Rapidly growing black holes and host galaxies in the distant Universe from theHerschelRadio Galaxy Evolution Project

Abstract

We present results from a comprehensive survey of 70 radio galaxies at redshifts 1 < z < 5.2 using the PACS and SPIRE instruments on board the Herschel Space Observatory. Combined with existing mid-IR photometry from the Spitzer Space Telescope, published 870 µm photometry, and new observations obtained with LABOCA on the APEX telescope, the spectral energy distributions (SEDs) of galaxies in our sample are continuously covered across 3.6-870 µm. The total 8-1000 µm restframe infrared luminosities of these radio galaxies are such that almost all of them are either ultra-(L IR tot > 10 12 L ) or hyper-luminous (L IR tot > 10 13 L ) infrared galaxies. We fit the infrared SEDs with a set of empirical templates which represent dust heated by a variety of starbursts (SB) and by an active galactic nucleus (AGN). We find that the SEDs of radio galaxies require the dust to be heated by both AGN and SB, but the luminosities of these two components are not strongly correlated. Assuming empirical relations and simple physical assumptions, we calculate the star formation rate (SFR), the black hole mass accretion rate (Ṁ BH ), and the black hole mass (M BH ) for each radio galaxy. We find that the host galaxies and their black holes are growing extremely rapidly, having SFR ≈ 100-5000 M yr −1 anḋ M BH ≈ 1-100 M yr −1 . The mean specific SFRs (sSFR) of radio galaxies at z > 2.5 are higher than the sSFR of typical star forming galaxies over the same redshift range, but are similar or perhaps lower than the galaxy population for radio galaxies at z < 2.5. By comparing the sSFR and the specificṀ BH (sṀ BH ), we conclude that black holes in radio loud AGN are already, or soon will be, overly massive compared to their host galaxies in terms of expectations from the local M BH -M Gal relation. In order to catch up with the black hole, the galaxies require about an order of magnitude more time to grow in mass at the observed SFRs compared to the time the black hole is actively accreting. However, during the current cycle of activity, we argue that this catching up is likely to be difficult because of the short gas depletion times. Finally, we speculate on how the host galaxies might grow sufficiently in stellar mass to ultimately fall onto the local M BH -M Gal relation.

Observational evidence that positive and negative AGN feedback depends on galaxy mass and jet power

“…Using Herschel data, Seymour et al (2011) found a mean SFR range of 80 to 600 M⊙ yr −1 for 1.2 < z < 3.0 radio-selected AGN. In the same context, Drouart et al (2014) estimated SFRs of a few hundred to a few thousand solar masses per year for 1 < z < 5 radio galaxies. Recently, Podigachoski et al (2015), comparing the SFR of 3C radio-loud AGN and radio quasars at z > 1, found similar SFRs for the two classes and at the same levels with the previous works.…”

Defying jet-gas alignment in two radio galaxies atz~ 2 with extended light profiles: Similarities to brightest cluster galaxies

“…This is remarkable, given that the general population of passively evolving early-type galaxies at high redshift tends to be more compact than their local analogs (e.g., Daddi et al 2005;van Dokkum et al 2008;Schawinski et al 2011;Whitaker et al 2012). Furthermore, although about 1/3 of HzRGs are still actively starforming (Drouart et al 2014), their molecular gas reservoirs of a few 10 10 M (e.g., Emonts et al 2014) compared to few 10 11 M of stellar mass (Seymour et al 2007;De Breuck et al 2010) suggest that most of their stellar mass growth has already been completed. Targett et al (2011) found extended broadband emission when stacking 13 HzRGs at redshifts between z = 1.5 and 2.0 (as did Best et al 1998, at slightly lower redshifts).…”

“…Drouart et al (2014) recently argued, based on Spitzer mid-infrared imaging and the core-tolobe fraction of the radio sources, that the unified model must hold for the global population of HzRGs, but the presence of nuclear broad Hα emission in a few sources does also suggest that this might not be the case for each individual HzRG (Nesvadba et al 2011a).…”

“…Powerful radio galaxies at redshifts z ≥ 1−2 (HzRGs) are particularly massive (several 10 11 M , e.g., Seymour et al 2007;De Breuck et al 2010), intensely star-forming (on the order of 1000 M yr −1 ; Archibald et al 2001;Reuland et al 2004;Seymour et al 2012;Ivison et al 2012;Barthel et al 2012;Drouart et al 2014), relatively evolved (Nesvadba et al 2011a) galaxies. Many are surrounded by overdensities of satellite galaxies, making them interesting tracers of overdense regions in the early Universe that may ultimately evolve into massive, rich galaxy clusters (e.g., Chambers et al 1996;Le Fevre et al 1996;Kurk et al 2004a;Venemans et al 2007;Galametz et al 2012;Wylezalek et al 2013).…”

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

“…For high (radio-quiet) AGN luminosities (L X > 10 44 erg s −1 ), Page et al (2012) reported suppression of star formation, consistent with the expectations from theoretical models, while Harrison et al (2012) found no clear evidence of suppression of star formation by extending the analyses to samples larger by an order of magnitude. Moreover, at the highest AGN luminosities (in excess of 10 46 erg s −1 ), recent studies based on decomposition of the IR emission to AGN and star formation contributions, have shown star formation rates (SFRs) of the order of several hundred solar masses per year in the hosts of some of the most powerful radio galaxies (Barthel et al 2012, B12 hereafter; Seymour et al 2012;Drouart et al 2014) and (radio-quiet) quasars (Leipski et al 2013(Leipski et al , 2014.…”