ALBERT

Description: We investigate the star formation properties of ~800 sources detected in one of the deepest radio surveys at 1.4 GHz. Our sample spans a wide redshift range (~0.1–4) and about four orders of magnitude in star formation rate (SFR). It includes both star-forming galaxies (SFGs) and active galactic nuclei (AGNs), further divided into radio-quiet (RQ) and radio-loud objects. We compare the SFR derived from the far-infrared luminosity, as traced by Herschel , with the SFR computed from their radio emission. We find that the radio power is a good SFR tracer not only for pure SFGs but also in the host galaxies of RQ AGNs, with no significant deviation with redshift or specific SFR. Moreover, we quantify the contribution of the starburst activity in the SFG population and the occurrence of AGNs in sources with different level of star formation. Finally, we discuss the possibility of using deep radio survey as a tool to study the cosmic star formation history.

Description: Using deep Herschel and ALMA observations, we investigate the star formation rate (SFR) distributions of X-ray-selected active galactic nucleus (AGN) host galaxies at 0.5 〈  z  〈 1.5 and 1.5 〈  z  〈 4, comparing them to that of normal, star-forming (i.e. ‘main-sequence’, or MS) galaxies. We find that 34–55 per cent of AGNs in our sample have SFRs at least a factor of 2 below that of the average MS galaxy, compared to 15 per cent of all MS galaxies, suggesting significantly different SFR distributions. Indeed, when both are modelled as lognormal distributions, the mass and redshift-normalized SFR distributions of X-ray AGNs are roughly twice as broad, and peak 0.4 dex lower, than that of MS galaxies. However, like MS galaxies, the normalized SFR distribution of AGNs in our sample appears not to evolve with redshift. Despite X-ray AGNs and MS galaxies having different SFR distributions, the linear-mean SFR of AGNs derived from our distributions is remarkably consistent with that of MS galaxies, and thus with previous results derived from stacked Herschel data. This apparent contradiction is due to the linear-mean SFR being biased by bright outliers, and thus does not necessarily represent a true characterization of the typical SFR of X-ray AGNs.

Description: We exploit Atacama Large Interferometer Array (ALMA) 870 μm observations to measure the star formation rates (SFRs) of eight X-ray detected active galactic nuclei (AGNs) in a z 3.1 protocluster, four of which reside in extended Lyα haloes (often termed Lyman-alpha blobs: LABs). Three of the AGNs are detected by ALMA and have implied SFRs of 220–410 M  yr –1 ; the non-detection of the other five AGNs places SFR upper limits of 210 M  yr –1 . The mean SFR of the protocluster AGNs (110–210 M  yr –1 ) is consistent (within a factor of 0.7–2.3) with that found for co-eval AGNs in the field, implying that the galaxy growth is not significantly accelerated in these systems. However, when also considering ALMA data from the literature, we find evidence for elevated mean SFRs (up-to a factor of 5.9 over the field) for AGNs at the protocluster core, indicating that galaxy growth is significantly accelerated in the central regions of the protocluster. We also show that all of the four protocluster LABs are associated with an ALMA counterpart within the extent of their Lyα emission. The SFRs of the ALMA sources within the LABs (150–410 M  yr –1 ) are consistent with those expected for co-eval massive star-forming galaxies in the field. Furthermore, the two giant LABs (with physical extents of 100 kpc) do not host more luminous star formation than the smaller LABs, despite being an order of magnitude brighter in Lyα emission. We use these results to discuss star formation as the power source of LABs.

Description: We present the results from a study of the morphologies of moderate luminosity X-ray-selected active galactic nuclei (AGN) host galaxies in comparison to a carefully mass-matched control sample at 0.5 〈 z 〈 3 in the CANDELS GOODS-S field. We apply a multiwavelength morphological decomposition analysis to these two samples and report on the differences between the morphologies as fitted from single Sérsic and multiple Sérsic models, and models which include an additional nuclear point-source component. Thus, we are able to compare the widely adopted single Sérsic fits from previous studies to the results from a full morphological decomposition, and address the issue of how biased the inferred properties of AGN hosts are by a potential nuclear contribution from the AGN itself. We find that the AGN hosts are indistinguishable from the general galaxy population except that beyond z ~= 1.5 they have significantly higher bulge fractions. Even including nuclear sources in our modelling, the probability of this result arising by chance is ~1 x 10 –5 , alleviating concerns that previous, purely single Sérsic, analyses of AGN hosts could have been spuriously biased towards higher bulge fractions. This data set also allows us to further probe the physical nature of these point-source components; we find no strong correlation between the point-source component and AGN activity. Our analysis of the bulge and disc fractions of these AGN hosts in comparison to a mass-matched control sample reveals a similar morphological evolutionary track for both the active and non-active populations, providing further evidence in favour of a model where AGN activity is triggered by secular processes.

Description: Using artificial neural network predictions of total infrared luminosities (L IR ), we compare the host galaxy star formation rates (SFRs) of ~21 000 optically selected active galactic nuclei (AGN), 466 low-excitation radio galaxies (LERGs) and 721 mid-IR-selected AGN. SFR offsets (SFR) relative to a sample of star-forming ‘main-sequence’ galaxies (matched in M * , z and local environment) are computed for the AGN hosts. Optically selected AGN exhibit a wide range of SFR, with a distribution skewed to low SFRs and a median SFR = –0.06 dex. The LERGs have SFRs that are shifted to even lower values with a median SFR = –0.5 dex. In contrast, mid-IR-selected AGN have, on average, SFRs enhanced by a factor of ~1.5. We interpret the different distributions of SFR amongst the different AGN classes in the context of the relative contribution of triggering by galaxy mergers. Whereas the LERGs are predominantly fuelled through low accretion rate secular processes which are not accompanied by enhancements in SFR, mergers, which can simultaneously boost SFRs, most frequently lead to powerful, obscured AGN.

Description: We present high-resolution (0.3 arcsec) Atacama Large Millimeter Array (ALMA) 870 μm imaging of five z 1.5–4.5 X-ray detected AGN (with luminosities of L 2–8keV 〉 10 42  erg s –1 ). These data provide a 20 times improvement in spatial resolution over single-dish rest-frame far-infrared (FIR) measurements. The sub-millimetre emission is extended on scales of FWHM  0.2 arcsec–0.5 arcsec, corresponding to physical sizes of 1–3 kpc (median value of 1.8 kpc). These sizes are comparable to the majority of z =1–5 sub-millimetre galaxies (SMGs) with equivalent ALMA measurements. In combination with spectral energy distribution analyses, we attribute this rest-frame FIR emission to dust heated by star formation. The implied star-formation rate surface densities are 20–200 M  yr –1  kpc –2 , which are consistent with SMGs of comparable FIR luminosities (i.e. L IR  [1–5] x 10 12  L ). Although limited by a small sample of AGN, which all have high-FIR luminosities, our study suggests that the kpc-scale spatial distribution and surface density of star formation in high-redshift star-forming galaxies is the same irrespective of the presence of X-ray detected AGN.

Description: We have investigated the gas content of a sample of several hundred AGN host galaxies at z  〈 1 and compared it with a sample of inactive galaxies, matched in bins of stellar mass and redshift. Gas masses have been inferred from the dust masses, obtained by stacked Herschel far-IR and sub-mm data in the GOODS and COSMOS fields, under reasonable assumptions and metallicity scaling relations for the dust-to-gas ratio. We find that AGNs are on average hosted in galaxies much more gas rich than inactive galaxies. In the vast majority of stellar mass bins, the average gas content of AGN hosts is higher than that in inactive galaxies. The difference is up to a factor of 10 higher in low-stellar-mass galaxies, with a significance of 6.5. In almost half of the AGN sample, the gas content is three times higher than that in the control sample of inactive galaxies. Our result strongly suggests that the probability of having an AGN activated is simply driven by the amount of gas in the host galaxy; this can be explained in simple terms of statistical probability of having a gas cloud falling into the gravitational potential of the black hole. The increased probability of an AGN being hosted by a star-forming galaxy, identified by previous works, may be a consequence of the relationship between gas content and AGN activity, found in this paper, combined with the Schmidt–Kennicutt law for star formation.

Description: Chandra data in the COSMOS, AEGIS-XD and 4 Ms Chandra Deep Field South are combined with multiwavelength photometry available in those fields to determine the rest-frame U  –  V versus V  –  J colours of X-ray AGN hosts in the redshift intervals 0.1 〈  z  〈 0.6 (mean $\overline{z}=0.40$ ) and 0.6 〈  z  〈 1.2 (mean $\overline{z}=0.85$ ). This combination of colours provides an effective and least model-dependent means of separating quiescent from star-forming, including dust reddened, galaxies. Morphological information emphasizes differences between AGN populations split by their U  –  V versus V  –  J colours. AGN in quiescent galaxies consist almost exclusively of bulges, while star-forming hosts are equally split between early- and late-type hosts. The position of AGN hosts on the U  –  V versus V  –  J diagram is then used to set limits on the accretion density of the Universe associated with evolved and star-forming systems independent of dust induced biases. It is found that most of the black hole growth at z 0.40 and 0.85 is associated with star-forming hosts. Nevertheless, a non-negligible fraction of the X-ray luminosity density, about 15–20 per cent, at both $\overline{z}=0.40$ and 0.85, is taking place in galaxies in the quiescent region of the U  –  V versus V  –  J diagram. For the low-redshift sub-sample, 0.1 〈  z  〈 0.6, we also find tentative evidence, significant at the 2 level, that AGN split by their U  –  V and V  –  J colours have different Eddington ratio distributions. AGN in blue star-forming hosts dominate at relatively high Eddington ratios. In contrast, AGN in red quiescent hosts become increasingly important as a fraction of the total population towards low Eddington ratios. At higher redshift, z  〉 0.6, such differences are significant at the 2 level only for sources with Eddington ratios 10 – 3 . These findings are consistent with scenarios in which diverse accretion modes are responsible for the build-up of supermassive black holes at the centres of galaxies. We compare these results with the predictions of the galform semi-analytic model for the cosmological evolution of AGN and galaxies. This model postulates two black hole fuelling modes, the first is linked to star formation events and the second takes place in passive galaxies. galform predicts that a substantial fraction of the black hole growth at z  〈 1 is associated with quiescent galaxies, in apparent conflict with the observations. Relaxing the strong assumption of the model that passive AGN hosts have zero star formation rate could bring those predictions in better agreement with the data.

Description: We present X-shooter at Very Large Telescope observations of a sample of 10 luminous, X-ray obscured quasi-stellar objects (QSOs) at z  ~ 1.5 from the XMM -COSMOS survey, expected to be caught in the transitioning phase from starburst to active galactic nucleus (AGN)-dominated systems. The main selection criterion is X-ray detection at bright fluxes ( L X   10 44  erg s –1 ) coupled to red optical-to-near-infrared-to-mid-infrared colours. Thanks to its large wavelength coverage, X-shooter allowed us to determine accurate redshifts from the presence of multiple emission lines for five out of six targets for which we had only a photometric redshift estimate, with an 80 per cent success rate, significantly larger than what is observed in similar programs of spectroscopic follow-up of red QSOs. We report the detection of broad and shifted components in the [O iii ] 5007, 4959 complexes for six out of eight sources with these lines observable in regions free from strong atmospheric absorptions. The full width at half-maximum (FWHM) associated with the broad components are in the range FWHM ~ 900–1600 km s –1 , larger than the average value observed in Sloan Digital Sky Survey type 2 AGN samples at similar observed [O iii ] luminosity, but comparable to those observed for QSO/ultraluminous infrared galaxies systems for which the presence of kpc scale outflows has been revealed through integral field unit spectroscopy. Although the total outflow energetics (inferred under reasonable assumptions) may be consistent with winds accelerated by stellar processes, we favour an AGN origin for the outflows given the high outflow velocities observed ( v  〉 1000 km s –1 ) and the presence of strong winds also in objects undetected in the far-infrared.

Description: We study the relation of AGN accretion, star formation rate (SFR) and stellar mass ( M * ) using a sample of 8600 star-forming galaxies up to z  = 2.5 selected with Herschel imaging in the GOODS and COSMOS fields. For each of them we derive SFR and M * , both corrected, when necessary, for emission from an active galactic nucleus (AGN), through the decomposition of their spectral energy distributions (SEDs). About 10 per cent of the sample are detected individually in Chandra observations of the fields. For the rest of the sample, we stack the X-ray maps to get average X-ray properties. After subtracting the X-ray luminosity expected from star formation and correcting for nuclear obscuration, we derive the average AGN accretion rate for both detected sources and stacks, as a function of M * , SFR and redshift. The average accretion rate correlates with SFR and with M * . The dependence on SFR becomes progressively more significant at z  〉 0.8. This may suggest that SFR is the original driver of these correlations. We find that average AGN accretion and star formation increase in a similar fashion with offset from the star-forming ‘main-sequence’. Our interpretation is that accretion on to the central black hole and star formation broadly trace each other, irrespective of whether the galaxy is evolving steadily on the main-sequence or bursting.