ALMA view of a massive spheroid progenitor: a compact rotating core of molecular gas in an AGN host at z=2.226

TL;DR

This study uses ALMA observations to reveal a compact, rapidly rotating molecular gas core in a z=2.226 galaxy hosting an obscured AGN, providing insights into early galaxy quenching and evolution.

Contribution

First measurement of dust and molecular gas size in a high-redshift galaxy hosting an AGN, linking compact gas reservoirs to galaxy quenching processes.

Findings

Gas emission size is ~1 kpc, indicating a highly compact structure.

High ISM density (~10^5.5 cm^-3) and low gas fraction (~0.2) suggest intense central star formation.

Evidence of rapid rotation (V_rot=320 km/s) in molecular gas.

Abstract

We present ALMA observations at 107.291 GHz (band 3) and 214.532 GHz (band 6) of GMASS 0953, a star-forming galaxy at z=2.226 hosting an obscured AGN that has been proposed as a progenitor of compact quiescent galaxies (QG). We measure for the first time the size of the dust and molecular gas emission of GMASS 0953 that we find to be extremely compact ($\sim$1 kpc). This result, coupled with a very high ISM density (n$\sim$10$^{5.5}$ cm$^{-3}$), a low gas mass fraction ($\sim$0.2) and a short gas depletion timescale ($\sim$150 Myr) imply that GMASS 0953 is experiencing an episode of intense star-formation in its central region that will rapidly exhaust its gas reservoirs, likely aided by AGN-induced feedback, confirming its fate as a compact QG. Kinematic analysis of the CO(6-5) line shows evidence of rapidly-rotating gas ($V_{rot}$=320$^{+92}_{-53}$ km s$^{-1}$), as observed also in a handful of similar sources at the same redshift. On-going quenching mechanisms could either destroy the rotation or leave it intact leading the galaxy to evolve into a rotating QG.