The CO-to-H ₂ Conversion Factor in the Milky Way’s Central Parsec

Abstract Carbon monoxide (CO) emission is a widely used tracer of molecular hydrogen (H 2 ) in the interstellar medium, owing to its abundance, low excitation energy, and ease of detection in cold molecular environments, in contrast to H 2 itself. While the CO-to-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-H 2 conversion factor on subkiloparsec scales. We calculate CO-to-H 2 conversion in the Milky Way’s circumnuclear disk/ring at ∼1 pc radius around the Galactic Center black hole. We derive a conversion factor of α CO ≃ 4.5 ± 2.5 M ⊙ (K km s −1 pc 2 ) −1 or X[CO] ≃ (2.1 ± 1.1) ×10 20 cm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">K</mml:mi> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">km</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> . This value is consistent with the Galactic disk but higher than the CMZ.