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Perley D.A.,California Institute of Technology | Perley D.A.,Copenhagen University | Tanvir N.R.,University of Leicester | Hjorth J.,Copenhagen University | And 11 more authors.
Astrophysical Journal | Year: 2016

We present rest-frame near-IR (NIR) luminosities and stellar masses for a large and uniformly selected population of gamma-ray burst (GRB) host galaxies using deep Spitzer Space Telescope imaging of 119 targets from the Swift GRB Host Galaxy Legacy Survey spanning 0.03 < z < 6.3, and we determine the effects of galaxy evolution and chemical enrichment on the mass distribution of the GRB host population across cosmic history. We find a rapid increase in the characteristic NIR host luminosity between z ∼ 0.5 and z ∼ 1.5, but little variation between z ∼ 1.5 and z ∼ 5. Dust-obscured GRBs dominate the massive host population but are only rarely seen associated with low-mass hosts, indicating that massive star-forming galaxies are universally and (to some extent) homogeneously dusty at high redshift while low-mass star-forming galaxies retain little dust in their interstellar medium. Comparing our luminosity distributions with field surveys and measurements of the high-z mass-metallicity relation, our results have good consistency with a model in which the GRB rate per unit star formation is constant in galaxies with gas-phase metallicity below approximately the solar value but heavily suppressed in more metal-rich environments. This model also naturally explains the previously reported "excess" in the GRB rate beyond z 2; metals stifle GRB production in most galaxies at z < 1.5 but have only minor impact at higher redshifts. The metallicity threshold we infer is much higher than predicted by single-star models and favors a binary progenitor. Our observations also constrain the fraction of cosmic star formation in low-mass galaxies undetectable to Spitzer to be small at z < 4. © 2016. The American Astronomical Society. All rights reserved..

Rodriguez-Gonzalvez C.,Us Planck Data Center | Muchovej S.,California Institute of Technology | Chary R.R.,Us Planck Data Center
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present 1-2 arcmin spatial resolution Combined Array for Research in Millimetre-wave Astronomy (CARMA)-8 31 GHz observations towards 19 unconfirmed Planck cluster candidates, selected to have significant galaxy overdensities from the WISE early data release and thought to be at z ≳ 1 from the WISE colours of the putative brightest cluster galaxy. We find a Sunyaev-Zeldovich (SZ) detection in the CARMA-8 data towards nine candidate clusters, where one detection is considered tentative. For each cluster candidate we present CARMA-8 maps, a study of their radio-source environment and we assess the reliability of the SZ detection. The CARMA SZ detections appear to be SZ bright, with the mean, primary-beam-corrected peak flux density of the decrement being -2.9 mJy beam-1 with a standard deviation of 0.8, and are typically offset from the Planck position by ≈80 arcsec. Using archival imaging data in the vicinity of the CARMA SZ centroids, we present evidence that one cluster matches Abell 586 - a known z ≈ 0.2 cluster; four candidate clusters are likely to have 0.3 ≈ z ≈ 0.7; and, for the remaining four, the redshift information is inconclusive. We also argue that the sensitivity limits resulting from the cross-correlation between Planck and WISE makes it challenging to use our selection criterion to identify clusters at z > 1. © 2014 The Authors.

Kanekar N.,Tata Institute of Fundamental Research | Wagg J.,European Southern Observatory | Wagg J.,University of Cambridge | Chary R.R.,Us Planck Data Center | Carilli C.L.,U.S. National Radio Astronomy Observatory
Astrophysical Journal Letters | Year: 2013

We report a Plateau de Bure Interferometer search for C II 158 μm emission from HCM 6A, a lensed Lyα emitter (LAE) at z = 6.56. Our non-detections of C II 158 μm line emission and 1.2 mm radio continuum emission yield 3σ limits of LȮ for the C II 158 μm line luminosity and S 1.2 mm < 0.68 mJy for the 1.2 mm flux density. The local conversion factor between L C II and the star formation rate (SFR) yields an SFR < 4.7 MȮ yr-1, ≈2 times lower than that inferred from the ultraviolet (UV) continuum, suggesting that the local factor may not be applicable in high-z LAEs. The non-detection of 1.2 mm continuum emission yields a total SFR < 28 MȮ yr-1; any obscured star formation is thus within a factor of two of the visible star formation. Our best-fit model to the rest-frame UV/optical spectral energy distribution of HCM 6A yields a stellar mass of 1.3 × 109 MȮ and an SFR of ≈10 MȮ yr-1, with negligible dust obscuration. We fortuitously detect CO J = 3-2 emission from a z = 0.375 galaxy in the foreground cluster A370, and obtain a CO line luminosity of L′(CO) > (8.95 ± 0.79) × 108 K km s-1 pc2 and a molecular gas mass of M(H2) > (4.12 ± 0.36) × 109 MȮ, for a CO-to-H2 conversion factor of 4.6 MȮ (K km s-1 pc2)-1. © 2013. The American Astronomical Society. All rights reserved.

Del Moro A.,Durham University | Alexander D.M.,Durham University | Mullaney J.R.,Durham University | Mullaney J.R.,University Paris Diderot | And 30 more authors.
Astronomy and Astrophysics | Year: 2012

Context. A tight correlation exists between far-infrared and radio emission for star-forming galaxies (SFGs), which seems to hold out to high redshifts (z≈2). Any excess of radio emission over that expected from star formation processes is most likely produced by an active galactic nucleus (AGN), often hidden by large amounts of dust and gas. Identifying these radio-excess sources will allow us to study a population of AGN unbiased by obscuration and thus find some of the most obscured, Compton-thick AGN, which are in large part unidentified even in the deepest X-ray and infrared (IR) surveys. Aims. We present here a new spectral energy distribution (SED) fitting approach that we adopt to select radio-excess sources amongst distant star-forming galaxies in the GOODS-Herschel (North) field and to reveal the presence of hidden, highly obscured AGN. Methods. Through extensive SED analysis of 458 galaxies with radio 1.4 GHz and mid-IR 24 μm detections using some of the deepest Chandra X-ray, Spitzer and Herschel infrared, and VLA radio data available to date, we have robustly identified a sample of 51 radio-excess AGN (∼1300 deg-2) out to redshift z≈3. These radio-excess AGN have a significantly lower far-IR/radio ratio (q<1.68, 3σ) than the typical relation observed for star-forming galaxies (q≈2.2). Results. We find that ≈45% of these radio-excess sources have a dominant AGN component in the mid-IR band, while for the remainders the excess radio emission is the only indicator of AGN activity. The presence of an AGN is also confirmed by the detection of a compact radio core in deep VLBI 1.4 GHz observations for eight of our radio-excess sources (≈16%; ≈66% of the VLBI detected sources in this field), with the excess radio flux measured from our SED analysis agreeing, to within a factor of two, with the radio core emission measured by VLBI. We find that the fraction of radio-excess AGN increases with X-ray luminosity reaching ∼60% at L X≈1044-1045 ergs-1, making these sources an important part of the total AGN population. However, almost half (24/51) of these radio-excess AGN are not detected in the deep Chandra X-ray data, suggesting that some of these sources might be heavily obscured. Amongst the radio-excess AGN we can distinguish three groups of objects: i) AGN clearly identified in infrared (and often in X-rays), a fraction of which are likely to be distant Compton-thick AGN; ii) moderate luminosity AGN (LX≲ 1043 ergs-1) hosted in strong star-forming galaxies; and iii) a small fraction of low accretion-rate AGN hosted in passive (i.e. weak or no star-forming) galaxies. We also find that the specific star formation rates (sSFRs) of the radio-excess AGN are on average lower that those observed for X-ray selected AGN hosts, indicating that our sources are forming stars more slowly than typical AGN hosts, and possibly their star formation is progressively quenching. © © ESO, 2012.

Stroe A.,Leiden University | Shimwell T.,Leiden University | Rumsey C.,Astrophysics Group | Van Weeren R.,Harvard - Smithsonian Center for Astrophysics | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2016

Radio relics are patches of diffuse synchrotron radio emission that trace shock waves. Relics are thought to form when intracluster medium electrons are accelerated by cluster mergerinduced shock waves through the diffusive shock acceleration mechanism. In this paper, we present observations spanning 150 MHz to 30 GHz of the 'Sausage' and 'Toothbrush' relics from the Giant Metrewave and Westerbork telescopes, the Karl G. Jansky Very Large Array, the Effelsberg telescope, the Arcminute Microkelvin Imager and Combined Array for Research in Millimeter-wave Astronomy.We detect both relics at 30 GHz, where the previous highest frequency detection was at 16 GHz. The integrated radio spectra of both sources clearly steepen above 2 GHz, at the ≳6σ significance level, supporting the spectral steepening previously found in the 'Sausage' and the Abell 2256 relic. Our results challenge the widely adopted simple formation mechanism of radio relics and suggest more complicated models have to be developed that, for example, involve re-acceleration of aged seed electrons. © 2015 The Authors.

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