The Host Galaxies of X-ray Quasars Are Not Strong Star Formers

TL;DR

This study uses deep submillimeter and radio observations to show that most host galaxies of high-redshift X-ray quasars exhibit little star formation, likely due to AGN feedback suppression.

Contribution

It provides new evidence that X-ray quasars' host galaxies are generally not strong star formers, contrasting with less luminous AGNs, based on deep multiwavelength data analysis.

Findings

X-ray quasars' host galaxies show low FIR flux, indicating little star formation.

FIR-radio and MIR-radio correlations hold for less luminous AGNs but not for quasars.

Stacking analyses may overestimate star formation in X-ray samples.

Abstract

We use ultradeep SCUBA-2 850um observations (~0.37 mJy rms) of the 2 Ms Chandra Deep Field-North (CDF-N) and 4 Ms Chandra Deep Field-South (CDF-S) X-ray fields to examine the amount of dusty star formation taking place in the host galaxies of high-redshift X-ray AGNs. Supplementing with COSMOS, we measure the submillimeter fluxes of the 4-8 keV sources at z>1, finding little flux at the highest X-ray luminosities but significant flux at intermediate luminosities. We determine gray body and MIR luminosities by fitting spectral energy distributions to each X-ray source and to each radio source in an ultradeep Karl G. Jansky Very Large Array (VLA) 1.4 GHz (11.5uJy at 5-sigma) image of the CDF-N. We confirm the FIR-radio and MIR-radio correlations to z=4 using the non-X-ray detected radio sources. Both correlations are also obeyed by the X-ray less luminous AGNs but not by the X-ray quasars. We interpret the low FIR luminosities relative to the MIR for the X-ray quasars as being due to a lack of star formation, while the MIR stays high due to the AGN contribution. We find that the FIR luminosity distributions are highly skewed and the means are dominated by a small number of high-luminosity galaxies. Thus, stacking or averaging analyses will overestimate the level of star formation taking place in the bulk of the X-ray sample. We conclude that most of the host galaxies of X-ray quasars are not strong star formers, perhaps because their star formation is suppressed by AGN feedback.