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A More Extreme View of Molecular Gas in the Center of the Milky Way galaxy

  • Author(s): Mills, Elisabeth
  • Advisor(s): Morris, Mark R
  • et al.
Abstract

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.

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