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

Mark Gorski, Lena Murchikova

TL;DR

This study tackles the problem of measuring the CO-to-H$_2$ conversion factor in the Milky Way's central parsec, where extreme environmental conditions may alter the standard disk value. Using ALMA CO($J=2-1$) observations of the circumnuclear disk around Sgr A*, the authors derive $\\alpha_{\mathrm{CO}}$ and $X_{\mathrm{CO}}$ without virial assumptions, via two independent approaches: area-based and HCN($J=1-0$) flux-based estimates. They find $\\alpha_{\mathrm{CO}} \approx 4.3$–$4.7\,M_\odot\,(\\mathrm{K\,km\,s^{-1}\,pc^{2}})^{-1}$ and $X_{\mathrm{CO}} \approx (2.1\pm1.1)\times10^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$, values that align with the Galactic disk but exceed CMZ values, implying the CND's gas properties differ from the CMZ. Foreground absorption is assessed and found to have only a minor effect ($\lesssim 1\%$) on the measurements. Overall, the results provide a crucial benchmark for molecular gas mass estimates in galactic centers and highlight the importance of region-specific conversion factors.

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 $α_\mathrm{CO} \simeq 4.5\pm2.5 \, M_\odot (\mathrm{K \, km \, s^{-1} pc^2})^{-1}$ 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.

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

TL;DR

This study tackles the problem of measuring the CO-to-H conversion factor in the Milky Way's central parsec, where extreme environmental conditions may alter the standard disk value. Using ALMA CO() observations of the circumnuclear disk around Sgr A*, the authors derive and without virial assumptions, via two independent approaches: area-based and HCN() flux-based estimates. They find – and cm (K km s), values that align with the Galactic disk but exceed CMZ values, implying the CND's gas properties differ from the CMZ. Foreground absorption is assessed and found to have only a minor effect () on the measurements. Overall, the results provide a crucial benchmark for molecular gas mass estimates in galactic centers and highlight the importance of region-specific conversion factors.

Abstract

Carbon monoxide (CO) emission is a widely used tracer of molecular hydrogen (H) in the interstellar medium (ISM), owing to its abundance, low excitation energy, and ease of detection in cold molecular environments, in contrast to itself. While the CO-to- 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- conversion factor on sub-kpc scales. We calculate CO-to- conversion in the Milky Way's Circumnuclear Disk/Ring (CND/CNR) at pc radius around the Galactic Center black hole. We derive a conversion factor of or X[CO] . This value is consistent with the Galactic disk but higher than CMZ.

Paper Structure

This paper contains 9 sections, 5 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Observations
  3. Calibration and Image quality
  4. Flux recovery
  5. CO--to--H$_{2}$ conversion factor in Milky Way's Galactic Center
  6. Caculating by area
  7. Calculating by fraction of HCN emission
  8. Foreground Absorption
  9. Summary

Figures (2)

  • Figure 1: Integrated flux map of the $^{12}{\rm CO}(J=2-1)$ emission around Sgr A$^*$. The map is primary beam corrected. The calculations of the CO--to--H$_{2}$ conversion factor use the region outlined with the white contours. The position of Sgr A$^*$ is marked with a white star.
  • Figure 2: The full map of the CND in ${\rm HCN}(J=1-0)$ emission from Hsieh2021. The dotted ellipse shows the extent of the CND Tsuboi2018. The part of the CND considered for the calculations of the CO--to--H$_{2}$ conversion factor is outlined with the white contours. The position of Sgr A$^*$ is marked with a white star.