The mean star-forming properties of QSO host galaxies

Abstract

Quasi-stellar objects (QSOs) occur in galaxies in which supermassive black holes (SMBHs) are growing substantially through rapid accretion of gas. Many popular models of the co-evolutionary growth of galaxies and black holes predict that QSOs are also sites of substantial recent star formation (SF), mediated by important processes, such as major mergers, which rapidly transform the nature of galaxies. A detailed study of the star-forming properties of QSOs is a critical test of these models. We present a far-infrared Herschel/PACS study of the mean star formation rate (SFR) of a sample of spectroscopically observed QSOs to z ∼ 2 from the COSMOS extragalactic survey. This is the largest sample to date of moderately luminous QSOs (with nuclear luminosities that lie around the knee of the luminosity function) studied using uniform, deep far-infrared photometry. We study trends of the mean SFR with redshift, black hole mass, nuclear bolometric luminosity, and specific accretion rate (Eddington ratio). To minimize systematics, we have undertaken a uniform determination of SMBH properties, as well as an analysis of important selection effects of spectroscopic QSO samples that influence the interpretation of SFR trends. We find that the mean SFRs of these QSOs are consistent with those of normal massive star-forming galaxies with a fixed scaling between SMBH and galaxy mass at all redshifts. No strong enhancement in SFR is found even among the most rapidly accreting systems, at odds with several co-evolutionary models. Finally, we consider the qualitative effects on mean SFR trends from different assumptions about the SF properties of QSO hosts and from redshift evolution of the SMBH-galaxy relationship. While currently limited by uncertainties, valuable constraints on AGN-galaxy co-evolution can emerge from our approach.

Far-Infrared Surveys of Galaxy Evolution

“…Cisternas et al 2011, Kocevski et al 2012, and references therein). It is also consistent with the absence of some correlations that are expected in at least some versions of the merger scenario: Obscured AGN as defined by either high X-ray obscuring column or by optical type 2 are not more star forming than unobscured AGN at the same redshift , Rovilos et al 2012, Merloni et al 2014, there is no evidence for enhanced star formation in the hosts of those AGN accreting with the highest Eddington ratios (Rosario et al 2013b), and HiBal and non-BAL QSOs do not differ significantly in their far-infrared properties (Cao Orjales et al 2012).…”

The cosmic growth of the active black hole population at 1 <z <2 in zCOSMOS, VVDS and SDSS

“…We do not include the type-1 AGN detected in the deep sample (Bongiorno et al 2012;Rosario et al 2013), since they do not constitute a well-defined sample, mainly due to the BzK colour preselection, targeting z > 1.5 galaxies. Targets have been cho- Figure 1.…”

A New Star Formation Rate Calibration From Polycyclic Aromatic Hydrocarbon Emission Features and Application to High-Redshift Galaxies

“…Not only do far-IR measurements reveal dust-obscured star formation, but even in luminous quasars, this emission appears to be dominated by the power from star formation in the host galaxies (Rosario et al 2013;Xu et al 2015). The Atacama Large Millimeter Array (ALMA) can measure rest-frame far-IR outputs for z  2, but at lower redshifts it becomes inefficient in this application.…”