ALMA observations of cold molecular gas filaments trailing rising radio bubbles in PKS0745-191

TL;DR

This study uses ALMA to observe cold molecular gas filaments in the galaxy PKS0745-191, revealing their properties, likely formation mechanisms, and relation to radio bubbles and cooling processes in the cluster core.

Contribution

First detailed ALMA observations of molecular filaments in PKS0745-191, linking their formation to buoyant radio bubbles and in situ cooling, challenging merger or free-fall origins.

Findings

Filaments contain ~4.6 billion solar masses of molecular gas.

Velocities are low, suggesting transient structures supported by magnetic fields.

Filaments likely formed from uplifted warm gas cooling in situ.

Abstract

We present ALMA observations of the CO(1-0) and CO(3-2) line emission tracing filaments of cold molecular gas in the central galaxy of the cluster PKS0745-191. The total molecular gas mass of 4.6 +/- 0.3 x 10^9 solar masses, assuming a Galactic X_{CO} factor, is divided roughly equally between three filaments each extending radially 3-5 kpc from the galaxy centre. The emission peak is located in the SE filament roughly 1 arcsec (2 kpc) from the nucleus. The velocities of the molecular clouds in the filaments are low, lying within +/-100 km/s of the galaxy's systemic velocity. Their FWHMs are less than 150 km/s, which is significantly below the stellar velocity dispersion. Although the molecular mass of each filament is comparable to a rich spiral galaxy, such low velocities show that the filaments are transient and the clouds would disperse on <10^7 yr timescales unless supported, likely by the indirect effect of magnetic fields. The velocity structure is inconsistent with a merger origin or gravitational free-fall of cooling gas in this massive central galaxy. If the molecular clouds originated in gas cooling even a few kpc from their current locations their velocities would exceed those observed. Instead, the projection of the N and SE filaments underneath X-ray cavities suggests they formed in the updraft behind bubbles buoyantly rising through the cluster atmosphere. Direct uplift of the dense gas by the radio bubbles appears to require an implausibly high coupling efficiency. The filaments are coincident with low temperature X-ray gas, bright optical line emission and dust lanes indicating that the molecular gas could have formed from lifted warmer gas that cooled in situ.