No description available.

No description available.

The purpose of this dissertation is to locate evolved massive stars within the central 50 pc of the Galactic Center. These stars are considered to be the descendants of O stars and should be less than 10 Myr old. They trace young star clusters within the Galactic Center. Through these stars and young star clusters, we hope to understand the star formation mode and history within the Galactic Center, as well as the properties of evolved massive stars in the high metallicity environment. We first study the Chandra X-ray deep survey of the Arches and Quintuplet clusters, two of the three young massive star clusters within the Galactic Center. The diffuse X-ray emission is used to constrain their initial mass function and we find a deficiency of low-mass stars, which could be explained by an ongoing collision between the clusters and the adjacent molecular clouds. We then perform a systematic search of young massive stars on a large scale within the Galactic Center through our new HST/NICMOS Paschen-alpha survey. We produce mosaic maps of the Paschen-alpha line and continuum emission, giving an unprecedentedly high resolution and high sensitivity panoramic view of stars and photo-ionized gas in the nuclear environment of the Galaxy. Many new HII regions and extended emission regions have been found. Combined with the archived HST snapshot observations and spectroscopic observations, we construct a sample of 180 potentially evolved massive stars. A multi-wavelength study of these stars is conducted. We find that young massive stars have continued to form within the Galactic Center during the last 10 Myr and some of the evolved massive stars may represent star formation in small groups or even in isolation, compared to the three massive star clusters within the Galactic Center

The mechanism by which massive stars form is not nearly as well understood as it is for lower mass stars. For instance, at the onset of massive star formation, it is still not clear whether the mass for a given massive star comes from the turbulent collapse of a dense core (i.e McKee & Tan, 2003) or whether the star continues to accrete material from the cores environment as it grows (i.e. Bonnell et al., 1998). From this point, it is suggested that the cold, massive core (an Infrared Dar Cloud) begins to heat up and form a Hot Core. Later in its protostellar evolution, an HII region forms from the ionizing radiation being produced by the massive star. How, or even whether, accretion onto the massive protostar can continue in the presence of the large outward thermal and radiation pressures from the star is also quite uncertain. Can the star continue to accrete ionized gas (i.e. Keto & Wood, 2006)? Are the accretion rates high enough early on to account for the final observed masses (i.e. Klaassen et al., 2006)? Or, is there some way of minimizing the radiation pressure affecting the infalling gas (i.e. McKee & Ostriker, 2007, and references therein). Here, we present observations which suggest that there is a statistically significant, although short, period in which rotation and infall of molecular gas (which powers a bipolar outflow) continue after the formation of an HII region. This continued infall of material is seen on both large and small scales, and appears to be continuing to produce outflows in many of the sources observed in this study. That it is not seen in all sources suggests that this stage is short lived. / Thesis / Doctor of Philosophy (PhD)

Galaxy properties are known to correlate with their environment, suggesting that environment plays a significant role in galaxy evolution. In particular, blue star forming spiral galaxies are preferentially found in low density regions while red, passive elliptical galaxies are found in the densest clusters. This suggests galaxies falling into groups and clusters experience a decrease in their star formation rate (SFR) and a morphological transformation from spiral to elliptical, but the timescales associated with these changes are not well constrained. This thesis explores the impact of environment on galaxy SFRs and morphologies for a large sample of galaxies from the Sloan Digital Sky Survey. We separate galaxies into two environments (groups and clusters) and use location in projected phase space as an estimate for how long a galaxy has been a part of its current environment. We calculate the timescales associated with the changes in galaxy SFRs and morphologies, and determine SFRs change more quickly than morphology. By comparing to a sample of field galaxies, we find evidence that prior group environments impact current galaxy properties via pre-processing. / Thesis / Master of Science (MSc)

No description available.

No description available.

Since high-mass stars form deeply embedded within dense molecular clumps, the evolution of young stars and of dense clumps is inextricably linked. Previous datasets, however, lack the information necessary to test the prevailing theories. Definitive tests require a sufficiently large sample of molecular clumps and maps of their gas temperatures, column densities, velocity dispersions, and velocities at a spatial resolution comparable to, or smaller than, the clump scale (~1 pc). The Radio Ammonia Mid-Plane Survey (RAMPS), a new molecular line survey of thermal NH3 and H2O masers, provides the necessary data. In this dissertation, I used RAMPS data and archival datasets to test several theories of high-mass star formation and to investigate the co-evolution of molecular clumps and high-mass stars. All theories of high-mass star formation make testable predictions regarding clump kinematics and gravitational stability. Analyses of RAMPS kinematic data revealed that the majority of molecular clumps, particularly those in early evolutionary stages, are unstable to gravitational collapse. Further, they display infall motions, a key prediction of the theory of competitive accretion. I also investigated the kinematics of molecular filaments by comparing their measured velocity gradients to those predicted by hydrodynamical simulations. The measured spatial distributions of velocity gradients are inconsistent with existing models. Feedback from protostars and stars is predicted to alter the properties of surrounding clumps. I investigated feedback size scales and found that high-mass protostellar and stellar feedback significantly changes the temperatures, chemical abundances, and velocity dispersions of clumps on scales of ~0.3 to 3 pc. Finally, I observed a massive molecular cloud filament undergoing an interaction with a supernova shock, which is accelerating, heating, and injecting turbulence into the filament's gas. Although the molecular cores hosted by the filament may remain gravitationally bound, the filament is gravitationally unbound and likely being dispersed. Given that the shock is removing a reservoir of gas that could have been accreted by the cores, these data suggest that the supernova is inhibiting star formation.

Ram pressure stripping is a well-known galactic quenching mechanism capable of removing star-forming gas from a galaxy as it falls into a group or cluster. However, prior to stripping, ram pressure can induce brief periods of enhanced star formation by compressing the gas on the leading side of an infalling galaxy. Studies of this phenomenon have focused primarily on a unique population of galaxies for which a stripped tail of gas opposing the direction of motion is visible, known as jellyfish galaxies. The role of this effect in galaxy evolution overall is currently unknown. This thesis investigates the importance of ram pressure-induced star formation across all infalling galaxies to generalize our understanding of the effect. We use several metrics to measure the star formation asymmetries of a large sample of group and cluster galaxies in the Sloan Digital Sky Survey using $u$-band imaging from the Canada-France Imaging Survey as a tracer for star formation rate. We find that the distributions of star formation asymmetries of satellite galaxies are indistinguishable from those of a control sample of isolated field galaxies. Subdividing the sample by host halo mass and time since infall, we still find no environmental dependence of ram pressure as an enhancer of star formation. We conclude that any statistical star formation enhancement is small for infalling galaxies, suggesting that this effect is either uncommon or short-lived. / Thesis / Master of Science (MSc)

Most galaxies are either actively forming stars or quenched, but there is a small number of galaxies in transition from one population to the other. These galaxies are "quenching" if they are in the process of becoming quenched or "rejuvenating" if they are returning to the star-forming main sequence after a period of being quenched. Quenching occurs when a galaxy’s limited cold gas supply is heated or removed, halting star formation, while rejuvenation refers to any process that reintroduces cold gas to quenched galaxies, reigniting star formation. Rejuvenating galaxies, which are significantly rarer and less well-studied than quenching galaxies, can offer valuable insights into galaxy evolution processes. This thesis investigates the properties of transitional galaxies, with a focus on their gas content, to explore the mechanisms driving quenching and rejuvenation. We employ a recent classification method using GALEX NUV and Sloan Digital Sky Survey H-alpha measurements to identify transitional galaxies and analyze the derived gas properties of those in the xGASS and xCOLD GASS surveys. We find that rejuvenating and quenching galaxies have intermediate gas fractions compared to actively star-forming and quenched galaxies, and that rejuvenating and quenching galaxies have similar depletion times to star-forming and quenched galaxies, respectively. We also find that the rejuvenating population, particularly at lower stellar mass, is efficient at converting its atomic gas supply to molecular hydrogen, which could be attributed to their high gas-phase metallicities at low stellar mass. / Thesis / Master of Science (MSc)