The CO-to-H₂ conversion factor in the Milky Way's central parsec

2025/11/20 by Mark Gorski, Gorski, Mark, E. M. Murchikova +1
Physics and Astronomy · #Astrophysics and Star Formation Studies #Astrophysical Phenomena and Observations #Galaxies: Formation, Evolution, Phenomena

paper · pdf · doi:10.48550/arxiv.2511.16720

Abstract

Carbon monoxide (CO) emission is a widely used tracer of molecular hydrogen (H₂) in the interstellar medium (ISM), owing to its abundance, low excitation energy, and ease of detection in cold molecular environments, in contrast to H₂ itself. While the CO-to-H₂ 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-H₂ conversion factor on sub-kpc scales. We calculate CO-to-H₂ conversion in the Milky Way's Circumnuclear Disk/Ring (CND/CNR) at ∼ 1 pc radius around the Galactic Center black hole. We derive a conversion factor of α_CO ≃ 4.5±2.5 M_\odot (\mathrmK km s^-1 pc²)^-1 or X[CO] ≃ (2.1±1.1)× 10²⁰ cm^-2 (\mathrmK km s^-1)^-1. This value is consistent with the Galactic disk but higher than CMZ.

Citations

• The CO-to-H₂ Conversion Factor in the Central Molecular Zone of the Milky Way using CO isotopologues
• CASA, the Common Astronomy Software Applications for Radio Astronomy
• The Astropy Project: Building an Open-science Project and Status of the v2.0 Core Package^*
• The CO-to-H₂ Conversion Factor

Cited by

• The Discovery of an Active Wind from the Milky Way’s Central Black Hole

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• The CO-H2 Conversion Factor in Galaxy Mergers
• Predictions for CO emission and the CO-to-H₂ conversion factor in galaxy simulations with non-equilibrium chemistry