# Quenching by gas compression and consumption: A case study of a massive radio galaxy at z = 2.57

## TL;DR

This study investigates how an active galactic nucleus influences star formation in a massive galaxy at z=2.57, revealing recent starburst activity, high star-formation efficiency, and gas consumption likely leading to quenching.

## Contribution

It provides detailed observational evidence of star formation history, gas properties, and AGN impact in a high-redshift radio galaxy, highlighting the role of gas compression and AGN activity in quenching.

## Key findings

- Recent star formation occurred in two bursts at 6 and >20 Myr ago.
- High star-formation efficiency of 26 Gyr$^{-1}$ and short depletion time of 38 Myr.
- AGN activity likely contributes to gas removal and star formation quenching.

## Abstract

The objective of this work is to study how active galactic nuclei (AGN) influence star formation in host galaxies. We present a detailed investigation of the star-formation history and conditions of a $z=2.57$ massive radio galaxy based on VLT/X-SHOOTER and ALMA observations. The deep rest-frame ultraviolet spectrum contains photospheric absorption lines and wind features indicating the presence of OB-type stars. The most significantly detected photospheric features are used to characterize the recent star formation: neither instantaneous nor continuous star-formation history is consistent with the relative strength of the Si II $\lambda$1485 and S V $\lambda$1502 absorption. Rather, at least two bursts of star formation took place in the recent past, at $6^{+1}_{-2}$ Myr and $\gtrsim20$ Myr ago, respectively. We deduce a molecular H$_{2}$ gas mass of $(3.9\pm1.0)\times10^{10}$ M$_{\odot}$ based on ALMA observations of the [C I] $^3$P$_{2}$-$^3$P$_{1}$ emission. The molecular gas mass is only 13 % of its stellar mass. Combined with its high star-formation rate of ($1020^{+190}_{-170}$) Myr, this implies a high star-formation efficiency of $(26\pm8$) Gyr$^{-1}$ and a short depletion time of $(38\pm12)$ Myr. We attribute the efficient star formation to compressive gas motions in order to explain the modest velocity dispersions ($\leqslant$ 55 km s$^{-1}$) of the photospheric lines and of the star-forming gas traced by [C I]. Because of the likely very young age of the radio source, our findings suggest that vigorous star formation consumes much of the gas and works in concert with the AGN to remove any residual molecular gas, and eventually quenching star formation in massive galaxies.