Robeson M. Herrnstein

We present the first large-scale mosaic performed with the Submillimeter Array (SMA) in the Galactic center. We have produced a 25-pointing mosaic, covering a ~2' x 2' area around Sgr A*. We have detected emission from two high-density molecular tracers, HCN(4-3) and CS(7-6), the latter never before reported in this region. The data have an angular resolution of 4.6" x 3.1", and the spectral window coverage is from -180 km/s to 1490 km/s for HCN(4-3) and from -1605 km/s to 129 km/s for CS(7-6). Both molecular tracers present a very clumpy distribution along the circumnuclear disk (CND), and are detected with a high signal-to-noise ratio in the southern part of the CND, while they are weaker towards the northern part. Assuming that the clumps are as close to the Galactic center as their projected distances, they are still dense enough to be gravitationally stable against the tidal shear produced by the supermassive black hole. Therefore, the CND is a non-transient structure. This geometrical distribution of both tracers suggests that the southern part of the CND is denser than the northern part. Also, by comparing the HCN(4-3) results with HCN(1-0) results we can see that the northern and the southern parts of the CND have different excitation levels, with the southern part warmer than the northern. Finally, we compare our results with those obtained with the detection of NH3, which traces the warmer and less dense material detected in the inner cavity of the CND. We suggest that we are detecting the origin point where a portion of the CND becomes destabilized and approaches the dynamical center of the Milky Way, possibly being impacted by the southern streamer and heated on its way inwards.

Using the NH₃ (6, 6) transition, which samples dense (~10⁵) molecular gas with an energy above ground of 412 K, we find hot gas at high velocities (-142 to -210 km s^-1) associated with the central 2 pc of the Galactic center. This material may be either infalling gas due to shocks or tidal stripping, or possibly gas swept from the nuclear region. We identify two high-velocity features, which we call the Southern Runner and the Cap, and correlate these features with others detected in various molecular observations of the Galactic center. The characteristic line widths of the Southern Runner and Cap, 10-15 km s^-1, are similar to those of other hot Galactic center clouds. The estimated H₂ masses of these clouds are 4 × 10³ and 2 × 10³ M_☉, consistent with the masses of the western streamer and northern ridge, NH₃ (6, 6) emission features detected within the central 10 pc at lower velocities. Three possible explanations for this emission are discussed, assuming that they lie at the Galactic center, including sweeping by the supernova remnant Sgr A East, infall and/or shock from the circumnuclear disk (CND), and stripping from the central rotating low-velocity NH ₃ (6, 6) cloud.

We present the first interferometric detection of extragalactic NH₃ (6, 6) emission in the nearby galaxy IC 342 made using the VLA. The data have a resolution of 78 × 50 and trace hot (T ~ 412 K) and dense (>10⁴ cm^-3) molecular gas. We have covered a 170'' × 300'' area and detect two very strong line emission peaks, likely associated with the two strongest star formation regions of the central part of the galaxy. We compare these emission peaks to CO (1-0) and (2-1) emission data, which are the most abundant CO transitions and trace spatially extended emission. The NH₃ (6, 6) emission is also compared to emission data from three high-density, nitrogen-bearing tracers: HNC (1-0), HC₃N (10-9), and N₂H⁺ (1-0). Our results suggest that the molecular mass in the nuclear region of IC 342 has at least two different components, a dense and cold component and a less dense and hotter component.

We present the first interferometric detection of extragalactic NH_3(6,6) emission in the nearby galaxy IC 342 made using the VLA. The data have a resolution of 7.8''x 5.0'' and trace hot (T ~ 412 K) and dense (> 10^{4} cm^{-3}) molecular gas. We have covered a 170''x 300'' area, and detect two very strong line emission peaks, likely associated with the two strongest star formation regions of the central part of the galaxy. We compare these emission peaks to CO (1-0) and (2-1) emission data, which are the most abundant CO transitions and trace spatially extended emission. The NH_3(6,6) emission is also compared to emission data from three high-density, nitrogen bearing tracers: HNC(1-0), HC_3N(10-9) and N_2H^{+}. Our results suggest that the molecular mass in the nuclear region of IC 342 has at least two different components, a dense and cold component and a less dense and hotter component.

Using the NH\3 (6,6) transition, which samples dense ($\sim 10^{5}$) molecular gas with an energy above ground of 412 K, we find hot gas at high velocities (--142 to --210 km s$^{-1}$) associated with the central 2 pc of the Galactic center. This material may be either infalling gas due to shocks or tidal stripping, or possibly gas swept from the nuclear region. We identify two high-velocity features, which we call the Southern Runner and the Cap, and correlate these features with others detected in various molecular observations of the Galactic center. The characteristic linewidths of the Southern Runner and Cap, 10 -- 15 \kms, are similar to those of other hot Galactic center clouds. The estimated H$_{2}$ masses of these clouds are 4$\times 10^{3}$ M$\sol$ and 2$\times 10^{3}$ M$\sol$, consistent with the masses of the western streamer and northern ridge, NH\3 (6,6) emission features detected within the central 10 pc at lower velocities. Three possible explanations for this emission are discussed assuming that they lie at the Galactic center, including sweeping by the supernova remnant Sgr A East, infall and/or shock from the circumnuclear disk (CND), and stripping from the central rotating low-velocity NH\3 (6,6) cloud.

We present the results of a 3.3-year project to monitor the flux density of Sagittarius A* at 2.0, 1.3, and 0.7 cm with the VLA. The fully calibrated light curves for Sgr A* at all three wavelengths are presented. Typical errors in the flux density are 6.1%, 6.2%, and 9.2% at 2.0, 1.3, and 0.7 cm, respectively. There is preliminary evidence for a bimodal distribution of flux densities, which may indicate the existence of two distinct states of accretion onto the supermassive black hole. At 1.3 and 0.7 cm, there is a tail in the distribution towards high flux densities. Significant variability is detected at all three wavelengths, with the largest amplitude variations occurring at 0.7 cm. The rms deviation of the flux density of Sgr A* is 0.13, 0.16, and 0.21 Jy at 2.0, 1.3, and 0.7 cm, respectively. During much of this monitoring campaign, Sgr A* appeared to be relatively quiescent compared to results from previous campaigns. At no point during the monitoring campaign did the flux density of Sgr A* more than double its mean value. The mean spectral index of Sgr A* is alpha=0.20+/-0.01, with a standard deviation of 0.14. The spectral index appears to depend linearly on the observed flux density at 0.7 cm with a steeper index observed during outbursts. This correlation is consistent with the expectation for outbursts that are self-absorbed at wavelengths of 0.7 cm or longer and inconsistent with the effects of simple models for interstellar scintillation. Much of the variability of Sgr A*, including possible time lags between flux density changes at the different wavelengths, appears to occur on time scales less than the time resolution of our observations (8 days). Future observations should focus on the evolution of the flux density on these time scales.