# Gravitational lensing reveals extreme dust-obscured star formation in quasar host galaxies

## TL;DR

This study uses Herschel/SPIRE observations of 104 gravitationally-lensed quasars to reveal that most host galaxies exhibit dust-obscured star formation with typical SFRs around 120 solar masses per year, supporting models of coexisting star formation and AGN activity.

## Contribution

It provides the largest sample of lensed quasars observed in the FIR, characterizing their dust properties and star formation rates, and investigates the relationship between FIR emission and radio activity.

## Key findings

- 66% of quasars show dust emission in their SEDs.
- Median dust temperature is approximately 38 K.
- Median SFR is about 120 solar masses per year.

## Abstract

We have observed 104 gravitationally-lensed quasars at $z\sim1-4$ with Herschel/SPIRE, the largest such sample ever studied. By targeting gravitational lenses, we probe intrinsic far-infrared (FIR) luminosities and star formation rates (SFRs) more typical of the population than the extremely luminous sources that are otherwise accessible. We detect 72 objects with Herschel/SPIRE and find 66 percent (69 sources) of the sample have spectral energy distributions (SEDs) characteristic of dust emission. For 53 objects with sufficiently constrained SEDs, we find a median effective dust temperature of $38^{+12}_{-5}$ K. By applying the radio-infrared correlation, we find no evidence for an FIR excess which is consistent with star-formation-heated dust. We derive a median magnification-corrected FIR luminosity of $3.6^{+4.8}_{-2.4}~\times 10^{11}~{\rm L_{\odot}}$ and median SFR of $120^{+160}_{-80}~{\rm M_{\odot}~yr^{-1}}$ for 94 quasars with redshifts. We find $\sim10$ percent of our sample have FIR properties similar to typical dusty star-forming galaxies at $z\sim2-3$ and a range of SFRs $<20-10000~{\rm M_{\odot}~yr^{-1}}$ for our sample as a whole. These results are in line with current models of quasar evolution and suggests a coexistence of dust-obscured star formation and AGN activity is typical of most quasars. We do not find a statistically-significant difference in the FIR luminosities of quasars in our sample with a radio excess relative to the radio-infrared correlation. Synchrotron emission is found to dominate at FIR wavelengths for $<15$ percent of those sources classified as powerful radio galaxies.