The Dense Gas in the Central Kiloparsec of NGC 6946

TL;DR

This study investigates dense molecular gas in the central kiloparsec of NGC 6946 using HCN and HCO+ observations, analyzing its distribution, density, and relation to galactic pressure, revealing insights into molecular gas behavior in galaxy centers.

Contribution

It provides new measurements of dense gas tracers in NGC 6946 and explores hypotheses about their distribution and relation to galactic pressure, applying PDR models and fitting empirical correlations.

Findings

Dense gas tracers are concentrated at galaxy center.

Hydrogen molecule density is roughly constant at 10^5 cm^-3.

Correlations between midplane pressure and dense gas tracers are established.

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 I_HCN/I_CO and I_ HCO/I_CO 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 K_S 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 I_HCN/I_HCO+ 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 10^5 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 I_HCN, I_HCO+, and I_CO. Correlations between the hydrostatic midplane pressure and I_HCN and I_HCO+ are demonstrated, and power law fits are provided. We confirm the validity of a relation found by Blitz & Rosolowsky (2006) between pressure and the molecular to atomic gas ratio in the high hydrostatic midplane pressure regime (10^6-10^8 cm^-3 K).