The Star Formation Rate - Dense Gas Relation in Galaxies as Measured by HCN (3-2) Emission

TL;DR

This study investigates the relationship between star formation rate and dense gas in galaxies using HCN (3-2) emission, finding a sub-linear correlation that supports models where line luminosity depends on average gas density.

Contribution

It provides new observational data on HCN (3-2) emission in nearby galaxies and refines the slope of the star formation rate-dense gas relation, supporting theoretical models with density-dependent line luminosity.

Findings

The slope of the IR-HCN (3-2) luminosity relation is approximately 0.79.

The relation is sub-linear, indicating non-linear tracing of dense gas by HCN (3-2).

Results align with models where line luminosity depends on average gas density.

Abstract

We present observations made with the 10m Heinrich Hertz Sub-Millimeter Telescope of HCN (3-2) emission from a sample of 30 nearby galaxies ranging in infrared luminosity from 10^10 - 10^12.5 L_sun and HCN (3-2) luminosity from 10^6 - 10^9 K km s^-1 pc^2. We examine the correlation between the infrared luminosity and HCN (3-2) luminosity and find that the best fit linear regression has a slope (in log-log space) of 0.74+/-0.12. Including recently published data from Gracia-Carpio et al. tightens the constraints on the best-fit slope to 0.79+/-0.09. This slope below unity suggests that the HCN (3-2) molecular line luminosity is not linearly tracing the amount of dense gas. Our results are consistent with predictions from recent theoretical models that find slopes below unity when the line luminosity depends upon the average gas density with a power-law index greater than a Kennicutt-Schmidt index of 1.5.