Unusual Ammonia Excitation Conditions in the Galactic Center
by: Betsy Mills (NRAO - Socorro)
Gas in the central 500 parsecs of our Galaxy is unusual in multiple ways. The molecular gas in this region is on average denser, hotter, and more turbulent than elsewhere in the Galactic disk, by 1-2 orders of magnitude. However, neither the reasons why the gas is not forming stars proportional to its increased density, nor the (likely related) mechanisms behind the increased temperature and turbulence, have been pinpointed. In this talk, I will focus on the our understanding of the unusually gas high temperatures in this region, including current observational constraints and biases, as well as the proposed heating mechanisms and how they can be tested. I will review recent surveys of the molecular gas temps with single-dish telescopes such as APEX and GBT which place new limits on the gas temperatures. I will primarily focus on ammonia, which is an ideal probe of the temperature of this gas, as its large abundances and multiple transitions in the radio regime make it possible to easily constrain a wide range of gas temperatures. I will present results from new, high-resolution observations of ammonia with the VLA which for the first time show the temperature structure of Galactic center gas on sub-parsec scales. The high spatial resolution of these observations of ammonia in the extreme Galactic center environment also highlight several factors which can complicate the use of ammonia as a temp probe, including maser action, and the impact of formational excitation. After correcting for these effects, these data show promise for distinguishing between multiple suggested heating mechanisms for this gas, including turbulent shocks, x-rays, and cosmic rays.
