The CO-to-H$_2$ conversion factor in the Milky Way's central parsec

TL;DR

This study estimates the CO-to-H2 conversion factor in the Milky Way's central parsec, revealing it is similar to the Galactic disk but higher than in the central molecular zone, with implications for molecular gas mass estimates.

Contribution

First measurement of the CO-to-H2 conversion factor at sub-kiloparsec scales in the Galactic Center's circumnuclear disk.

Findings

Conversion factor of approximately 4.5 M_sun (K km s^{-1} pc^2)^{-1}

Value consistent with Galactic disk, higher than CMZ

Provides a basis for more accurate molecular gas mass estimates in galactic centers.

Abstract

Carbon monoxide (CO) emission is a widely used tracer of molecular hydrogen (H$_2$) in the interstellar medium (ISM), owing to its abundance, low excitation energy, and ease of detection in cold molecular environments, in contrast to $\mathrm{H}_2$ itself. While the CO-to-$\mathrm{H}_2$ conversion factor is often assumed to be constant across the disks of galaxies, deviations are observed in extreme environments such as the central molecular zone (CMZ) in galactic nuclei. Here we present the first estimate of the CO-to-$\mathrm{H}_2$ conversion factor on sub-kpc scales. We calculate CO-to-$\mathrm{H}_2$ conversion in the Milky Way's Circumnuclear Disk/Ring (CND/CNR) at $\sim 1$ pc radius around the Galactic Center black hole. We derive a conversion factor of $\alpha_\mathrm{CO} \simeq 4.5\pm2.5 \, M_\odot (\mathrm{K \, km \, s^{-1} pc^2})^{-1}$ or X[CO] $\simeq (2.1\pm1.1)\times 10^{20} \, \mathrm{cm}^{-2} (\mathrm{K \, km \, s^{-1}})^{-1}$. This value is consistent with the Galactic disk but higher than CMZ.