ALMA Observations of Molecular Absorption in the Gravitational Lens PMN 0134-0931

TL;DR

This study reports the detection of molecular absorption lines in a gravitational lens system at z=0.7645, revealing complex gas kinematics and physical conditions in the lensing galaxy, with implications for understanding molecular gas in distant galaxies.

Contribution

First detection of molecular absorption lines in a gravitational lens system at cosmological distance, providing insights into the gas dynamics and physical conditions in the lensing galaxy.

Findings

Absorption lines detected in two lines of sight separated by 5 kpc.

Wide absorption profiles suggest a highly inclined, possibly interacting galaxy.

Column densities and abundance ratios indicate denser molecular gas in one line of sight.

Abstract

We report the detection of molecular absorption lines at z=0.7645 towards the radio-loud QSO PMN 0134-0931. The CO J=2-1 and HCO+ J=2-1 lines are seen in absorption along two different lines of sight to lensed images of the background QSO. The lines of sight are separated by ~0.7", corresponding to 5 kpc in the lens plane. PMN 0134-0931 represents one out of only five known molecular absorption line systems at cosmologically significant distances. Moreover, it is also one of three such systems where the absorption occurs in a galaxy acting as a gravitational lens. The absorption lines through the two lines of sight are shifted by 215+/-8 km/s, possibly representing rotational motion in one of the lensing galaxies. The absorption profiles are wide, ~200 km/s, suggesting that the absorption occurs in a highly inclined disk galaxy with a flat rotation curve and a cloud-cloud velocity dispersion ~30 km/s. Gravitational lens models require two equal mass galaxies to account for the observed configuration of lensed images. The presence of two galaxies in close proximity means that they might be interacting and potentially merging and the kinematics of the molecular gas may not reflect ordered rotational motion. The column densities of both CO and HCO+ are normal for diffuse molecular gas towards one of the lensed images, but significantly higher towards the other. Also, the abundance ratio N(CO)/N(HCO+) is 2-3 times higher than in typical diffuse molecular gas. It is plausible that the second line of sight probes denser molecular gas than what is normally the case for absorption.