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Abstract: We present observations of the HCN and HCO+ J=1-0 transitions in the centerof the nearby spiral galaxy NGC 6946 made with the BIMA and CARMAinterferometers. Using the BIMA SONG CO map, we investigate the change in theI HCN-I CO and I HCO-I CO integrated intensity ratios as a function of radiusin the central kiloparsec of the galaxy, and find that they are stronglyconcentrated at the center. We use the 2MASS K S band image to find the stellarsurface density, and then construct a map of the hydrostatic midplane pressure.We apply a PDR model to the observed I HCN-I HCO+ integrated intensity ratio tocalculate the number density of molecular hydrogen in the dense gas traceremitting region, and find that it is roughly constant at 10^5 cm^-3 across ourmap. We explore two hypotheses for the distribution of the dense gas. If theHCN and HCO+ emission comes from self-gravitating density peaks inside of aless dense gas distribution, there is a linear proportionality between theinternal velocity dispersion of the dense gas and the size of the density peak.Alternatively, the HCN and HCO+ emission could come from dense gashomogeneously distributed throughout the center and bound by ambient pressure,similar to what is observed toward the center of the Milky Way. We find both ofthese hypotheses to be plausible. We fit the relationships between I HCN,I HCO+, and I CO. Correlations between the hydrostatic midplane pressure andI HCN and I HCO+ are demonstrated, and power law fits are provided. We confirmthe validity of a relation found by Blitz and Rosolowsky 2006 between pressureand the molecular to atomic gas ratio in the high hydrostatic midplane pressureregime 10^6-10^8 cm^-3 K.

Autor: E.S. Levine, Tamara T. Helfer, R. Meijerink, Leo Blitz


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