ALBERT

Description: Context. Galaxy clusters in the local universe descend from high-redshift overdense regions known as protoclusters. The large gas reservoirs and high rate of galaxy interaction in protoclusters are expected to enhance star-formation activity and trigger luminous supermassive black-hole accretion in the nuclear regions of the host galaxies. Aims. We investigated the active galactic nucleus (AGN) content of a gas-rich and starbursting protocluster at z = 4.002, known as the Distant Red Core (DRC). In particular, we search for luminous and possibly obscured AGN in 13 identified members of the structure, and compare the results with protoclusters at lower redshifts. We also test whether a hidden AGN can power the Lyα blob (LAB) detected with VLT/MUSE in the DRC. Methods. We observed all of the identified members of the structure with 139 ks of Chandra ACIS-S imaging. Being less affected by absorption than optical and IR bands, even in the presence of large column densities of obscuring material, X-ray observations are the best tools to detect ongoing nuclear activity in the DRC galaxies. Results. We detect obscured X-ray emission from the two most gas-rich members of the DRC, named DRC-1 and DRC-2. Both of them are resolved into multiple interacting clumps in high-resolution Atacama Large Millimeter Array and Hubble Space Telescope observations. In particular, DRC-2 is found to host a luminous (L2−10 keV ≈ 3 × 1045 erg s−1 ) Compton-thick (NH ≳ 1024 cm−2) quasar (QSO) candidate, comparable to the most luminous QSOs known at all cosmic times. The AGN fraction among DRC members is consistent with results found for lower redshift protoclusters. However, X-ray stacking analysis reveals that supermassive black hole (SMBH) accretion is likely also taking place in other DRC galaxies that are not detected individually by Chandra. Conclusions. The luminous AGN detected in the most gas-rich galaxies in the DRC and the widespread SMBH accretion in the other members, which is suggested by stacking analysis, point toward the presence of a strong link between large gas reservoirs, galaxy interactions, and luminous and obscured nuclear activity in protocluster members. The powerful and obscured QSO detected in DRC-2 is likely powering the nearby LAB detected with VLT/MUSE, possibly through photoionization; however, we propose that the diffuse Lyα emission may be due to gas shocked by a massive outflow launched by DRC-2 over a ≈10 kpc scale.

Description: The overdense environments of protoclusters of galaxies in the early Universe ( z  〉 2) are expected to accelerate the evolution of galaxies, with an increased rate of stellar mass assembly and black hole accretion compared to co-eval galaxies in the average density ‘field’. These galaxies are destined to form the passive population of massive galaxies that dominate the cores of rich clusters today. While signatures of the accelerated growth of galaxies in the SSA22 protocluster ( z  = 3.1) have been observed, the mechanism driving this remains unclear. Here we show an enhanced rate of galaxy–galaxy mergers could be responsible. We morphologically classify Lyman-break Galaxies (LBGs) in the SSA22 protocluster and compare these to those of galaxies in the field at z  = 3.1 as either active mergers or non-merging using Hubble Space Telescope ACS/ F 814 W imaging, probing the rest-frame ultraviolet stellar light. We measure a merger fraction of 48 ± 10 per cent for LBGs in the protocluster compared to 30 ± 6 per cent in the field. Although the excess is marginal, an enhanced rate of mergers in SSA22 hints that galaxy–galaxy mergers are one of the key channels driving accelerated star formation and AGN growth in protocluster environments.

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 Very Large Telescope VIMOS, Keck DEIMOS and Keck LRIS multi-object spectra of 367 sources in the field of the z   3.09 protocluster SSA22. Sources are spectroscopically classified via template matching, allowing new identifications for 206 extragalactic sources, including 36 z  〉 2 Lyman-break galaxies (LBGs) and Lyman α emitters (LAEs), eight protocluster members, and 94 X-ray sources from the ~400 ks Chandra deep survey of SSA22. Additionally, in the area covered by our study, we have increased by 4, 13, and 6 times the number of reliable redshifts of sources at 1.0 〈  z  〈 2.0, at z  〉 3.4, and with X-ray emission, respectively. We compare our results with past spectroscopic surveys of SSA22 to investigate the completeness of the LBGs and the X-ray properties of the new spectroscopically classified sources in the SSA22 field.

Description: We present the results of a joint Swift -NuSTAR spectroscopy campaign on M31. We focus on the five brightest globular cluster X-ray sources in our fields. Two of these had previously been argued to be black hole candidates on the basis of apparent hard-state spectra at luminosities above those for which neutron stars are in hard states. We show that these two sources are likely to be Z-sources (i.e. low magnetic field neutron stars accreting near their Eddington limits), or perhaps bright atoll sources (low magnetic field neutron stars which are just a bit fainter than this level) on the basis of simultaneous Swift and NuSTAR spectra which cover a broader range of energies. These new observations reveal spectral curvature above 6–8 keV that would be hard to detect without the broader energy coverage the NuSTAR data provide relative to Chandra and XMM–Newton . We show that the other three sources are also likely to be bright neutron star X-ray binaries, rather than black hole X-ray binaries. We discuss why it should already have been realized that it was unlikely that these objects were black holes on the basis of their being persistent sources, and we re-examine past work which suggested that tidal capture products would be persistently bright X-ray emitters. We discuss how this problem is likely due to neglecting disc winds in older work that predict which systems will be persistent and which will be transient.

Description: We present a Herschel /Spectral and Photometric Imaging Receiver (SPIRE) survey of three protoclusters at z = 2–3 (2QZCluster, HS1700, SSA22). Based on the SPIRE colours ( S 350 / S 250 and S 500 / S 350 ) of 250 μm sources, we selected high-redshift dusty star-forming galaxies potentially associated with the protoclusters. In the 2QZCluster field, we found a 4 overdensity of six SPIRE sources around 4.5 arcmin (~2.2 Mpc) from a density peak of H α emitters at z = 2.2. In the HS1700 field, we found a 5 overdensity of eight SPIRE sources around 2.1 arcmin (~1.0 Mpc) from a density peak of Lyman-break galaxies at z = 2.3. We did not find any significant overdensities in SSA22 field, but we found three 500 μm sources are concentrated 3 arcmin (~1.4 Mpc) east to the Ly α emitters overdensity. If all the SPIRE sources in these three overdensities are associated with protoclusters, the inferred star formation rate densities are 10 3 –10 4 times higher than the average value at the same redshifts. This suggests that dusty star formation activity could be very strongly enhanced in z ~ 2–3 protoclusters. Further observations are needed to confirm the redshifts of the SPIRE sources and to investigate what processes enhance the dusty star formation activity in z ~ 2–3 protoclusters.

Description: Ly α blobs (LABs) offer insight into the complex interface between galaxies and their circumgalactic medium. Whilst some LABs have been found to contain luminous star-forming galaxies and active galactic nuclei that could potentially power the Ly α emission, others appear not to be associated with obvious luminous galaxy counterparts. It has been speculated that LABs may be powered by cold gas streaming on to a central galaxy, providing an opportunity to directly observe the ‘cold accretion’ mode of galaxy growth. Star-forming galaxies in LABs could be dust obscured and therefore detectable only at longer wavelengths. We stack deep Submillimetre Common User Bolometer Array 2 (SCUBA-2) observations of the Small Selected Area 22 h field to determine the average 850 μm flux density of 34 LABs. We measure S 850 = 0.6 ± 0.2 mJy for all LABs, but stacking the LABs by size indicates that only the largest third (area ≥1794 kpc 2 ) have a mean detection, at 4.5, with S 850 = 1.4 ± 0.3 mJy. Only two LABs (1 and 18) have individual SCUBA-2 〉3.5 detections at a depth of 1.1 mJy beam –1 . We consider two possible mechanisms for powering the LABs and find that central star formation is likely to dominate the emission of Ly α, with cold accretion playing a secondary role.

Description: We exploit the 7 Ms Chandra observations in the Chandra  Deep Field-South (CDF-S), the deepest X-ray survey to date, coupled with CANDELS/GOODS-S data, to measure the total X-ray emission arising from 2076 galaxies at 3.5 ≤ z 〈 6.5. This aim is achieved by stacking the Chandra data at the positions of optically selected galaxies, reaching effective exposure times of ≥10 9 s. We detect significant (〉3.7) X-ray emission from massive galaxies at z 4. We also report the detection of massive galaxies at z 5 at a 99.7 per cent confidence level (2.7), the highest significance ever obtained for X-ray emission from galaxies at such high redshifts. No significant signal is detected from galaxies at even higher redshifts. The stacking results place constraints on the BHAD associated with the known high-redshift galaxy samples, as well as on the SFRD at high redshift, assuming a range of prescriptions for X-ray emission due to X- ray binaries. We find that the X-ray emission from our sample is likely dominated by processes related to star formation. Our results show that low-rate mass accretion on to SMBHs in individually X-ray-undetected galaxies is negligible, compared with the BHAD measured for samples of X-ray detected AGN, for cosmic SMBH mass assembly at high redshift. We also place, for the first time, constraints on the faint-end of the AGN X-ray luminosity function (logL X ~ 42) at z 〉 4, with evidence for fairly flat slopes. The implications of all of these findings are discussed in the context of the evolution of the AGN population at high redshift.

Description: We have discovered a persistent, but highly variable X-ray source in the nearby starburst galaxy NGC 253. The source varies at the level of a factor of about 5 in count rate on time-scales of a few hours. Two long observations of the source with Chandra and XMM – Newton show suggestive evidence for the source having a period of about 14–15 hours, but the time sampling in existing data is insufficient to allow a firm determination that the source is periodic. Given the amplitude of variation and the location in a nuclear starburst, the source is likely to be a Wolf–Rayet X-ray binary, with the tentative period being the orbital period of the system. In light of the fact that we have demonstrated that careful examination of the variability of moderately bright X-ray sources in nearby galaxies can turn up candidate Wolf–Rayet X-ray binaries, we discuss the implications of Wolf–Rayet X-ray binaries for predictions of the gravitational wave source event rate, and, potentially, interpretations of the events.

Description: We present results from a Chandra study of ultraluminous X-ray sources (ULXs) in a sample of 17 nearby ( D L  〈 60 Mpc) luminous infrared galaxies (LIRGs), selected to have star formation rates (SFRs) in excess of 7 M  yr –1 and low foreground Galactic column densities ( N H 5 x 10 20  cm –2 ). A total of 53 ULXs were detected and we confirm that this is a complete catalogue of ULXs for the LIRG sample. We examine the evolution of ULX spectra with luminosity in these galaxies by stacking the spectra of individual objects in three luminosity bins, finding a distinct change in spectral index at luminosity ~2 10 39 erg s –1 . This may be a change in spectrum as 10 M black holes transit from an ~ Eddington to a super-Eddington accretion regime, and is supported by a plausible detection of partially ionized absorption imprinted on the spectrum of the luminous ULX ( L X   5 10 39 erg s –1 ) CXOU J024238.9-000055 in NGC 1068, consistent with the highly ionized massive wind that we would expect to see driven by a super-Eddington accretion flow. This sample shows a large deficit in the number of ULXs detected per unit SFR (0.2 versus 2 ULXs, per M yr –1 ) compared to the detection rate in nearby ( D L  〈 14.5 Mpc) normal star-forming galaxies. This deficit also manifests itself as a lower differential X-ray luminosity function normalization for the LIRG sample than for samples of other star-forming galaxies. We show that it is unlikely that this deficit is a purely observational effect. Part of this deficit might be attributable to the high metallicity of the LIRGs impeding the production efficiency of ULXs and/or a lag between the star formation starting and the production of ULXs; however, we argue that the evidence – including very low N ULX / L FIR , and an even lower ULX incidence in the central regions of the LIRGs – shows that the main culprit for this deficit is likely to be the high column of gas and dust in these galaxies, that fuels the high SFR but also acts to obscure many ULXs from our view.