Star formation in the hosts of high-z QSOs: Evidence from Spitzer PAH detections

TL;DR

This study uses Spitzer mid-infrared spectroscopy to detect PAH emission in high-redshift QSOs, revealing intense star formation in their host galaxies, often exceeding 1000 solar masses per year, and exploring the relationship between star formation and AGN luminosity.

Contribution

It provides the first evidence of PAH emission in high-z QSOs, linking star formation rates to AGN activity and extending local QSO correlations to high redshift.

Findings

PAH emission detected in 9 out of 12 QSOs.

Star formation rates often exceed 1000 Msun/yr.

Correlation between PAH luminosity and far-infrared luminosity.

Abstract

We present Spitzer rest-frame mid-infrared spectroscopy of twelve z~2 mm-bright type 1 QSOs, selected from unlensed and lensed QSO samples and covering a range of AGN optical luminosities L_5100=10^45 to 10^47 erg/s. On top of the AGN continuum, we detect PAH emission from luminous star formation in nine objects individually as well as in the composite spectrum for the full sample. PAH luminosity and rest frame far-infrared luminosity correlate and extend the similar correlation for lower luminosity local QSOs. This provides strong evidence for intense star formation in the hosts of these mm-bright QSOs, sometimes exceeding 1000 Msun/yr and dominating their rest frame far-infrared emission. The PAH-based limit on star formation rates is lower for luminous z~2 QSOs that are not preselected for their mm emission. Partly dependent on systematic changes of the AGN dust covering factor and the dust spectral energy distribution of the AGN proper, the spectral energy distributions of mm-faint high-z QSOs may be AGN dominated out to rest frame far-infrared wavelengths. Towards the most luminous high-z QSOs, there is a flattening of the relation between star formation and AGN luminosity that is observed for lower redshift QSOs. No QSO in our sample has a PAH-measured star formation rate in excess of 3000 Msun/yr.