An ALMA view of the interstellar medium of the z=4.77 lensed starburst SPT-S J213242-5802.9

TL;DR

This paper presents ALMA observations of a highly star-forming galaxy at z=4.77, revealing its interstellar medium properties, gas mass, and evidence of an evolved ISM with possible AGN activity, advancing understanding of early galaxy evolution.

Contribution

First detailed ALMA study of the interstellar medium in a z=4.77 starburst galaxy, providing insights into its gas content, excitation conditions, and ISM evolution.

Findings

Detected [NII], CO(12-11), and tentative [CI] lines in SPT2132-58.

Derived a gas mass of approximately 3.8×10^10 M☉ and a short depletion timescale.

Found a high [CII]/[NII] ratio indicating an evolved and possibly PDR-dominated ISM.

Abstract

We present ALMA detections of the [NII] 205$\mu$m and CO(12-11) emission lines, and the tentative detection of [CI] $^3$P$_1$ - $^3$P$_0$ for the strongly lensed ($\mu$=5.7$\pm$0.5) dusty, star-forming galaxy SPT-S J213242-5802.9 (hereafter SPT2132-58) at z=4.77. The [NII] and CO(12-11) lines are detected at 11.5 and 8.5 $\sigma$ level, respectively, by our band-6 observations. The [CI] line is detected at 3.2 $\sigma$ after a re-analysis of existing band-3 data. The [CI] luminosity implies a gas mass of 3.8$\pm$1.2$\times$10$^{10}$ M$_\odot$, and consequently a very short depletion timescale of 34$\pm$13 Myr and a CO-luminosity-to-gas-mass conversion factor $\alpha_{\rm CO}$ of 1.0$\pm$0.3 M$_\odot$ (K km s$^{-1}$ pc$^{2}$)$^{-1}$. SPT2132-58 is an extreme starburst with an intrinsic star formation rate of 1100$\pm$200 M$_\odot$/yr. We find a [CII]/[NII] ratio of 26$\pm$7, which is the highest reported at z$>$4. This suggests that SPT2132-58 hosts an evolved interstellar medium (0.5 Z$_\odot$<Z<1.5 Z$_\odot$), which may be dominated by photodissociation regions. The CO(2-1) and CO(5-4) transitions have lower CO-to-far-infrared ratios than local and high-redshift samples, while CO(12-11) is similar to these samples, suggesting the presence of an additional very excited component or an AGN.