The Dense Gas in the Central Kiloparsec of NGC 6946

Abstract

We present observations of the HCN and HCO -->+ -->J = 1–0 transitions in the center of the nearby spiral galaxy NGC 6946 made with the BIMA and CARMA interferometers. Using the BIMA SONG CO map, we investigate the change in the -->IHCN/ICO and -->IHCO+/ICO integrated intensity ratios as a function of radius in the central kiloparsec of the galaxy and find that they are strongly concentrated at the center. We use the 2MASS -->Ks-band image to find the stellar surface density and then construct a map of the hydrostatic midplane pressure. We apply a PDR model to the observed -->IHCN/IHCO+ integrated intensity ratio to calculate the number density of molecular hydrogen in the dense gas tracer emitting region and find that it is roughly constant at -->105 cm -->−3 across our map. We explore two hypotheses for the distribution of the dense gas. If the HCN and HCO -->+ emission comes from self-gravitating density peaks inside of a less dense gas distribution, there is a linear proportionality between the internal velocity dispersion of the dense gas and the size of the density peak. Alternatively, the HCN and HCO -->+ emission could come from dense gas homogeneously distributed throughout the center and bound by ambient pressure, similar to what is observed toward the center of the Milky Way. We find both of these hypotheses to be plausible. We fit the relationships between -->IHCN, IHCO+, and -->ICO. Correlations between the hydrostatic midplane pressure and -->IHCN and -->IHCO+ are demonstrated, and power-law fits are provided. We confirm the validity of a relation found by Blitz & Rosolowsky between pressure and the molecular to atomic gas ratio in the high hydrostatic midplane pressure regime ( -->106- -->108 cm -->−3 K).