Molecular gas mass measurements of an active, starburst galaxy at $z\approx2.6$ using ALMA observations of the [CI], CO and dust emission

TL;DR

This study uses ALMA observations of multiple emission lines and dust continuum to measure the molecular gas mass in a high-redshift starburst galaxy with an active nucleus, revealing spatial variations and emphasizing the importance of deep observations.

Contribution

It provides new spatially resolved measurements of cold gas in a $z oughly 2.6$ galaxy using multiple tracers, highlighting the need for improved systematic uncertainties.

Findings

Molecular gas mass estimated around 10^11 solar masses.

Different emission lines show varied spatial morphologies.

Deep ALMA observations are essential for accurate cold gas characterization.

Abstract

We present new ALMA observations of a starburst galaxy at cosmic noon hosting a radio-loud active galactic nucleus: PKS 0529-549 at $z=2.57$. To investigate the conditions of its cold interstellar medium, we use ALMA observations which spatially resolve the [CI] fine-structure lines, [CI] (2-1) and [CI] (1-0), CO rotational lines, CO (7-6) and CO (4-3), and the rest-frame continuum emission at 461 and 809 GHz. The four emission lines display different morphologies, suggesting spatial variation in the gas excitation conditions. The radio jets have just broken out of the molecular gas but not through the more extended ionized gas halo. The [CI] (2-1) emission is more extended ($\approx8\,{\rm kpc}\times5\,{\rm kpc}$) than detected in previous shallower ALMA observations. The [CI] luminosity ratio implies an excitation temperature of $44\pm16$ K, similar to the dust temperature. Using the [CI] lines, CO (4-3), and 227 GHz dust continuum, we infer the mass of molecular gas $M_{\mathrm{mol}}$ using three independent approaches and typical assumptions in the literature. All approaches point to a massive molecular gas reservoir of about $10^{11}$ $M_{\odot}$, but the exact values differ by up to a factor of 4. Deep observations are critical in correctly characterizing the distribution of cold gas in high-redshift galaxies, and highlight the need to improve systematic uncertainties in inferring accurate molecular gas masses.