We present a catalogue of ∼3000 submillimetre sources detected (≥3.5σ) at 850 μm over ∼5 deg² surveyed as part of the James Clerk Maxwell Telescope (JCMT) SCUBA-2 Cosmology Legacy Survey (S2CLS). This is the largest survey of its kind at 850 μm, increasing the sample size of 850 μm selected submillimetre galaxies by an order of magnitude. The wide 850 μm survey component of S2CLS covers the extragalactic fields: UKIDSS-UDS, COSMOS, Akari-NEP, Extended Groth Strip, Lockman Hole North, SSA22 and GOODS-North. The average 1σ depth of S2CLS is 1.2 mJy beam⁻¹, approaching the SCUBA-2 850 μm confusion limit, which we determine to be σ_c ≈ 0.8 mJy beam⁻¹. We measure the 850 μm number counts, reducing the Poisson errors on the differential counts to approximately 4 per cent at S₈₅₀ ≈ 3 mJy. With several independent fields, we investigate field-to-field variance, finding that the number counts on 0.5°–1° scales are generally within 50 per cent of the S2CLS mean for S₈₅₀ > 3 mJy, with scatter consistent with the Poisson and estimated cosmic variance uncertainties, although there is a marginal (2σ) density enhancement in GOODS-North. The observed counts are in reasonable agreement with recent phenomenological and semi-analytic models, although determining the shape of the faint-end slope (S₈₅₀ < 3 mJy) remains a key test. The large solid angle of S2CLS allows us to measure the bright-end counts: at S₈₅₀ > 10 mJy there are approximately 10 sources per square degree, and we detect the distinctive up-turn in the number counts indicative of the detection of local sources of 850 μm emission, and strongly lensed high-redshift galaxies. All calibrated maps and the catalogue are made publicly available.

In this study, we investigate the relationship between the star formation rate (SFR) and AGN luminosity ($$\mathrel {L_{\rm AGN}}$$) for ∼2000 X-ray detected AGN. The AGN span over three orders of magnitude in X-ray luminosity ($$10^{42} <\, \mathrel {L_{2{\rm -}8\,\mathrm{keV}}}\,< 10^{45.5} \mathrel {\,\rm erg \; s^{-1}}$$) and are in the redshift range z = 0.2–2.5. Using infrared (IR) photometry (8–500 $$\mathrel {\,\rm \mu m}$$), including deblended Spitzer and Herschel images and taking into account photometric upper limits, we decompose the IR spectral energy distributions into AGN and star formation components. Using the IR luminosities due to star formation, we investigate the average SFRs as a function of redshift and AGN luminosity. In agreement with previous studies, we find a strong evolution of the average SFR with redshift, tracking the observed evolution of the overall star-forming galaxy population. However, we find that the relationship between the average SFR and AGN luminosity is broadly flat at all redshifts and across all the AGN luminosities investigated; in comparison to previous studies, we find less scatter amongst the average SFRs across the wide range of AGN luminosities investigated. By comparing to empirical models, we argue that the observed flat relationship is due to short time-scale variations in AGN luminosity, driven by changes in the mass accretion rate, which wash out any underlying correlations between SFR and $$\mathrel {L_{\rm AGN}}$$. Furthermore, we show that the exact form of the predicted relationship between SFR and AGN luminosity (and its normalization) is highly sensitive to the assumed intrinsic Eddington ratio distribution.

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.

We investigate the mean star formation rates (SFRs) in the host galaxies of ∼3000 optically selected quasi-stellar objects (QSOs) from the Sloan Digital Sky Survey within the Herschel-ATLAS fields, and a radio-luminous subsample covering the redshift range of z = 0.2–2.5. Using Wide-field Infrared Survey Explorer (WISE) and Herschel photometry (12–500 μm) we construct composite spectral energy distributions (SEDs) in bins of redshift and active galactic nucleus (AGN) luminosity. We perform SED fitting to measure the mean infrared luminosity due to star formation, removing the contamination from AGN emission. We find that the mean SFRs show a weak positive trend with increasing AGN luminosity. However, we demonstrate that the observed trend could be due to an increase in black hole (BH) mass (and a consequent increase of inferred stellar mass) with increasing AGN luminosity. We compare to a sample of X-ray selected AGN and find that the two populations have consistent mean SFRs when matched in AGN luminosity and redshift. On the basis of the available virial BH masses, and the evolving BH mass to stellar mass relationship, we find that the mean SFRs of our QSO sample are consistent with those of main sequence star-forming galaxies. Similarly the radio-luminous QSOs have mean SFRs that are consistent with both the overall QSO sample and with star-forming galaxies on the main sequence. In conclusion, on average QSOs reside on the main sequence of star-forming galaxies, and the observed positive trend between the mean SFRs and AGN luminosity can be attributed to BH mass and redshift dependencies.

We present the infrared (IR) and X-ray properties of a sample of 33 mid-IR luminous quasars (νL_6 μm ≥ 6 × 10⁴⁴ erg s⁻¹) at redshift z ≈ 1–3, identified through detailed spectral energy distribution analyses of distant star-forming galaxies, using the deepest IR data from Spitzer and Herschel in the GOODS–Herschel fields. The aim is to constrain the fraction of obscured, and Compton-thick (CT, N_H > 1.5 × 10²⁴ cm⁻²) quasars at the peak era of nuclear and star formation activities. Despite being very bright in the mid-IR band, ≈30 per cent of these quasars are not detected in the extremely deep 2 and 4 Ms Chandra X-ray data available in these fields. X-ray spectral analysis of the detected sources reveals that the majority (≈67 per cent) are obscured by column densities N_H > 10²² cm⁻²; this fraction reaches ≈80 per cent when including the X-ray-undetected sources (9 out of 33), which are likely to be the most heavily obscured, CT quasars. We constrain the fraction of CT quasars in our sample to be ≈24–48 per cent, and their space density to be Φ = (6.7 ± 2.2) × 10⁻⁶ Mpc⁻³. From the investigation of the quasar host galaxies in terms of star formation rates (SFRs) and morphological distortions, as a sign of galaxy mergers/interactions, we do not find any direct relation between SFRs and quasar luminosity or X-ray obscuration. On the other hand, there is tentative evidence that the most heavily obscured quasars have, on average, more disturbed morphologies than the unobscured/moderately obscured quasar hosts, which preferentially live in undisturbed systems. However, the fraction of quasars with disturbed morphology amongst the whole sample is ≈40 per cent, suggesting that galaxy mergers are not the main fuelling mechanism of quasars at z ≈ 2.

Hot dust-obscured galaxies (hot DOGs), selected from Wide-Field Infrared Survey Explorer’s all-sky infrared survey, host some of the most powerful active galactic nuclei known and may represent an important stage in the evolution of galaxies. Most known hot DOGs are located at , due in part to a strong bias against identifying them at lower redshift related to the selection criteria. We present a new selection method that identifies 153 hot DOG candidates at , where they are significantly brighter and easier to study. We validate this approach by measuring a redshift z = 1.009 and finding a spectral energy distribution similar to that of higher-redshift hot DOGs for one of these objects, WISE J1036+0449 ( ). We find evidence of a broadened component in Mg ii, which would imply a black hole mass of and an Eddington ratio of . WISE J1036+0449 is the first hot DOG detected by the Nuclear Spectroscopic Telescope Array, and observations show that the source is heavily obscured, with a column density of . The source has an intrinsic 2–10 keV luminosity of , a value significantly lower than that expected from the mid-infrared/X-ray correlation. We also find that other hot DOGs observed by X-ray facilities show a similar deficiency of X-ray flux. We discuss the origin of the X-ray weakness and the absorption properties of hot DOGs. Hot DOGs at could be excellent laboratories to probe the characteristics of the accretion flow and of the X-ray emitting plasma at extreme values of the Eddington ratio.