Massive galaxies on the road to quenching: ALMA observations of powerful high redshift radio galaxies

TL;DR

This study uses deep ALMA observations to analyze high-redshift radio galaxies, revealing that many are quenched in star formation and suggesting a feedback mechanism involving star formation and radio jets that rapidly quench massive galaxies.

Contribution

It provides the first detailed disentanglement of synchrotron and dust emission in high-redshift radio galaxies, revises star formation rate estimates, and proposes a feedback mechanism for galaxy quenching.

Findings

Many host galaxies are below the main sequence, indicating quenching.

Synchrotron emission significantly affects millimeter flux measurements.

Star formation likely depletes gas before radio jets expel remaining gas.

Abstract

We present 0.3" (band 6) and 1.5" (band 3) ALMA observations of the (sub)millimeter dust continuum emission for 25 radio galaxies at 1<z<5.2. Our survey reaches a rms flux density of ~50$\mu$Jy in band 6 and ~20$\mu$Jy in band 3. This is an order of magnitude deeper than single-dish 850 $\mu$m observations, and reaches fluxes where synchrotron and thermal dust emission are expected to be of the same order of magnitude. Combining our sensitive ALMA observations with radio data from ATCA, VLA, and IR photometry from Herschel and Spitzer, we have disentangled the synchrotron and thermal dust emission. We determine the star-formation rates (SFR) and AGN IR luminosities using our newly developed spectral energy distribution fitting code MrMoose. We find that synchrotron emission contributes substantially at ~1 mm. Through our sensitive flux limits and accounting for a contribution from synchrotron emission in the mm, we revise downward the median SFR by a factor of 7 compared to previous estimates based solely on Herschel and Spitzer data. The hosts of these radio-loud AGN appear predominantly below the main sequence of star-forming galaxies, indicating that the star formation in many of the host galaxies has been quenched. Future growth of the host galaxies without substantial black hole mass growth will be needed to bring these objects on the local relation between the supermassive black holes and their host galaxies. Given the mismatch in the timescales of any star formation that took place in the host galaxies and lifetime of the AGN, we hypothesize that a key role is played by star formation in depleting the gas before the action of the powerful radio jets quickly drives out the remaining gas. This positive feedback loop of efficient star formation rapidly consuming the gas coupled to the action of the radio jets in removing the residual gas is how massive galaxies are rapidly quenched.