Anisotropy in the carbon monoxide (CO) line emission across the Milky Way’s disk

¹ Istituto di Astrofisica e Planetologia Spaziali (IAPS). INAF. Via Fosso del Cavaliere 100, 00133 Roma, Italy
² Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
³ Laboratoire AIM, Paris-Saclay, CEA/IRFU/SAp – CNRS – Université Paris Diderot, 91191 Gif-sur-Yvette Cedex, France
⁴ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁵ Universität Heidelberg, Interdiszipliäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
⁸ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁹ SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
¹⁰ École Polytechnique Fédérale de Lausanne, Observatoire de Sauverny, Chemin Pegasi 51, 1290 Versoix, Switzerland

^★ Corresponding author: juandiegosolerp@gmail.com

Received: 4 April 2025
Accepted: 14 September 2025

Abstract

We present a study of the CO line emission anisotropy across the Milky Way’s disk to examine the effect of stellar feedback and Galactic dynamics on the distribution of the dense interstellar medium. We used the Hessian matrix method to characterize the ¹²CO(1–0) line emission distribution and identify the preferential orientation across line-of-sight velocity channels in the Dame et al. (2001, ApJ, 547, 792) composite Galactic plane survey, which covers the Galactic latitude range |b| < 5^°. The structures sampled with this tracer are predominantly parallel to the Galactic plane toward the inner Galaxy, in clear contrast to the predominantly perpendicular orientation of the structures traced by neutral atomic hydrogen (HI) emission toward the same regions. The analysis of the Galactic plane portions sampled at higher angular resolution with other surveys reveals that the alignment with the Galactic plane is also prevalent at smaller scales. We find no preferential orientation in the CO emission toward the outer Galaxy, in contrast to the preferential alignment with the Galactic plane displayed by HI in that portion of the Milky Way. We interpret these results as the combined effect of the decrease in midplane pressure with increasing Galactocentric radius and SN feedback, which lifts diffuse gas more efficiently than dense gas off the Galactic plane.