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The objective of this work is to study how active galactic nuclei (AGN)influence star formation in host galaxies. We present a detailed investigationof the star-formation history and conditions of a $z=2.57$ massive radio galaxybased on VLT/X-SHOOTER and ALMA observations. The deep rest-frame ultravioletspectrum contains photospheric absorption lines and wind features indicatingthe presence of OB-type stars. The most significantly detected photosphericfeatures are used to characterize the recent star formation: neitherinstantaneous nor continuous star-formation history is consistent with therelative 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 molecularH$_{2}$ gas mass of $(3.9\pm1.0)\times10^{10}$ M$_{\odot}$ based on ALMAobservations of the [C I] $^3$P$_{2}$-$^3$P$_{1}$ emission. The molecular gasmass is only 13 % of its stellar mass. Combined with its high star-formationrate of ($1020^{+190}_{-170}$) Myr, this implies a high star-formationefficiency of $(26\pm8$) Gyr$^{-1}$ and a short depletion time of $(38\pm12)$Myr. We attribute the efficient star formation to compressive gas motions inorder 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]. Becauseof the likely very young age of the radio source, our findings suggest thatvigorous star formation consumes much of the gas and works in concert with theAGN to remove any residual molecular gas, and eventually quenching starformation in massive galaxies.

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