A molecular absorption line survey toward the AGN of Hydra-A

TL;DR

This study uses ALMA to observe molecular absorption lines in Hydra-A's nucleus, revealing a clumpy, dynamic interstellar medium with at least 12 molecular clouds and potential variability over two years.

Contribution

First detailed molecular absorption line survey of Hydra-A's nucleus, uncovering a complex, clumpy interstellar medium with evidence of cloud motions and variability.

Findings

Detected multiple molecular absorption lines indicating at least 12 clouds.

Found clouds have properties similar to Milky Way clouds at several kpc.

Observed potential absorption profile variability over two years.

Abstract

We present Atacama Large Millimeter/submillimeter Array observations of the brightest cluster galaxy Hydra-A, a nearby ($z=0.054$) giant elliptical galaxy with powerful and extended radio jets. The observations reveal CO(1-0), CO(2-1), $^{13}$CO(2-1), CN(2-1), SiO(5-4), HCO$^{+}$(1-0), HCO$^{+}$(2-1), HCN(1-0), HCN(2-1), HNC(1-0) and H$_{2}$CO(3-2) absorption lines against the galaxy's bright and compact active galactic nucleus. These absorption features are due to at least 12 individual molecular clouds which lie close to the centre of the galaxy and have velocities of approximately $-50$ to $+10$ km/s relative to its recession velocity, where positive values correspond to inward motion. The absorption profiles are evidence of a clumpy interstellar medium within brightest cluster galaxies composed of clouds with similar column densities, velocity dispersions and excitation temperatures to those found at radii of several kpc in the Milky Way. We also show potential variation in a $\sim 10$ km/s wide section of the absorption profile over a two year timescale, most likely caused by relativistic motions in the hot spots of the continuum source which change the background illumination of the absorbing clouds.