometric scaling relations
We investigate the global photometric scaling relations traced by early-type galaxies in different environments, ranging from dwarf spheroidals, over dwarf elliptical galaxies, up to giant ellipticals (−8 mag ≳ MV ≳ −24 mag). These results are based in part on our new HST/ACS F555W and F814W imagery of dwarf spheroidal galaxies in the Perseus Cluster. We show that at MV ~ −14 mag, the slopes of the photometric scaling relations involving the Sérsic parameters change significantly. We argue that these changes in slope reflect the different physical processes that dominate the evolution of early-type galaxies in different mass regimes. We present N-body/SPH simulations of the formation and evolution of dwarf spheroidals that reproduce these slope changes and discuss the underlying physics. As such, these scaling relations contain a wealth of information that can be used to test models for the formation of early-type galaxies.
Journal Article
Low-mass cluster galaxies: A cornerstone of galaxy evolution
Low-Mass cluster galaxies are the most common galaxy type in the universe and are important objects for understanding galaxy formation, luminosity functions, dark matter and the formation of large scale structure. In this short summary I describe the properties and likely origins of low-mass cluster galaxies and what they reveal about broader cosmological issues.
Journal Article