Predicting HCN, HCO⁺, multitransition CO, and dust emission of star-forming galaxies

Extension to luminous infrared galaxies and the role of cosmic-ray ionization

¹ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
² Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
³ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland

^⋆ Corresponding author; Bernd.Vollmer@astro.unistra.fr

Received: 22 July 2024
Accepted: 27 November 2024

Abstract

The specific star formation rate of star-forming main sequence galaxies significantly decreased since z ∼ 1.5 because the molecular gas fraction and star formation efficiency decreased. The gas velocity dispersion decreased within the same redshift range and is apparently correlated with the star formation efficiency (inverse of the molecular gas depletion time). However, the radio–infrared (IR) correlation has not changed significantly since z ∼ 1.5. The theory of turbulent clumpy star-forming gas disks together with the scaling relations of the interstellar medium describes the large- and small-scale properties of galactic gas disks. We extend our previous work on the IR multitransition molecular line, and radio continuum emission of local and high-z star-forming and starburst galaxies to local and z ∼ 0.5 luminous IR galaxies. The model reproduces the IR luminosities, CO, HCN, and HCO⁺ line luminosities, and the CO spectral line energy distributions of these galaxies. We derived CO(1–0) and HCN(1–0) conversion factors for all galaxy samples. The relation between the star formation rate per unit area and the H₂ surface density cannot be fit simply for all redshifts. The star formation efficiency, the product of the gas turbulent velocity dispersion, and the angular velocity of the galaxies are tightly correlated. Galaxies with lower stellar masses can in principle compensate their gas consumption via star formation by radial viscous gas accretion. The limiting stellar mass increases with redshift. The radio continuum emission is directly proportional to the density of cosmic-ray (CR) electrons, but the molecular line emission depends on the CR ionization rate via the gas chemistry. The normalization of the CR ionization rate we found for the different galaxy samples is higher by about a factor of three to five than the normalization for the solar neighborhood. This means that the mean yield of low-energy CR particles for a given star formation rate per unit area is higher by about three to ten times in external galaxies than was observed by Voyager I.