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

We present Spitzer rest frame mid-infrared spectroscopy of 12 z ∼ 2 millimeter-bright type 1 QSOs, selected from unlensed and lensed QSO samples and covering a range of AGN optical luminosities L₅₁₀₀ = 10 ⁴⁵-10⁴⁷ erg s^-1. 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 a similar correlation for lower luminosity local QSOs. This provides strong evidence for intense star formation in the hosts of these millimeter-bright QSOs, sometimes exceeding 1000 M_⊙ yr^-1 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 millimeter 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 millimeter-faint high-z QSOs may be AGN dominated out to rest frame far-infrared wavelengths. Toward 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 M_⊙ yr ^-1.