Central molecular zones in galaxies: ¹³CO(6–5) and molecular gas conditions in bright nearby galaxies

¹ Sterrewacht Leiden, PO Box 9513 2300 RA Leiden, The Netherlands
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Aix-Marseille Université, CNRS, LAM, Marseille 13388, France

^⋆ Corresponding author; israel@strw.leidenuniv.nl

Received: 20 February 2024
Accepted: 22 August 2024

Abstract

This paper summarizes all presently available J_upp ≥ 5  ¹³CO and accompanying  ¹²CO measurements of galaxy centers including new J = 6–5  ¹³CO and  ¹²CO observations of eleven galaxies with the Atacama Pathfinder EXperiment (APEX) telescope and also Herschel high-J measurements of both species in five galaxies. The observed J = 6–5/J = 1–0  ¹²CO integrated temperature ratios range from 0.10 to 0.45 in matching beams. Multi-aperture data indicate that the emission of  ¹³CO(6–5) is more centrally concentrated than that of  ¹²CO(6–5). The intensities of  ¹²CO(6–5) suggest a correlation with those of HCO⁺ but not with those of HCN. The new data are essential in refining and constraining the parameters of the observed galaxy center molecular gas in a simple two-phase model to approximate its complex multi-phase structure. In all galaxies except the Seyfert galaxy NGC 1068, high-J emission from the center is dominated by a dense (n ∼ 10⁵ cm⁻³) and relatively cool (20–60 K) high-pressure gas. In contrast, the low-J lines are dominated by low-pressure gas of a moderate density (n ∼ 10³ cm⁻³) and more elevated temperature (60–150 K) in most galaxies. The three exceptions with significant high-pressure gas contributions to the low-J emission are all associated with active central star formation.