This thesis examines the molecular gas properties in the central 600 parsecs of the Galaxy (the CMZ). I first present a study of a group of HII regions in the central 10 parsecs embedded within the M-0.02-0.07 cloud, adjacent to the Sgr A East supernova remnant. To better depict the physical relationship between these sources, I use archival VLA observations and Paschen-alpha images from the Hubble Space telescope to determine the extinction toward the HII regions. From the measured extinctions and source morphologies, I determine that three of the HII regions lie on the front side of the cloud and formed in the eastern part of the cloud which is unaffected by the supernova remnant's expansion. The higher extinction of the fourth HII region indicates it is embedded in the supernova-compressed ridge, and though younger than the other HII regions, is still older than the supernova. This work is an important determination of the their line-of-sight placement of these sources which helps to define their interaction and strengthens the case that the supernova did not trigger the formation of the HII regions.

I next present results from a project I led to survey a sample of CMZ clouds for hot gas using the Green Bank Telescope. I observe multiple highly-excited lines of ammonia, and detect emission from the (9,9) line of ammonia, (excitation energy = 840 K) in 13 of 17 clouds, many of which have no associated star formation. For the three strongest sources, I derive rotation temperatures of 400-500 K, substantially higher than previous temperatures of 200-300 K measured for these clouds. The widespread detections of gas hotter than 400 K indicates for the first time that his hot gas must be heated by global processes in the CMZ. These extremely high temperatures also suggest that cosmic rays are not responsible for the heating of this gas.

I also present a determination of the density of the Circumnuclear disk (CND) in the central two parsecs using multiple transitions of HCN and \hco with the APEX telescope, a 12m ALMA prototype dish. These molecules have critical densities > 10^7 cm^-3 and directly constrain the existence of high-density gas. The excitation analysis of HCN and HCO+ yields significantly tighter limits on the typical densities in this gas n = 10^5 -10^7 cm^-3, however I cannot rule out the possibility that a small number of individual gas clumps are tidally stable or in virial equilibrium. The detection of the v_2=1 vibrationally-excited J=4-3 HCN line also indicates that radiative excitation may play a role in the excitation of HCN in this source, but it is not clear if radiative excitation in the CND is a localized or global phenomenon.

Finally, I present a summary of early results from a survey of four molecular cloud complexes in the CMZ , which I led using the new capabilities of the VLA. The goal of this work is to determine the variation in temperature in these clouds on 0.1 pc scales to ultimately identify their heating sources. An unanticipated discovery of this study has been hundreds of weak, collisionally-excited methanol masers which are detected in all of the clouds we survey. The masers are distributed non-uniformly in the clouds, with concentrations indicating regions of shock activity which may be driving the evolution of physical conditions in these clouds. I also find that the ammonia (3,3) line is weakly masing in several sources, which can be used to put constraints on the density of the gas clumps in these clouds.

A report on the relative abundances of the three stable isotopes of silicon, 28Si, 29Si and 30Si, across the Galaxy using the v = 0, J = 1 - 0 transition of silicon monoxide. The chosen sources represent a range in Galactocentric radii (RGC) from 0 to 9.8 kpc. The high spectral resolution and sensitivity afforded by the Robert C. Byrd Green Bank Telescope (GBT) permit isotope ratios to be corrected for optical depth, using a novel method developed as part of this study. The optical-depth-corrected data indicate that the secondary-to-primary silicon isotope ratios [29Si]/[28Si] and [30Si]/[28Si] vary much less than predicted on the basis of other stable isotope ratios in the Galaxy. Indeed, there is no detectable variation in Si isotope ratios with RGC. This lack of an isotope ratio gradient stands in stark contrast to the monotonically decreasing trend with RGC in published secondary-to-primary oxygen isotope ratios. These results, when considered in the context of the expectations for galactic chemical evolution, suggest that the reported isotope ratio trends in oxygen, and likely carbon as well, may be in error and require further investigation. The methods developed in this study for SiO isotopologue ratio measurements are equally applicable to Galactic oxygen, carbon and nitrogen isotope ratio measurements, and should prove useful for future observations of these isotope systems.

Turbulence is a prevalent phenomenon in the interstellar medium, and in particular, in the environment at the centers of galaxies. For example, detailed observations of the Milky Way’s Central Molecular Zone (CMZ) have revealed that it has a complex and turbulent structure. It is suspected that turbulence is responsible for maintaining the relatively high temperatures in the CMZ, as well as for suppressing star formation. Therefore, turbulence is a key phenomenon for understanding the dynamics and energetics of this region.

In galactic-scale simulations, turbulence is often modeled by invoking star formation and supernovae feedback. However, these phenomena do not appear to be sufficient for explaining the high-velocity dispersion observed in the CMZ, indicating that additional gas-stirring processes are likely to be operating. In this thesis, I present a new numerical method to driveturbulence in galactic centers. Instead of relying on a particular physical mechanism, I have adopted a Fourier forcing module, which has the advantage of being independent of the actual sources of turbulence, but which is adjustable to the scales needed to match observations. I have applied this method using a Smoothed Particle Hydrodynamics (SPH) code. This turbulence injection method is capable of balancing the self-gravity of gas in different physical scenarios, which allows the simulations to be run for long timescales, thereby enabling my study of the effects of turbulence on the gas dynamics.

First, I introduce this new numerical tool and present simulations of a simplistic model of the CMZ to showcase its performance and numerical convergence. I find that turbulence induces a flocculent spiral pattern similar to that observed in the centers of other galaxies. Furthermore, the driving module induces a turbulent viscosity which enhances mass inflowto the center.

To further show the flexibility and applicability of this method, I present larger-scale simulations of galactic nuclear rings. Nuclear rings are a common feature of galaxies which form due to the effects of bar potentials, and the CMZ contains such a ring. I find that turbulence thickens nuclear rings and enhances mass accretion towards the center, both effects causedby turbulent viscosity. This mass inflow could explain the feeding of material from the CMZ to the inner few parsecs of the Galaxy.

As a final example of my method’s versatility, I performed simulations of the Circumnuclear Disk (CND) at the center of the Milky Way. The transient vs long-lived nature of the CND is widely debated in the literature. Some authors have argued that the observed stream morphology and non-axisymmetric density distribution implies that the CND is a transientstructure. However, the results of my simulations suggest that these arguments are not correct. I find that turbulent perturbation on sufficiently large scales, such as those that would be produced by supernova blast waves, creates a disk with a non-uniform density structure, and promotes the creation of streams that endure for longer than a few dynamical timescales.

In conclusion, the work I present in this thesis shows that my injection method is a flexible tool that can be applied to a variety of physical scenarios. Furthermore, this method can be adapted to different hydrodynamical codes based on alternative hydrodynamics techniques.