Browsing by Subject "Galaxy evolution"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Connecting the dots : tracking galaxy evolution using constant cumulative number density at 3(2015-12) Jaacks, Jason Dale; Finkelstein, Steven L.; Bromm, VolkerUsing the cosmological smoothed particle hydrodynamical code GADGET-3 we make a realistic assessment of the technique of using constant cumulative number density as a tracer of galaxy evolution. We find that over a redshift range of 3Item Probing galaxy evolution by unveiling the structure of massive galaxies across cosmic time and in diverse environments(2013-08) Weinzirl, Timothy Michael; Jogee, ShardhaHow galaxies form and evolve is one of the primary outstanding problems in extragalactic astronomy. I conduct a quantitative census of the relative importance of the major structural components (flattened and dynamically cold disk-dominated components versus puffy and dynamically hot spheroidal or triaxial bulges/ellipticals) in massive galaxies over cosmic time and across different environments in order to explore how galaxies evolve under the action of the various assembly mechanisms (major mergers, minor mergers, gas accretion, and internal secular processes) in these different regimes. I perform three inter-related analyses focusing on massive galaxies from z ~ 0 - 3 in both field and rich cluster environments. Important strengths of this thesis include the use of high-resolution, panchromatic imaging from some of the largest and deepest galaxy surveys with the Hubble Space Telescope (HST), Spitzer, and Chandra space telescopes, and also the inclusion of detailed comparisons between the empirical data and hierarchical ΛCDM-based models of galaxy evolution.Item Pseudobulges in disk galaxies : growth, structure and frequency in the local Universe(2010-08) Fisher, David Bradley; Kormendy, J. (John); Evans II, Neal J.; Bender, Ralf; Gebhardt, Karl; Jogee, ShardhaContrary to historic assumptions, bulges in the local Universe present a heterogeneous class of objects. Observations indicate that bulges are bimodal in structure, interstellar medium, stellar populations and dynamical state. Using observations in the UV, optical, near-infrared and mid-infrared we study the nature of local bulge-disk galaxies. The aim is first to find consistent means to differentiate different bulge types. Then we can use these diagnostic methods to study the properties of bulges of each type, thereby better understanding the possible formation mechanisms of each type. Finally, we will use these diagnostic methods to determine how many of each type of bulge exists in the local Universe, and thus understand how the heterogeneity of bulges may affect our understanding of galaxy evolution. Using 3.6-8.0 micron colors we show that dichotomy in bulge morphology is closely tied to the dichotomy in bulge interstellar medium. We find that those bulges with active interstellar medium, per unit stellar mass, have morphological features commonly found in disks (e.g. nuclear spirals, bars and rings). We follow this up with more robust star formation rates, as measured by linear combining UV and 24 micron luminosity, and determine that the boundary is near specific star formation rate ~30 Gyr⁻¹. We also find that the shape of bulge surface brightness profiles correlates well with morphology. When parameterized by a Sérsic function, we find that bulges with n[subscript b]<2 have disk-like morphology and those bulges with n[subscript b]>2 have morphology that is very similar to that of an elliptical galaxy. We thus conclude that bulges with disk-like nuclear morphology, specific star formation rate that is less than 30 Gyr⁻¹, and/or Sérsic index n[subscript b]<2 represents a distinct class of object. We refer to these bulges as "pseudobulges" and the complimentary set of bulges that are inactive, with high Sérsic index, and morphologically like elliptical galaxies is referred to as "classical bulges." We find that a significant amount of evidence points to pseudobulges and classical bulges originating from separate formation mechanisms. First, we rule out the possibility that pseudobulges are the result solely from mass dependent phenomenon. Rather, pseudobulges and classical bulges over lap significantly in mass, luminosity and size. Also, they are found in galaxies of similar mass, luminosity and size. Therefore, pseudobulges are not simply a low-mass phenomenon of the same process. Also, we find that many of the properties of pseudobulges are connected to properties of the outer disk. We find that the half-light radius of pseudobulges correlates linearly with the scale-length of the outer disk. Furthermore, this correlation does not exist for classical bulges. Also, the mass of pseudobulges correlates with the mass of the outer disk. We find that the star formation rate density of pseudobulges is a function of the stellar mass of the exponential outer disk such that pseudobulges with high star formation rate densities only occur more massive stellar disks. Thus it appears that both structure and growth of pseudobulges is a function of the properties of the outer disk. However, classical bulges do not show the same correlations. Also, we find that the star formation rate density of pseudobulges positively correlates with the mass density, classical bulges do not show an analogous correlation. If secular growth were responsible for the formation of pseudobulges, such a correlation should exist. Furthermore, we find that the specific star formation rates of most pseudobulges are high enough to account for the stellar mass within the typical ages of disk (~10 Gyr). We also show that classical bulges participate in the same structural parameter correlations as elliptical galaxies. Just like elliptical galaxies, as classical bulges become brighter they also become larger in radius, lower in surface density, and have higher Sérsic index. However pseudobulges behave very differently. There is little-to-no correlation between the size of pseudobulges and the luminosity, surface brightness or Sérsic index. We stress that this observation extends of 9 magnitudes in brightness. Therefore the size of pseudobulges, has thus far only been found to correlate with the size of the outer disk. Furthermore we find that pseudobulges show a positive correlation between surface density and luminosity. The behavior of pseudobulges in these parameter correlations implies that they are not virialized stellar systems that have experienced violent relaxation. Thus it is likely that the formation of pseudobulges is not like that of elliptical galaxies and classical bulges. Furthermore, the connection between pseudobulge properties and those of their associated outer disk seem to favor long-term growth that is more likely to be driven by disk processes, commonly called "secular evolution." Finally we show that the dichotomy of bulge types has a strong influence on our understanding of galaxy evolution. We find that global galaxy properties are tied to the bulge dichotomy. Galaxies with pseudobulges are found to be in "blue sequenc" galaxies and those with classical bulges are found to be in "red sequence" galaxies. A large body of literature has shown that blue and red galaxies appear to be distinct classifications of galaxies. The correlation with bulge type implies that the bulge dichotomy may be also be a consequence of the bimodal nature of galaxy evolution. Finally, we show that in the local Universe pseudobulges are by far the most common type of massive galaxy. We find that only 17% of galaxies have a detectable classical bulge. Also we show that over 3/4 of the star formation in spiral and elliptical galaxies in the local Universe occurs in galaxies with pseudobulges. Thus understanding pseudobulges is a necessary step to understanding the processes that have lead to the population of galaxies in the nearby Universe.Item Studying star formation at low and high redshift with integral field spectroscopy(2011-05) Blanc, Guillermo; Gebhardt, Karl; Evans, Neal J.; Hill, Gary J.; Bromm, Volker; Gawiser, EricIn this thesis I focus mainly in studying the process of star formation in both high redshift, and local star forming galaxies, by using an observational technique called integral field spectroscopy (IFS). Although these investigations are aimed at studying the star formation properties of these objects, throughout this work I will also discuss the geometric, kinematic, and chemical structures in the inter-stellar medium of these galaxies, which are intimately connected with the process of star formation itself. The studies presented here were conducted under the umbrella of two different projects. First, the HETDEX Pilot Survey for Emission Line Galaxies, where I have studied the properties of Ly-alpha emitting galaxies across the 2