Three regimes of CO emission in galaxy mergers

¹ Department of Astronomy and Theoretical Physics, Lund Observatory, Box 43, 221 00 Lund, Sweden
e-mail: florent@astro.lu.se
² Laboratoire AIM Paris-Saclay, CEA/IRFU/SAp, Université Paris Diderot, 91191, Gif-sur-Yvette Cedex, France
³ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 16, 53121 Bonn, Germany

Received: 8 October 2018
Accepted: 15 November 2018

Abstract

The conversion factor α_CO from the observable CO(1-0) luminosity to the mass of molecular gas is known to vary between isolated galaxies and some mergers, but the underlying reasons are not clearly understood. Thus, the value(s) of α_CO that are to be adopted remain highly uncertain. To provide better constraints, we applied the large velocity gradient method to a series of hydrodynamical simulations of galaxies and derived the evolution of α_CO. We report significant variations of α_CO, and identify three distinct regimes: disk galaxies, starbursts, and post-burst phases. We show that estimating the star formation rate over 20Myr smoothes out some of these differences, but still maintains a distinction between disks and starbursts. We find a tighter correlation of α_CO with the gas depletion time than with star formation rate, but deviations are induced by the transitions to and from the starburst episodes. We conclude that α_CO fluctuates because of both feedback energy and velocity dispersion. Identifying the phase of an interaction by classical means (e.g., morphology or luminosity) could then help to select the relevant conversion factor that is to be used and to obtain more accurate estimates of the molecular masses of galaxies.