Densitometry and Thermometry of Starburst Galaxies

TL;DR

This study uses molecular line observations to measure physical conditions in starburst galaxies, revealing consistent dense gas densities and highlighting the importance of direct gas temperature measurements for understanding star formation.

Contribution

First measurements of spatial density and kinetic temperature in starburst galaxies using formaldehyde and ammonia lines, extending techniques from Galactic star formation studies.

Findings

Spatial density in starburst galaxies is roughly constant (~10^{4.5} to 10^{5.5} cm^{-3}).

Kinetic temperature measurements differ from dust-based estimates, emphasizing the need for direct gas temperature probes.

The star formation rate correlates with dense gas mass, not average density.

Abstract

With a goal toward deriving the physical conditions in external galaxies, we present a survey of formaldehyde (H2CO) and ammonia (NH3) emission and absorption in a sample of starburst galaxies using the Green Bank Telescope. By extending well-established techniques used to derive the spatial density in star formation regions in our own Galaxy, we show how the relative intensity of the 1(10)-1(11) and 2(11)-2(12) K-doublet transitions of H2CO can provide an accurate densitometer for the active star formation environments found in starburst galaxies (c.f. Mangum et al. 2008). Similarly, we employ the well-established technique of using the relative intensities of the (1,1), (2,2), and (4,4) transitions of NH3 to derive the kinetic temperature in starburst galaxies. Our measurements of the kinetic temperature constrained spatial density in our starburst galaxy sample represent the first mean density measurements made toward starburst galaxies. We note a disparity between kinetic temperature measurements derived assuming direct coupling to dust and those derived from our NH3 measurements which points to the absolute need for direct gas kinetic temperature measurements using an appropriate molecular probe. Finally, our spatial density measurements point to a rough constancy to the spatial density (10^{4.5} to 10^{5.5} cm^{-3}) in our starburst galaxy sample. This implies that the Schmidt-Kennicutt relation between L_{IR} and M_{dense}: (1) Is a measure of the dense gas mass reservoir available to form stars, and (2) Is not directly dependent upon a higher average density driving the star formation process in the most luminous starburst galaxies.