N. A. Hatch

University of Nottingham, Nottigham, England, United Kingdom

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Publications (70)224.19 Total impact

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    E. Contini · G. De Lucia · N. Hatch · S. Borgani · X. Kang
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    ABSTRACT: We investigate the galaxy population in simulated protocluster regions using a semi-analytic model of galaxy formation, coupled to merger-trees extracted from N-body simulations. We select the most massive clusters at redshift z = 0 from our set of simulations, and follow their main progenitors back in time. The analysis shows that protocluster regions are dominated by central galaxies and their number decreases with time as many become satellites, clustering around the central object. In agreement with observations, we find an increasing velocity dispersion with cosmic time, the increase being faster for satellites. The analysis shows that protoclusters are very extended regions, ≳20 Mpc at z ≳ 1. The fraction of galaxies in protocluster regions that are not progenitor of cluster galaxies varies with redshift, stellar mass and area considered. It is about 20–30 per cent for galaxies with stellar mass ∼109 M⊙, while negligible for the most massive galaxies considered. Nevertheless, these objects have properties similar to those of progenitors. We investigate the building-up of the passive sequence in clusters, and find that their progenitors are on average always active at any redshift of interest of protoclusters. The main mechanism which quenches their star formation is the removal of the hot gas reservoir at the time of accretion. The later galaxies are accreted (become satellite), and the more the cold gas available, the longer the time spent as active. Central galaxies are active over all redshift range considered, although a non-negligible fraction of them become passive at redshift z < 1, due to strong feedback from active galactic nuclei.
    Preview · Article · Dec 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We report on the discovery of a z=1.58 mature cluster around the high-redshift radio galaxy 7C1753+6311, first identified in the Clusters Around Radio-Loud AGN survey. Two-thirds of the excess galaxies within the central 1Mpc lie on a red sequence with a colour that is consistent with an average formation redshift of zf~3. We show that 80+/-6% of the red sequence galaxies in the cluster core are quiescent, while the remaining 20% are red due to dusty star formation. We demonstrate that the cluster has an enhanced quiescent galaxy fraction that is three times that of the control field. We also show that this enhancement is mass dependent: 91+/-9% of the M* >10^{10.5}Msun cluster galaxies are quiescent, compared to only 36+/-2% of field galaxies, whereas the fraction of quiescent galaxies with lower masses is the same in the cluster and field environments. The presence of a dense core and a well-formed, quiescent red sequence suggest that this is a mature cluster. This means that distant radio galaxies do not solely reside in young, uncollapsed protoclusters, rather they can be found in clusters in a wide range of evolutionary states.
    Preview · Article · Nov 2015 · The Astrophysical Journal

  • No preview · Article · Aug 2015 · Proceedings of the International Astronomical Union
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    ABSTRACT: We use a sample of 37 of the densest clusters and protoclusters across 1.3 ≤ z ≤ 3.2 from the Clusters Around Radio-Loud AGN (CARLA) survey to study the formation of massive cluster galaxies. We use optical i′-band and infrared 3.6 and 4.5 μm images to statistically select sources within these protoclusters and measure their median observed colours; 〈i′ − [3.6]〉. We find the abundance of massive galaxies within the protoclusters increases with decreasing redshift, suggesting these objects may form an evolutionary sequence, with the lower redshift clusters in the sample having similar properties to the descendants of the high-redshift protoclusters. We find that the protocluster galaxies have an approximately unevolving observed-frame i′ − [3.6] colour across the examined redshift range. We compare the evolution of the 〈i′ − [3.6]〉 colour of massive cluster galaxies with simplistic galaxy formation models. Taking the full cluster population into account, we show that the formation of stars within the majority of massive cluster galaxies occurs over at least 2 Gyr, and peaks at z ∼ 2–3. From the median i′ − [3.6] colours, we cannot determine the star formation histories of individual galaxies, but their star formation must have been rapidly terminated to produce the observed red colours. Finally, we show that massive galaxies at z > 2 must have assembled within 0.5 Gyr of them forming a significant fraction of their stars. This means that few massive galaxies in z > 2 protoclusters could have formed via dry mergers.
    Preview · Article · Jul 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We explore the structures of protoclusters and their relationship with high redshift clusters using the Millennium Simulation combined with a semi-analytic model. We find that protoclusters are very extended, with 90 per cent of their mass spread across $\sim35\,h^{-1}{\rm Mpc}$ comoving at $z=2$ ($\sim30\, \rm{arcmin}$). The `main halo', which can manifest as a high redshift cluster or group, is only a minor feature of the protocluster, containing less than 20 per cent of all protocluster galaxies at $z=2$. Furthermore, many protoclusters do not contain a main halo that is massive enough to be identified as a high redshift cluster. Protoclusters exist in a range of evolutionary states at high redshift, independent of the mass they will evolve to at $z=0$. We show that the evolutionary state of a protocluster can be approximated by the mass ratio of the first and second most massive haloes within the protocluster, and the $z=0$ mass of a protocluster can be estimated to within 0.2 dex accuracy if both the mass of the main halo and the evolutionary state is known. We also investigate the biases introduced by only observing star-forming protocluster members within small fields. The star formation rate required for line-emitting galaxies to be detected is typically high, which leads to the artificial loss of low mass galaxies from the protocluster sample. This effect is stronger for observations of the centre of the protocluster, where the quenched galaxy fraction is higher. This loss of low mass galaxies, relative to the field, distorts the size of the galaxy overdensity, which in turn can contribute to errors in predicting the $z=0$ evolved mass.
    Preview · Article · Jun 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Powerful AGN are known to be found in overdense environments at high-redshifts. A joint selection using WISE mid-IR and NVSS radio detections of galaxies over most of the sky has highlighted a new very luminous population of about 150 powerful, physically-compact AGN, which might involve the most powerful feedback processes taking place at any epoch. We propose to probe the environments the 33 examples which have spectroscopically confirmed redshifts z>1.3, to allow Spitzer-based color selection of companions over the 5.2-arcmin wide IRAC field. All have new JVLA radio imaging, all but three have ALMA photometry, 25 have VLBA resolution radio images. From SCUBA2 submillimeter imaging observations of about 50 of these galaxies, including 3 in this proposal, we know that the density of ultraluminous dusty companions in 3-arcmin fields around these AGN is unprecedentedly large: up to a factor of 6 in excess of field surveys, the largest overdensity seen for any population of high-redshift galaxies. We propose to exploit the same techniques pioneered by the CARLA program to probe the spatial distribution and stellar populations of high-redshift galaxies in the environments of our AGN. We will determine whether the dramatic overdensity of ultraluminous galaxies around these AGN is matched in more normal sub-L* objects probed by Spitzer, determine the relationship between the modelled stellar populations in our targets and companions, and the AGN/ environmental properties, and provide accurate positions for spectroscopic investigation using new multi-object spectrographs. The 2.5-Mpc regions probed by Spitzer at z>1.3 are ideal to encompass the protocluster in which the AGNs reside, and to understand their evolution and the relationship with their environments.
    No preview · Article · Nov 2014
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    ABSTRACT: We present APEX LABOCA 870 micron observations of the field around the high-redshift radio galaxy MRC1138-262 at z=2.16. We detect 16 submillimeter galaxies in this ~140 square arcmin bolometer map with flux densities in the range 3-11 mJy. The raw number counts indicate a density of submillimeter galaxies (SMGs) that is up to four times that of blank field surveys. Based on an exquisite multiwavelength database, including VLA 1.4 GHz radio and infrared observations, we investigate whether these sources are members of the protocluster structure at z=2.2. Using Herschel PACS+SPIRE and Spitzer MIPS photometry, we derive reliable far-infrared photometric redshifts for all sources. Follow-up VLT ISAAC and SINFONI near-infrared spectra confirm that four of these SMGs have redshifts of z=2.2. We also present evidence that another SMG in this field, detected earlier at 850 micron, has a counterpart that exhibits Halpha and CO(1-0) emission at z=2.15. Including the radio galaxy and two SMGs with far-IR photometric redshifts at z=2.2, we conclude that at least eight submm sources are part of the protocluster at z=2.16 associated with the radio galaxy MRC1138-262. We measure a star formation rate density SFRD ~1500 Msun yr^-1 Mpc^-3, four magnitudes higher than the global SFRD of blank fields at this redshift. Strikingly, these eight sources are concentrated within a region of 2 Mpc (the typical size of clusters in the local universe) and are distributed within the filaments traced by the Halpha emitters at z=2.2. This concentration of massive, dusty starbursts is not centered on the submillimeter-bright radio galaxy which could support the infalling of these sources into the cluster center. Approximately half (6/11) of the SMGs that are covered by the Halpha imaging data are associated with Halpha emitters, demonstrating the potential of tracing SMG counterparts with this population (abridged).
    Full-text · Article · Oct 2014 · Astronomy and Astrophysics
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    ABSTRACT: Distant powerful radio-loud active galactic nuclei (RLAGN) tend to reside in dense environments and are commonly found in protoclusters at z > 1.3. We examine whether this occurs because RLAGN are hosted by massive galaxies, which preferentially reside in rich environments. We compare the environments of powerful RLAGN at 1.3 < z < 3.2 from the Clusters Around Radio-Loud AGN survey to a sample of radio-quiet galaxies matched in mass and redshift. We find that the environments of RLAGN are significantly denser than those of radio-quiet galaxies, implying that not more than 50 per cent of massive galaxies in this epoch can host powerful radio-loud jets. This is not an observational selection effect as we find no evidence to suggest that it is easier to observe the radio emission when the galaxy resides in a dense environment. We therefore suggest that the dense Mpc-scale environment fosters the formation of a radio jet from an AGN. We show that the number density of potential RLAGN host galaxies is consistent with every >1014 M⊙ cluster having experienced powerful radio-loud feedback of duration ∼60 Myr during 1.3 < z < 3.2. This feedback could heat the intracluster medium to the extent of 0.5–1 keV per gas particle, which could limit the amount of gas available for further star formation in the protocluster galaxies.
    Preview · Article · Sep 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present optical and near-infrared (NIR) photometry for three gamma-ray burst supernovae (GRB-SNe): GRB 120729A, GRB 130215A / SN 2013ez and GRB 130831A / SN 2013fu. In the case of GRB 130215A / SN 2013ez, we also present optical spectroscopy at t-t0=16.1 d, which covers rest-frame 3000-6250 Angstroms. Based on Fe II (5169) and Si (II) (6355), our spectrum indicates an unusually low expansion velocity of 4000-6350 km/s, the lowest ever measured for a GRB-SN. Additionally, we determined the brightness and shape of each accompanying SN relative to a template supernova (SN 1998bw), which were used to estimate the amount of nickel produced via nucleosynthesis during each explosion. We find that our derived nickel masses are typical of other GRB-SNe, and greater than those of SNe Ibc that are not associated with GRBs. For GRB 130831A / SN 2013fu, we use our well-sampled R-band light curve (LC) to estimate the amount of ejecta mass and the kinetic energy of the SN, finding that these too are similar to other GRB-SNe. For GRB 130215A, we take advantage of contemporaneous optical/NIR observations to construct an optical/NIR bolometric LC of the afterglow. We fit the bolometric LC with the millisecond magnetar model of Zhang & Meszaros (2001), which considers dipole radiation as a source of energy injection to the forward shock powering the optical/NIR afterglow. Using this model we derive an initial spin period of P=12 ms and a magnetic field of B=1.1 x 10^15 G, which are commensurate with those found for proposed magnetar central engines of other long-duration GRBs.
    Full-text · Article · May 2014
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    ABSTRACT: We present results from a survey of 70 radio galaxies (RGs) at redshifts 1<z<5.2 using the PACS and SPIRE on-board Herschel. Combined with existing mid-IR photometry from Spitzer and observations obtained with LABOCA, the SEDs of galaxies in our sample are continuously covered across 3.6-870um. The total infrared luminosities of these RGs are such that they almost all are either ultra-or hyper-luminous infrared galaxies. We fit the infrared SEDs with a set of empirical templates which represent dust heated (1) by a variety of SB and (2) by a AGN. We find that the SEDs of RGs require the dust to be heated by both AGN and SB, but the luminosities of these two components are not strongly correlated. Assuming empirical relations and simple physical assumptions, we calculate the SFR, the black hole mass accretion rate (MdotBH), and the black hole mass (MBH) for each RG. We find that the host galaxies and their BHs are growing extremely rapidly, having SFR~100-5000 Msun/yr and MdotBH~1-100 Msun/yr. The mean sSFR of RGs at z>2.5 are higher than the sSFR of typical star-forming galaxies over the same redshift range but are similar or perhaps lower than the galaxy population for RGs at z<2.5. By comparing the sSFR and the specific black hole mass accretion rate, we conclude that BHs in radio loud AGN are already, or soon will be, overly massive compared to their host galaxies in terms of expectations from the local MBH-MGal relation. In order to ``catch up'' with the BH, the galaxies require about an order-of magnitude more time to grow in mass, at the observed SFRs, compared to the time the BH is actively accreting. However, during the current cycle of activity, we argue that this catching-up is likely to be difficult due to the short gas depletion times. Finally, we speculate on how the host galaxies might grow sufficiently in stellar mass to ultimately fall onto the local MBH-MGal relation.
    Full-text · Article · Apr 2014 · Astronomy and Astrophysics
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    ABSTRACT: We present results from a narrow-band survey of the field around the high-redshift radio galaxy MRC 2104−242. We have selected Hα emitters in a 7 arcmin2 field and compared the measured number density with that of a field sample at similar redshift. We find that MRC 2104−242 lies in an overdensity of galaxies that is 8.0 ± 0.8 times the average density of a blank field, suggesting it resides in a large-scale structure that may eventually collapse to form a massive cluster. We find that there is more dust obscured star formation in the protocluster galaxies than in similarly selected control field galaxies and there is tentative evidence of a higher fraction of starbursting galaxies in the denser environment. However, on average we do not find a difference between the star formation rate (SFR)–mass relations of the protocluster and field galaxies and so conclude that the SFR of these galaxies at z ∼ 2.5 is governed predominantly by galaxy mass and not the host environment. We also find that the stellar mass distribution of the protocluster galaxies is skewed towards higher masses and there is a significant lack of galaxies at M < 1010 M⊙ within our small field of view. Based on the level of overdensity we expect to find ∼22 star-forming galaxies below 1010 M⊙ in the protocluster and do not detect any. This lack of low-mass galaxies affects the level of overdensity which we detect. If we only consider high-mass (M > 1010.5 M⊙) galaxies, the density of the protocluster field increases to ∼55 times the control field density.
    Preview · Article · Mar 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present 4.5 {\mu}m luminosity functions for galaxies identified in 178 candidate galaxy clusters at 1.3 < z < 3.2. The clusters were identified as Spitzer/IRAC color-selected overdensities in the Clusters Around Radio-Loud AGN (CARLA) project, which imaged 421 powerful radio-loud AGN at z > 1.3. The luminosity functions are derived for different redshift and richness bins, and the IRAC imaging reaches depths of m*+2, allowing us to measure the faint end slopes of the luminosity functions. We find that {\alpha} = -1 describes the luminosity function very well in all redshifts bins and does not evolve significantly. This provides evidence that the rate at which the low mass galaxy population grows through star formation, gets quenched and is replenished by in-falling field galaxies does not have a major net effect on the shape of the luminosity function. Our measurements for m* are consistent with passive evolution models and high formation redshifts z_f ~ 3. We find a slight trend towards fainter m* for the richest clusters, implying that the most massive clusters in our sample could contain older stellar populations, yet another example of cosmic downsizing. Modelling shows that a contribution of a star-forming population of up to 40% cannot be ruled out. This value, found from our targeted survey, is significantly lower than the values found for slightly lower redshift, z ~ 1, clusters found in wide-field surveys. The results are consistent with cosmic downsizing, as the clusters studied here were all found in the vicinity of radio-loud AGNs -- which have proven to be preferentially located in massive dark matter halos in the richest environments at high redshift -- they may therefore be older and more evolved systems than the general protocluster population.
    Full-text · Article · Mar 2014 · The Astrophysical Journal
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    ABSTRACT: Protoclusters, the high-redshift ancestors of local galaxy clusters, are powerful laboratories for tracing the emergence of large-scale structure, and studying the evolution of galaxies in dense environments. This article presents the results of the first far-infrared, wide-field survey of protoclusters, the ancestors of local galaxy clusters, carried out using the SPIRE instrument on-board the Herschel Space Observatory, over the key redshift range 2 < z < 4. Examination of the environment within 6 comoving Mpc of the central radio galaxy in each field, reveals that ˜11% of fields contain a >3σ far-infrared excess in source numbers, and there is a tentative trend for the most powerful radio galaxies to host the largest galaxy overdensities. These protocluster candidates are generally contained within 6 comoving Mpc, which is in agreement with simulations and previous work.
    No preview · Article · Nov 2013
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    ABSTRACT: This paper presents the first results of a far-infrared search for protocluster-associated galaxy overdensities using the Spectral and Photometric Imaging REciever (SPIRE) instrument on-board the Herschel Space Observatory. Large (∼400 arcmin2) fields surrounding 26 powerful high-redshift radio galaxies (2.0 < z < 4.1; L500 MHz > 1028.5 WHz−1) are mapped at 250, 350 and 500 μm to give a unique wide-field sample. On average, the fields have a higher than expected, compared to blank fields, surface density of 500 μm sources within 6 comoving Mpc of the radio galaxy. The analysis is then restricted to potential protocluster members only, which are identified using a far-infrared colour selection; this reveals significant overdensities of galaxies in two fields, neither of which are previously known protoclusters. The probability of finding two overdensities of this size by chance, given the number of fields observed, is 5 × 10−4. Overdensities here exist around radio galaxies with L500 MHz ≳ 1029 WHz−1 and z < 3. The radial extent of the average far-infrared overdensity is found to be ∼6 comoving Mpc. Comparison with predictions from numerical simulations shows that the overdensities are consistent with having masses >1014 M⊙. However, the large uncertainty in the redshift estimation means that it is possible that these far-infrared overdensities consist of several structures across the redshift range searched.
    Full-text · Article · Oct 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present the spectroscopic confirmation of a structure of galaxies surrounding the radio galaxy MRC0156-252 at z = 2.02. The structure was initially discovered as an overdensity of both near-infrared selected z > 1.6 and mid-infrared selected z > 1.2 galaxy candidates. We used the VLT/FORS2 multi-object spectrograph to target ~80 high-redshift galaxy candidates, and obtain robust spectroscopic redshifts for more than half the targets. The majority of the confirmed sources are star-forming galaxies at z > 1.5. In addition to the radio galaxy, two of its close-by companions (< 6'') also show AGN signatures. Ten sources, including the radio galaxy, lie within |z - 2.020 | < 0.015 (i.e., velocity offsets < 1500 km/s) and within projected 2 Mpc comoving of the radio galaxy. Additional evidence suggests not only that the galaxy structure associated with MRC0156-252 is a forming galaxy cluster but also that this structure is most probably embedded in a larger scale structure.
    Full-text · Article · Sep 2013 · Astronomy and Astrophysics
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    ABSTRACT: We present multicolour Hubble Space Telescope images of the powerful z=2.4 radio galaxy MRC 0406-244 and model its complex morphology with several components including a host galaxy, a point source, and extended nebular and continuum emission. We suggest that the main progenitor of this radio galaxy was a normal, albeit massive (M ~10^{11} solar masses), star-forming galaxy. The optical stellar disc of the host galaxy is smooth and well described by a S\'ersic profile, which argues against a recent major merger, however there is also a point-source component which may be the remnant of a minor merger. The half-light radius of the optical disc is constrained to lie in the range 3.5 to 8.2kpc, which is of similar size to coeval star forming galaxies. Biconical shells of nebular emission and UV-bright continuum extend out from the host galaxy along the radio jet axis, which is also the minor axis of the host galaxy. The origin of the continuum emission is uncertain, but it is most likely to be young stars or dust-scattered light from the AGN, and it is possible that stars are forming from this material at a rate of 200^{+1420}_{-110} solar masses per year.
    Preview · Article · Sep 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We have used the SINFONI near-infrared integral field unit on the Very Large Telescope to resolve the optical emission line structure of one of the brightest (L Lyα ≈ 1044 erg s–1) and nearest (z ≈ 2.38) of all Lyα blobs (LABs). The target, known in the literature as object "B1", lies at a redshift where the main optical emission lines are accessible in the observed near-infrared. We detect luminous [O III] λλ4959, 5007 and Hα emission with a spatial extent of at least 32 × 40 kpc (4'' × 5''). The dominant optical emission line component shows relatively broad lines (600-800 km s–1, FWHM) and line ratios consistent with active galactic nucleus (AGN) photoionization. The new evidence for AGN photoionization, combined with previously detected C IV and luminous, warm infrared emission, suggest that B1 is the site of a hidden quasar. This is confirmed by the fact that [O II] is relatively weak compared with [O III] (extinction-corrected [O III]/[O II] of about 3.8), which is indicative of a high, Seyfert-like ionization parameter. From the extinction-corrected [O III] luminosity we infer a bolometric AGN luminosity of ~3 × 1046 erg s–1, and further conclude that the obscured AGN may be Compton-thick given existing X-ray limits. The large line widths observed are consistent with clouds moving within the narrow-line region of a luminous QSO. The AGN scenario is capable of producing sufficient ionizing photons to power the Lyα, even in the presence of dust. By performing a census of similar objects in the literature, we find that virtually all luminous LABs harbor obscured quasars. Based on simple duty-cycle arguments, we conclude that AGNs are the main drivers of the Lyα in LABs rather than the gravitational heating and subsequent cooling suggested by cold stream models. We also conclude that the empirical relation between LABs and overdense environments at high redshift must be due to a more fundamental correlation between AGNs (or massive galaxies) and environment.
    No preview · Article · Jun 2013 · The Astrophysical Journal
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    ABSTRACT: We report the first results from the Clusters Around Radio-Loud AGN program, a Cycle 7 and 8 Spitzer Space Telescope snapshot program to investigate the environments of a large sample of obscured and unobscured luminous radio-loud active galactic nuclei (AGNs) at 1.2 < z –0.1 (AB), which efficiently selects high-redshift (z > 1.3) galaxies of all types, we identify galaxy cluster member candidates in the fields of the radio-loud AGN. The local density of these Infrared Array Camera (IRAC)-selected sources is compared to the density of similarly selected sources in blank fields. We find that 92% of the radio-loud AGN reside in environments richer than average. The majority (55%) of the radio-loud AGN fields are found to be overdense at a ≥2σ level; 10% are overdense at a ≥5σ level. A clear rise in surface density of IRAC-selected sources toward the position of the radio-loud AGN strongly supports an association of the majority of the IRAC-selected sources with the radio-loud AGN. Our results provide solid statistical evidence that radio-loud AGN are likely beacons for finding high-redshift galaxy (proto-)clusters. We investigate how environment depends on AGN type (unobscured radio-loud quasars versus obscured radio galaxies), radio luminosity and redshift, finding no correlation with either AGN type or radio luminosity. We find a decrease in density with redshift, consistent with galaxy evolution for this uniform, flux-limited survey. These results are consistent with expectations from the orientation-driven AGN unification model, at least for the high radio luminosity regimes considered in this sample.
    No preview · Article · May 2013
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    ABSTRACT: We have used the SINFONI near-infrared integral field unit on the VLT to resolve the optical emission line structure of one of the brightest (L~1e44 erg/s) and nearest (z=2.38) of all Lya blobs (LABs). The target, known in the literature as object 'B1' (Francis et al. 1996), lies at a redshift where the main optical emission lines are accessible in the observed near-infrared. We detect luminous [OIII]4959,5007A and Ha emission with a spatial extent of at least 32x40 kpc (4"x5"). The dominant optical emission line component shows relatively broad lines (600-800 km/s, FWHM) and line ratios consistent with AGN-photoionization. The new evidence for AGN photoionization, combined with previously detected CIV and luminous, warm infrared emission, suggest that B1 is the site of a hidden quasar. This is confirmed by the fact that [OII] is relatively weak compared to [OIII] (extinction-corrected [OIII]/[OII] of about 3.8), which is indicative of a high, Seyfert-like ionization parameter. From the [OIII] luminosity we infer a bolometric AGN luminosity of ~3e46 erg/s, and further conclude that the obscured AGN may be Compton-thick given existing X-ray limits. The large line widths observed are consistent with clouds moving within the narrow line region of a luminous QSO. The AGN scenario is capable of producing sufficient ionizing photons to power the Lya, even in the presence of dust. By performing a census of similar objects in the literature, we find that virtually all luminous LABs harbor obscured quasars. Based on simple duty-cycle arguments, we conclude that AGN are the main drivers of the Lya in LABs rather than the gravitational heating and subsequent cooling suggested by cold stream models. We also conclude that the empirical relation between LABs and overdense environments at high redshift must be due to a more fundamental correlation between AGN (or massive galaxies) and environment.
    Full-text · Article · May 2013
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    ABSTRACT: We report the first results from the Clusters Around Radio-Loud AGN program, a Cycle 7 and 8 Spitzer Space Telescope snapshot program to investigate the environments of a large sample of obscured and unobscured luminous radio-loud active galactic nuclei (AGNs) at 1.2 < z < 3.2. These data, obtained for 387 fields, reach 3.6 and 4.5 μm depths of [3.6]AB = 22.6 and [4.5]AB = 22.9 at the 95% completeness level, which is two to three times fainter than L* in this redshift range. By using the color cut [3.6] – [4.5] > –0.1 (AB), which efficiently selects high-redshift (z > 1.3) galaxies of all types, we identify galaxy cluster member candidates in the fields of the radio-loud AGN. The local density of these Infrared Array Camera (IRAC)-selected sources is compared to the density of similarly selected sources in blank fields. We find that 92% of the radio-loud AGN reside in environments richer than average. The majority (55%) of the radio-loud AGN fields are found to be overdense at a ≥2σ level; 10% are overdense at a ≥5σ level. A clear rise in surface density of IRAC-selected sources toward the position of the radio-loud AGN strongly supports an association of the majority of the IRAC-selected sources with the radio-loud AGN. Our results provide solid statistical evidence that radio-loud AGN are likely beacons for finding high-redshift galaxy (proto-)clusters. We investigate how environment depends on AGN type (unobscured radio-loud quasars versus obscured radio galaxies), radio luminosity and redshift, finding no correlation with either AGN type or radio luminosity. We find a decrease in density with redshift, consistent with galaxy evolution for this uniform, flux-limited survey. These results are consistent with expectations from the orientation-driven AGN unification model, at least for the high radio luminosity regimes considered in this sample.
    Preview · Article · May 2013 · The Astrophysical Journal

Publication Stats

1k Citations
224.19 Total Impact Points

Institutions

  • 2010-2015
    • University of Nottingham
      • School of Physics and Astronomy
      Nottigham, England, United Kingdom
  • 2007-2010
    • Leiden University
      • Leiden Observartory
      Leyden, South Holland, Netherlands
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, ENG, United Kingdom