12CO(3-2) Emission in Spiral Galaxies: Warm Molecular Gas in Action?

TL;DR

This study investigates 12CO(3-2) emission in five nearby spiral galaxies, finding emission mainly in high surface brightness galaxies with prominent bars, suggesting bars influence molecular gas excitation.

Contribution

It provides new observational evidence linking bar prominence and surface brightness to molecular gas excitation in spiral galaxies.

Findings

12CO(3-2) detected in two high surface brightness galaxies

Emission confined to galaxy bulges with high velocity dispersions

Bars may dynamically enhance molecular gas excitation

Abstract

Using the APEX sub-millimeter telescope we have investigated the 12CO(3-2) emission in five face-on nearby barred spiral galaxies, where three of them are high surface brightness galaxies (HSBs) lying at the Freeman limit, and two are low surface brightness galaxies (LSBs). We have positive detections for two of three HSB spirals and non-detections for the LSBs. For the galaxies with positive detection (NGC0521 and PGC070519), the emission is confined to their bulges, with velocity dispersions of ~90 and ~73 km/s and integrated intensities of 1.20 and 0.76 K/km/s, respectively. For the non-detections, the estimated upper limit for the integrated intensity is ~0.54 K/km/s. With these figures we estimate the H2 masses as well as the atomic-to-molecular mass ratios. Although all the galaxies are barred, we observe 12CO(3-2) emission only for galaxies with prominent bars. We speculate that bars could dynamically favor the 12CO(3-2) emission, as a second parameter after surface brightness. Therefore, secular evolution could play a major role in boosting collisional transitions of molecular gas, such as 12CO(3-2), especially in LSBs.