Cosmic rays, gas, and dust in the central molecular zone

I. X_CO factors, cosmic-ray densities, and dust opacities

¹ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
² Université de Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
³ Department for Physics and Technology, University of Bergen, Allegaten 55, Bergen 5020, Norway

^★ Corresponding authors: helena.ren@mpi-hd.mpg.de; quentin.remy@mpi-hd.mpg.de

Received: 4 February 2025
Accepted: 14 March 2025

Abstract

Context. The central molecular zone (CMZ) is a unique environment in our Galaxy, with extreme conditions to test our understanding of the gas, dust, and cosmic-ray (CR) physics.

Aims. Our goal is to estimate the total gas mass in the direction of the Galactic centre (GC), quantify the various associated uncertainties, and discuss the implications for the estimates of CR energy densities and dust opacities.

Methods. The H_I 21 cm line and the carbon monoxide isotopes (¹²CO (J = 1 → 0), ¹³CO (J = 1 → 0; J = 2 → 1), and C¹⁸O (J = 2 → 1)) line emission maps were used to derive the total gas column density. The gas in the CMZ is separated from the disk contribution in position and velocity thanks to its different properties in terms of the velocity dispersion and brightness ratio of CO isotopes. The variations of the X_CO factors were modelled relying on both theoretical trends from simulations and empirical corrections. We used the new gas column density estimated together with gamma-ray and dust emission measurements to derive the CR energy density and dust opacities, respectively.

Results. The X_CO values in the CMZ range from (0.32–1.37) × 10²⁰ cm⁻² K⁻¹ km⁻¹ s, with a distribution that is highly asymmetric and skewed. The median value is X̅_CO^CMZ = 0.39 × 10²⁰ cm⁻² K⁻¹ km⁻¹ s. The total gas mass in the CMZ is estimated to be 2.3_−0.3^+0.3 × 10⁷ M_⊙ with an ∼10% contribution from the atomic phase. Without removing the disk contamination, the total mass is about twi-ce as higher, and the atomic gas fraction increases to ∼30%. The CR energy density in the CMZ, assuming a 1/r profile, is higher by a factor of two compared to the previous calculations at TeV energies.

Conclusions. Towards the GC the contamination from both atomic and molecular gas in the disk is not negligible. Using molecular gas tracers, which probes only the densest molecular cores, leads to an overestimation of the CR energy density, while ignoring the foreground and background contribution leads to an underestimation of the CR energy density in the CMZ.