STAR FORMATION IN THE EARLY UNIVERSE: BEYOND THE TIP OF THE ICEBERG
- Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH (United Kingdom)
- Department of Physics, University of Warwick, Coventry CV4 7AL (United Kingdom)
- Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
- Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen O (Denmark)
- HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL (United Kingdom)
- School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287 (United States)
- Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavik (Iceland)
- Astrophysics Research Institute, Liverpool John Moores University, Liverpool (United Kingdom)
- Department of Astronomy, Yale University, New Haven, CT 06511-208101 (United States)
- Max-Planck-Institut fuer extraterrestrische Physik, Garching bei Muenchen (Germany)
- Instituto de Astrofsica de Andalucia (IAA-CSIC), Glorieta de la Astronomia s/n, 18.008 Granada (Spain)
We present late-time Hubble Space Telescope (HST) imaging of the fields of six Swift gamma-ray bursts (GRBs) lying at 5.0 {approx}< z {approx}< 9.5. Our data include very deep observations of the field of the most distant spectroscopically confirmed burst, GRB 090423, at z = 8.2. Using the precise positions afforded by their afterglows, we can place stringent limits on the luminosities of their host galaxies. In one case, that of GRB 060522 at z 5.11, there is a marginal excess of flux close to the GRB position which may be a detection of a host at a magnitude J{sub AB} Almost-Equal-To 28.5. None of the others are significantly detected, meaning that all the hosts lie below L* at their respective redshifts, with star formation rates (SFRs) {approx}< 4 M{sub Sun} yr{sup -1} in all cases. Indeed, stacking the five fields with WFC3-IR data, we conclude a mean SFR <0.17 M{sub Sun} yr{sup -1} per galaxy. These results support the proposition that the bulk of star formation, and hence integrated UV luminosity, at high redshifts arises in galaxies below the detection limits of deep-field observations. Making the reasonable assumption that GRB rate is proportional to UV luminosity at early times allows us to compare our limits with expectations based on galaxy luminosity functions (LFs) derived from the Hubble Ultra-Deep Field and other deep fields. We infer that an LF, which is evolving rapidly toward steeper faint-end slope ({alpha}) and decreasing characteristic luminosity (L*), as suggested by some other studies, is consistent with our observations, whereas a non-evolving LF shape is ruled out at {approx}> 90% confidence. Although it is not yet possible to make stronger statements, in the future, with larger samples and a fuller understanding of the conditions required for GRB production, studies like this hold great potential for probing the nature of star formation, the shape of the galaxy LF, and the supply of ionizing photons in the early universe.
- OSTI ID:
- 22039291
- Journal Information:
- Astrophysical Journal, Vol. 754, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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