A $^{13}$CO Detection in a Brightest Cluster Galaxy

TL;DR

This study reports the first detection of $^{13}$CO emission in a galaxy cluster's brightest galaxy, revealing detailed molecular gas properties and suggesting a lower CO-to-H$_2$ conversion factor than the Galactic standard.

Contribution

It provides the first $^{13}$CO detection in a galaxy cluster's brightest galaxy and analyzes its implications for molecular gas mass estimates.

Findings

$^{13}$CO emission confined to two molecular gas clumps

Derived CO-to-H$_2$ conversion factor is about half the Galactic value

Molecular gas mass estimated at $2.1 imes 10^9$ solar masses

Abstract

We present ALMA Cycle 4 observations of CO(1-0), CO(3-2), and $^{13}$CO(3-2) line emission in the brightest cluster galaxy of RXJ0821+0752. This is one of the first detections of $^{13}$CO line emission in a galaxy cluster. Half of the CO(3-2) line emission originates from two clumps of molecular gas that are spatially offset from the galactic center. These clumps are surrounded by diffuse emission that extends $8~{\rm kpc}$ in length. The detected $^{13}$CO emission is confined entirely to the two bright clumps, with any emission outside of this region lying below our detection threshold. Two distinct velocity components with similar integrated fluxes are detected in the $^{12}$CO spectra. The narrower component ($60~{\rm km}~{\rm s}^{-1}$ FWHM) is consistent in both velocity centroid and linewidth with $^{13}$CO(3-2) emission, while the broader ($130-160~{\rm km}~{\rm s}^{-1}$), slightly blueshifted wing has no associated $^{13}$CO(3-2) emission. A simple local thermodynamic model indicates that the $^{13}$CO emission traces $2.1\times 10^{9}~{\rm M}_\odot$ of molecular gas. Isolating the $^{12}$CO velocity component that accompanies the $^{13}$CO emission yields a CO-to-H$_2$ conversion factor of $\alpha_{\rm CO}=2.3~{\rm M}_{\odot}~({\rm K~km~s^{-1}})^{-1}$, which is a factor of two lower than the Galactic value. Adopting the Galactic CO-to-H$_2$ conversion factor in brightest cluster galaxies may therefore overestimate their molecular gas masses by a factor of two. This is within the object-to-object scatter from extragalactic sources, so calibrations in a larger sample of clusters are necessary in order to confirm a sub-Galactic conversion factor.