Suchergebnisse

Whether stars could have driven the reionization of the intergalactic medium depends critically on the proportion of ionizing radiation that escapes the galaxies in which it is produced. Spectroscopy of gamma-ray burst (GRB) afterglows can be used to estimate the opacity to extreme ultraviolet (EUV) radiation along the lines-of-sight to the bursts. Assuming that long-duration GRBs trace the locations of the massive stars dominating EUV production, the average escape fraction of ionizing radiation can be calculated independently of galaxy size or luminosity. Here we present a compilation of H i column density (N HI ) measures for 140 GRBs in the range 1.6 < z < 6.7. Although the sample is heterogeneous, in terms of spectral resolution and signal-to-noise ratio, fits to the Ly α absorption line provide robust constraints on N HI , even for spectra of insufficient quality for other purposes. Thus we establish an escape fraction at the Lyman limit of (f esc ) ≈ 0.005, with a 98 per cent confidence upper limit of (f esc ) ≈ 0.015. This analysis suggests that stars provide a small contribution to the ionizing radiation budget at z < 5. At higher redshifts firm conclusions are limited by the small size of the GRB sample (7/140), but any decline in average H i column density seems to be modest. We also find no significant correlation of N HI with galaxy UV luminosity or host stellar mass. We discuss in some detail potential biases and argue that, while not negligible, systematic errors in f esc are unlikely to be more than a factor ~2 in either direction, and so would not affect the primary conclusions. Given that many GRB hosts are low-metallicity dwarf galaxies with high specific star-formation rates, these results present a particular problem for the hypothesis that such galaxies dominated the reionization of the Universe. © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. ; JJ acknowledges support from Nederlandse Onderzoekschool Voor Astronomie(NOVA) and The Netherlands Organisation for Scientific Research (NWO) - The Sao Paulo Research Foundation (FAPESP) grant for advanced instrumentation in astronomy. KEH acknowledges support by a Project Grant (162948-051) from The Icelandic Research Fund. DAK acknowledges support from the Spanish research project AYA2014-58381-P and Juan de la Cierva Incorporacion IJCI-2015-26153. AJL and ERS acknowledge Science and Technology Facilities Council (STFC) consolidated grant ST/L000733/1. NRT and KW acknowledge Science and Technology Facilities Council (STFC) consolidated grant ST/N000757/1. AC acknowledges National Aeronautics and Space Administration (NASA) grant NNX15AP95A. AdUP acknowledges support from a Ramon y Cajal fellowship (RyC-2012-09975), a 2016 Banco Bilbao Vizcaya Argentaria (BBVA) Foundation Grant for Researchers and Cultural Creators, and from the Spanish research project AYA 2014-58381-P. RC acknowledges partial support from National Aeronautics and Space Administration (NASA) Swift grant NNX16AB04G.This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 725246). ; Peer Reviewed

We present Hubble Space Telescope (HST) and Chandra imaging, combined with Very Large Telescope MUSE integral field spectroscopy of the counterpart and host galaxy of the first binary neutron star merger detected via gravitational-wave emission by LIGO and Virgo, GW170817. The host galaxy, NGC 4993, is an S0 galaxy at z - 0.009783. There is evidence for large, face-on spiral shells in continuum imaging, and edge-on spiral features visible in nebular emission lines. This suggests that NGC 4993 has undergone a relatively recent (less than or similar to 1 Gyr) "dry" merger. This merger may provide the fuel for a weak active nucleus seen in Chandra imaging. At the location of the counterpart, HST imaging implies there is no globular or young stellar cluster, with a limit of a few thousand solar masses for any young system. The population in the vicinity is predominantly old with less than or similar to 1% of any light arising from a population with ages <500 Myr. Both the host galaxy properties and those of the transient location are consistent with the distributions seen for short-duration gamma-ray bursts, although the source position lies well within the effective radius (r(e) similar to 3 kpc), providing an r(e)-normalized offset that is closer than similar to 90% of short GRBs. For the long delay time implied by the stellar population, this suggests that the kick velocity was significantly less than the galaxy escape velocity. We do not see any narrow host galaxy interstellar medium features within the counterpart spectrum, implying low extinction, and that the binary may lie in front of the bulk of the host galaxy. ; We thank the referee for a prompt and highly constructive report that improved the content and clarity of the manuscript. We also thank the editor, Fred Rasio, for helpful comments. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 099.D-0668 (A.J.L.), and on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. These observations are associated with programs GO 14771 (N.R.T.), GO 14804 (A.J.L.), and GO 14850 (E.T.). We thank the staff at ESO and STScI for their excellent support of these observations. A.J.L. acknowledges that this project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 725246) A.J.L., D.S., and J.D.L. acknowledge support from STFC via grant ST/P000495/1. N.R.T., K.W., P.T.O., J.L.O., and S.R. acknowledge support from STFC. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). A.d.U.P., C.T., Z.C., and D.A.K. acknowledge support from the Spanish project AYA 2014-58381-P. Z.C. also acknowledges support from the Juan de la Cierva Incorporacion fellowship IJCI-2014-21669, and D.A.K. from Juan de la Cierva Incorporacion fellowship IJCI-2015-26153. M.I. was supported by the NRFK grant, No. 2017R1A3A3001362. E.T. acknowledges support from grants GO718062A and HSTG014850001A. S.R. has been supported by the Swedish Research Council (VR) under grant number 2016-03657_3, by the Swedish National Space Board under grant number Dnr. 107/16 and by the research environment grant "Gravitational Radiation and Electromagnetic Astrophysical Transients (GREAT)" funded by the Swedish Research council (VR) under Dnr 2016-06012. P.A.E. acknowledges UKSA support. ; Peer Reviewed