PREPRINT
92141EB0-3609-4B4C-A851-3276BA5FDDB9

# A 4~Kpc Molecular Gas Lane in Cygnus A

C. L. Carilli, R. A. Perley, D. A. Perley, V. Dhawan, R. Decarli, A. Evans, K. Nyland
arXiv:2209.01278

Submitted on 2 September 2022

## Abstract

We present the discovery of a 4 kpc molecular gas lane in the Cygnus A host galaxy, using ALMA CO 2-1 observations. The gas lane is oriented roughly perpendicular to the projected radio jet axis. The CO emission generally follows the clumpy dust lanes seen in HST I-band images. The total molecular gas mass is $30×{10}^{8}$ M${}_{\odot }$ for Milky Way type clouds, and $3.6×{10}^{8}$ M${}_{\odot }$ for starburst conditions. There is a velocity change from the northern to southern CO peaks of about $±175$~km~s${}^{-1}$, and an apparently smooth velocity gradient between the peaks, although the emission in the central region is weak. In the inner $\sim 0.5"$ projected distance from the radio core, comparison of the CO velocities to those observed for H${}_{2}$ 2.1218 $\mu$m emission shows higher velocities for the vibrationally excited warm molecular gas than the cooler CO 2-1 line emitting gas at similar projected radii. A possible explanation for these different projected velocities at a given radius is that the cooler CO gas is distributed in a clumpy ring at radius $\sim 1.5"$ to $2"$, while the warm H${}_{2}$ 2.12$\mu$m emitting gas is interior to this ring. Of course, the current data cannot rule-out a clumpy, amorphous molecular gas distribution linearly distributed perpendicular to the radio jet axis. We consider surface brightness properties on scales down to $\sim 265$~pc, and discuss the Cygnus A results in the context of other radio galaxies with CO emission.

## Preprint

Comment: 5 Figures, 23 pages, 2 tables, accepted for publication in the ApJ

Subject: Astrophysics - Astrophysics of Galaxies