What is the relevance of major mergers and interactions as triggering mechanisms for active galactic nuclei (AGN) activity? To answer this long-standing question, we analyze 140 XMM-Newton-selected AGN host galaxies and a matched control sample of 1264 inactive galaxies over z ∼ 0.3-1.0 and M * < 10 11.7 M ⊙ with high-resolution HST /ACS imaging from the COSMOS field. The visual analysis of their morphologies by 10 independent human classifiers yields a measure of the fraction of distorted morphologies in the AGN and control samples, i.e., quantifying the signature of recent mergers which might potentially be responsible for fueling/triggering the AGN. We find that (1) the vast majority (>85%) of the AGN host galaxies do not show strong distortions, and (2) there is no significant difference in the distortion fractions between active and inactive galaxies. Our findings provide the best direct evidence that, since z ∼ 1, the bulk of black hole (BH) accretion has not been triggered by major galaxy mergers, therefore arguing that the alternative mechanisms, i.e., internal secular processes and minor interactions, are the leading triggers for the episodes of major BH growth. We also exclude an alternative interpretation of our results: a substantial time lag between merging and the observability of the AGN phase could wash out the most significant merging signatures, explaining the lack of enhancement of strong distortions on the AGN hosts. We show that this alternative scenario is unlikely due to: (1) recent major mergers being ruled out for the majority of sources due to the high fraction of disk-hosted AGN, (2) the lack of a significant X-ray signal in merging inactive galaxies as a signature of a potential buried AGN, and (3) the low levels of soft X-ray obscuration for AGN hosted by interacting galaxies, in contrast to model predictions.

We present new constraints on the ratio of black hole (BH) mass to total galaxy stellar mass at 0.3 < z < 0.9 for a sample of 32 type-1 active galactic nuclei (AGNs) from the XMM-COSMOS survey covering the range M BH ∼ 10 7.2−8.7 M ⊙ . Virial M BH estimates based on Hβ are available from the COSMOS Magellan/IMACS survey. We use high-resolution Hubble Space Telescope (HST) imaging to decompose the light of each type-1 AGN and host galaxy, and employ a specially-built mass-to-light ratio to estimate the stellar masses (M * ). The M BH − M * ratio shows a zero offset with respect to the local relation for galactic bulge masses, and we also find no evolution in the mass ratio M BH /M * ∝ (1 + z) 0.02±0.34 up to z ∼ 0.9. Interestingly, at the high-M BH end there is a positive offset from the z = 0 relation, which can be fully explained by a mass function bias with a cosmic scatter of σ µ = 0.3, reaffirming that the intrinsic distribution is consistent with zero evolution. From our results we conclude that since z ∼ 0.9 no substantial addition of stellar mass is required: the decline in star formation rates and merger activity at z < 1 support this scenario. Nevertheless, given that a significant fraction of these galaxies show a disk component, their bulges are indeed undermassive. This is a direct indication that for the last 7 Gyr the only essential mechanism required in order that these galaxies obey the z = 0 relation is a redistribution of stellar mass to the bulge, likely driven by secular processes, i.e., internal instabilities and minor merging.

We constrain the ratio of black hole (BH) mass to total stellar mass of type-1 AGN in the COSMOS survey at 1 < z < 2. For 10 AGN at mean redshift z ∼ 1.4 with both HST/ACS and HST/NICMOS imaging data we are able to compute the total stellar mass M * ,total , based on restframe UV-to-optical host galaxy colors which constrain mass-to-light ratios. All objects have virial M BH -estimates available from the COSMOS Magellan/IMACS and zCOSMOS surveys. We find within errors zero difference between the M BH -M * ,total -relation at z ∼ 1.4 and the M BH -M * ,bulge -relation in the local Universe. Our interpretation is: (a) If our objects were purely bulge-dominated, the M BH -M * ,bulge -relation has not evolved since z ∼ 1.4. However, (b) since we have evidence for substantial disk components, the bulges of massive galaxies (M * ,total = 11.1 ± 0.3 or log M BH ∼ 8.3 ± 0.2) must have grown over the last 9 Gyrs predominantly by redistribution of disk-into bulge-mass. Since all necessary stellar mass exists in the galaxy at z = 1.4, no star-formation or addition of external stellar material is required, only a redistribution e.g. induced by minor and major merging or through disk instabilities. Merging, in addition to redistributing mass in the galaxy, will add both BH and stellar/bulge mass, but does not change the overall final M BH /M * ,bulge ratio.Since the overall cosmic stellar and BH mass buildup trace each other tightly over time, our scenario of bulge-formation in massive galaxies is independent of any strong BH-feedback and means that the mechanism coupling BH and bulge mass until the present is very indirect.

We use deep optical, infrared and radio observations to explore the symbiosis between nuclear activity and galaxy evolution in the southern compact radio source PKS 1549−79 (z= 0.1523). The optical imaging observations reveal the presence of tidal tail features which provide strong evidence that the host galaxy has undergone a major merger in the recent past. The merger hypothesis is further supported by the detection of a young stellar population (YSP), which, on the basis of spectral synthesis modelling of our deep Very Large Telescope (VLT) optical spectra, was formed 50–250 Myr ago and makes up a significant fraction of the total stellar mass (1–30 per cent). Despite the core‐jet structure of the radio source, which is consistent with the idea that the jet is pointing close to our line of sight, our H i 21‐cm observations reveal significant H i absorption associated with both the core and the jet. Moreover, the luminous, quasar‐like active galactic nucleus (AGN) (MV < −23.5) is highly extinguished (Av > 6.4) at optical wavelengths and show many properties in common with narrow‐line Seyfert 1 galaxies (NLS1), including relatively narrow permitted lines [full width at half‐maximum (FWHM) ∼ 1940 km s−1], highly blueshifted [O iii]λλ5007,4959 lines (ΔV∼ 680 km s−1) and evidence that the putative supermassive black hole is accreting at a high Eddington ratio (0.3 < Lbol/Ledd < 11). The results suggest that accretion at high Eddington ratio does not prevent the formation of powerful relativistic jets. Together, the observations lend strong support to the predictions of some recent numerical simulations of galaxy mergers in which the black hole grows rapidly through merger‐induced accretion following the coalescence of the nuclei of two merging galaxies, and the major growth phase is largely hidden at optical wavelengths by the natal gas and dust. Although the models also predict that AGN‐driven outflows will eventually remove the gas from the bulge of the host galaxy, the visible warm outflow in PKS 1549−79 is not currently capable of doing so. However, much of the outflow may be hidden by the material obscuring the quasar and/or tied up in hotter or cooler phases of the interstellar medium. By combining our estimates of the reddening of the quasar with the H i column derived from the 21‐cm radio observations, we have also made the first direct estimate of the H i spin temperature in the vicinity of a luminous AGN: Tspin > 3000 K.

We present deep Spitzer/IRS spectra for complete samples of 46 2Jy radio galaxies (0.0575%) than their more extended counterparts (≈15 -25%). We discuss this result in the context of a possible bias towards the selection of compact radio sources triggered in gas-rich environments.