Enhanced star formation rates in AGN hosts with respect to inactive galaxies from PEP-Herschelobservations

Abstract

We compare the average star formation (SF) activity in X-ray selected AGN hosts with a mass-matched control sample of inactive galaxies, including both star forming and quiescent sources, in the 0.5 < z < 2.5 redshift range. Recent observations carried out by PACS, the 60−210 μm photometric camera on board the Herschel Space Observatory, in GOODS-S, GOODS-N and COSMOS allow us to obtain an unbiased estimate of the far-IR luminosity, and hence of the SF properties, of the two samples. Accurate AGN host stellar mass estimates are obtained by decomposing their total emission into the stellar and the nuclear components. We report evidence of a higher average SF activity in AGN hosts with respect to the control sample of inactive galaxies. The level of SF enhancement is modest (∼0.26 dex at ∼3σ confidence level) at low X-ray luminosities (L X 10 43.5 erg s −1 ) and more pronounced (0.56 dex at >10σ confidence level) in the hosts of luminous AGNs. However, when comparing to star forming galaxies only, AGN hosts are found broadly consistent with the locus of their "main sequence". We investigate the relative far-IR luminosity distributions of active and inactive galaxies, and find a higher fraction of PACS detected, hence normal and highly star forming systems among AGN hosts. Although different interpretations are possible, we explain our findings as a consequence of a twofold AGN growth path: faint AGNs evolve through secular processes, with instantaneous AGN accretion not tightly linked to the current total SF in the host galaxy, while the luminous AGNs co-evolve with their hosts through periods of enhanced AGN activity and star formation, possibly through major mergers. While an increased SF activity with respect to inactive galaxies of similar mass is expected in the latter, we interpret the modest SF offsets measured in low-L X AGN hosts as either a) generated by non-synchronous accretion and SF histories in a merger scenario or b) due to possible connections between instantaneous SF and accretion that can be induced by smaller scale (non-major merger) mechanisms. Far-IR luminosity distributions favour the latter scenario.

Star formation inz> 1 3CR host galaxies as seen byHerschel

“…It is clear that the 3CR FIR-detected objects have, on average, SFRs comparable to those of their equally massive non-AGN counterparts. The majority of FIRdetected objects are thus also located near the main sequence of star-forming galaxies, similarly to what has been found in deep Herschel/PACS studies of less powerful, X-ray selected, high-z AGN Santini et al 2012;Rosario et al 2013). In comparison to the FIR-detected objects, the nondetected objects have similar stellar mass but significantly lower Fig.…”

Herschel Observed Stripe 82 Quasars and Their Host Galaxies: Connections Between Agn Activity and Host Galaxy Star Formation

“…It seems that AGN evolution is dominated by a nonmerger process. Other studies about AGN hosts reach similar conclusions, such as Kocevski et al (2012), Santini et al (2012), and Schawinski et al (2011). Their results provide evidence that the starburst galaxies only account for 10% of the cosmic SFR density at z ∼ 2 (Rodighiero et al 2011;Lamastra et al 2013).…”

Hidden starbursts and active galactic nuclei at 0 < z < 4 from theHerschel-VVDS-CFHTLS-D1 field: Inferences on coevolution and feedback

“…Combining these results with the results of the previous section, it becomes apparent that not only are starbursts more common in those galaxies that host all types of AGN, but the starbursts occurring in those galaxies are considerably more powerful than the average starburst at all redshifts. Such a trend is also seen in AGN hosts selected by a variety of other methods (e.g., Santini et al 2012;Gruppioni et al 2013), which suggests this phenomenon is common among nearly all types of AGN.…”

“…Already, observations from Herschel have been used to great effect to investigate the relationship between various types of AGN and their starbursting host galaxies. In galaxies with X-ray-selected AGN, numerous studies have found correlations between the strength of the starburst in galaxies, as determined by Herschel, and the strength of the AGN they host (Shao et al 2010;Harrison et al 2012b;Rovilos et al 2012;Santini et al 2012; though for an alternative viewpoint see Page et al 2012). The host galaxies of radio-and IR-selected AGN have also been the subjects of investigation with Herschel (e.g., Hardcastle et al 2012;Del Moro et al 2013), though the connection between these AGNs and their host galaxies is less clear than in the case of X-ray-selected AGN.…”