Quasars, their host galaxies and their central black holes

Abstract

We present the final results from our deep Hubble Space Telescope (HST) imaging study of the host galaxies of radio‐quiet quasars (RQQs), radio‐loud quasars (RLQs) and radio galaxies (RGs). We describe and analyse new Wide Field & Planetary Camera 2 (WFPC2) R‐band observations for 14 objects, which when combined with the first tranche of HST imaging reported in McLure et al., provide a complete and consistent set of deep, red, line‐free images for statistically matched samples of 13 RQQs, 10 RLQs and 10 RGs in the redshift band 0.1 < z < 0.25. We also report the results of new deep VLA imaging that has yielded a 5‐GHz detection of all but one of the 33 active galactic nuclei (AGN) in our sample. Careful modelling of our images, aided by a high dynamic‐range point spread function, has allowed us to determine accurately the morphology, luminosity, scalelength and axial ratio of every host galaxy in our sample. Armed with this information we have undertaken a detailed comparison of the properties of the hosts of these three types of powerful AGN, both internally and with the galaxy population in general. We find that spheroidal hosts become more prevalent with increasing nuclear luminosity such that, for nuclear luminosities MV < −23.5, the hosts of both radio‐loud and radio‐quiet AGN are virtually all massive ellipticals. Moreover, we demonstrate that the basic properties of these hosts are indistinguishable from those of quiescent, evolved, low‐redshift ellipticals of comparable mass. This result rules out the possibility that radio‐loudness is determined by host‐galaxy morphology, and also sets severe constraints on evolutionary schemes that attempt to link low‐z ultraluminous infrared galaxies with RQQs. Instead, we show that our results are as expected given the relationship between black hole and spheroid mass established for nearby galaxies, and apply this relation to estimate the mass of the black hole in each object. The results agree remarkably well with completely independent estimates based on nuclear emission‐line widths; all the quasars in our sample have Mbh > 5 × 108 M⊙, while the radio‐loud objects are confined to Mbh > 109 M⊙. This apparent mass‐threshold difference, which provides a natural explanation for why RQQs outnumber RLQs by a factor of 10, appears to reflect the existence of a minimum and a maximum level of black hole radio output, which is a strong function of black hole mass (∝M2−2.5bh). Finally, we use our results to estimate the fraction of massive spheroids/black holes that produce quasar‐level activity. This fraction is ≃0.1 per cent at the present day, rising to >10 per cent at z≃ 2–3.

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

“…latter sample from low-level AGN activity will likely only serve to decrease the observed differences between the two samples. As a result of the previous arguments regarding the purity of the radio AGN host sample, despite the large redshift baseline of this sample (0 < z < 4), the redshift evolution of the radio power density (e.g., Condon 1989Condon , 2002Dunlop et al 2003;Smolčić et al 2009) has little effect on our results. Or, rather, this evolution will simply result in more radio AGN being selected at high redshifts as the redshift distribution of the ∼10% of the radio AGN selected hosts whose L TIR subtracted P ν,1.4 GHz is less than log(P ν,1.4 GHz ) = 23 (see above) is indistinguishable from that of the remaining 90% of the sample.…”

Do we expect most AGN to live in discs?

“…7 We therefore compare the predicted "stochastically fueled" fraction of AGN with the fraction of disk-dominated AGN hosts, as a function of luminosity and redshift. At low redshifts, we compare with the PG quasar sample of Dunlop et al (2003); Floyd et al (2004) (we plot the fraction with best-fit bulge-to-total mass ratio B/T < 0.5 in bins of Lbol, estimated from the observed nuclear Vband luminosities, with Poisson errors). At z ∼ 0.5 − 1, we compare the low-luminosity sample from COSMOS studied in Gabor et al (2009) and Cisternas et al (2011); we plot the "final" quoted fraction of disk-dominated galaxies in the sample (with the approximate ∼ 10% systematic difference between classifiers quoted therein) and 90% range of Lbol estimated from the hard X-ray luminosities.…”

“…Since the duty cycle of "stochastically" fueled systems is dominated by gas-rich disks, and most major merger-triggered systems that induce strong bulge and BH growth will appear as spheroids, we compare the observed fraction of disk-dominated AGN host galaxies in different luminosity/redshift intervals. At low redshift, measured from the PG quasar sample of Dunlop et al (2003); Floyd et al (2004, circles). At intermediate redshift, measured in low-luminosity AGN in COSMOS in Gabor et al (2009);Cisternas et al (2011, triangle), and in true (Type 2) quasars in Zakamska et al (2006Zakamska et al ( , 2008; Liu et al (2009, inverted triangle).…”

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

“…Dunlop et al 2003;Floyd et al 2004), while for red quasars the merger fraction increases to ∼ 85 per cent. Assuming that the merger fraction might rises with bolometric luminosity (e.g.…”