Modelling mechanical heating in star-forming galaxies: CO and ¹³CO Line ratios as sensitive probes

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: mher@strw.leidenuniv.nl
² Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 13 February 2014
Accepted: 6 June 2016

Abstract

We apply photo-dissociation region (PDR) molecular line emission models, that have varying degrees of enhanced mechanical heating rates, to the gaseous component of simulations of star-forming galaxies taken from the literature. Snapshots of these simulations are used to produce line emission maps for the rotational transitions of the CO molecule and its ¹³CO isotope up to J = 4−3. We use these maps to investigate the occurrence and effect of mechanical feedback on the physical parameters obtained from molecular line intensity ratios. We consider two galaxy models: a small disk galaxy of solar metallicity and a lighter dwarf galaxy with 0.2 Z_⊙ metallicity. Elevated excitation temperatures for CO(1−0) correlate positively with mechanical feedback, that is enhanced towards the central region of both model galaxies. The emission maps of these model galaxies are used to compute line ratios of CO and ¹³CO transitions. These line ratios are used as diagnostics where we attempt to match them These line ratios are used as diagnostics where we attempt to match them to mechanically heated single component (i.e. uniform density, Far-UV flux, visual extinction and velocity gradient) equilibrium PDR models. We find that PDRs ignoring mechanical feedback in the heating budget over-estimate the gas density by a factor of 100 and the far-UV flux by factors of ~10−1000. In contrast, PDRs that take mechanical feedback into account are able to fit all the line ratios for the central <2 kpc of the fiducial disk galaxy quite well. The mean mechanical heating rate per H atom that we recover from the line ratio fits of this region varies between 10^-27–10^-26 erg s^-1. Moreover, the mean gas density, mechanical heating rate, and the A_V are recovered to less than half dex. On the other hand, our single component PDR model fit is not suitable for determining the actual gas parameters of the dwarf galaxy, although the quality of the fit line ratios are comparable to that of the disk galaxy.