Andrew Bunker

Abstract:

We present the discovery of an absorption-line redshift of z = 2.609 for GRB 090426, establishing the first firm lower limit to a redshift for a gamma-ray burst (GRB) with an observed duration of <2 s. With a rest-frame burst duration of T90z = 0.35 s and a detailed examination of the peak energy of the event, we suggest that this is likely (at >90 per cent confidence) a member of the short/hard phenomenological class of GRBs. From analysis of the optical-afterglow spectrum we find that the burst originated along a very low H i column density sightline, with NH i < 3.2 × 1019 cm-2. Our GRB 090426 afterglow spectrum also appears to have weaker low-ionization absorption (Si ii, C ii) than ∼95 per cent of previous afterglow spectra. Finally, we also report the discovery of a blue, very luminous, star-forming putative host galaxy (∼2L *) at a small angular offset from the location of the optical afterglow. We consider the implications of this unique GRB in the context of burst duration classification and our understanding of GRB progenitor scenarios. © 2009 RAS.

Abstract:

We present the first results of a new Keck spectroscopic survey of UV faint Lyman break galaxies in the redshift range 3 < z < 7. Combined with earlier Keck and published European Southern Observatory (ESO) VLT data, our spectroscopic sample contains more than 600 dropouts offering new insight into the nature of sub-L* sources typical of those likely to dominate the cosmic reionization process. In this first paper, in a series discussing these observations, we characterize the fraction of strong Lyα emitters within the continuum-selected dropout population. By quantifying how the 'Lyα fraction', xLyα, varies with redshift, we seek to constrain changes in Lyα transmission associated with reionization. In order to distinguish the effects of reionization from other factors which affect the Lyα fraction [e.g. dust, interstellar medium (ISM) kinematics], we study the luminosity and redshift-dependence of the Lyα fraction over 3 ≲z≲ 6, when the intergalactic medium (IGM) is known to be ionized. These results reveal that low-luminosity galaxies show strong Lyα emission much more frequently (xLyα= 0.47 ± 0.16 at MUV=-19) than luminous systems (xLyα= 0.08 ± 0.02 at MUV=-21), and that at fixed luminosity, the prevalence of strong Lyman α emission increases moderately with redshift over 3 < z < 6 (d xLyα/d z= 0.05 ± 0.03). Based on the bluer mean UV slopes of the strong Lyα emitting galaxies in our data set (〈Β〉Lyα-〈Β〉noLyα=-0.33 ± 0.09 at MUV=-20.5) we argue that the Lyα fraction trends are governed by redshift and luminosity-dependent variations in the dust obscuration, with likely additional contributions from trends in the kinematics and covering fraction of neutral hydrogen. Using the limited infrared spectroscopy of candidate z{reversed tilde} 7 galaxies, we find a tentative decrease in the Lyα fraction by a factor of >1.9 with respect to the predicted z{reversed tilde} 7 value, a result which, if confirmed with future surveys, would suggest an increase in the neutral fraction by this epoch. Given the abundant supply of z and Y drops now available from deep Hubble WFC3/IR surveys, we show it will soon be possible to significantly improve estimates of the Lyα fraction using optical and near-infrared multi-object spectrographs, thereby extending the study conducted in this paper to 7 ≲z≲ 8. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

We have searched for star-forming galaxies at z- 7-10 by applying the Lyman-break technique to newly released Y-, J- and H-band images (1.1, 1.25 and 1.6 μm) from Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. By comparing these images of the Hubble Ultra Deep Field with the Advanced Camera for Surveys (ACS) z'-band (0.85 μm) images, we identify objects with red colours, (z'-Y)AB > 1.3, consistent with the Lyman α forest absorption at z≈ 6.7-8.8. We identify 12 of these z'-drops down to a limiting magnitude YAB < 28.5 (equivalent to a star formation rate of 1.3-M--yr-1 at z= 7.1), all of which are undetected in the other ACS filters. We use the WFC3 J-band image to eliminate contaminant low-mass Galactic stars, which typically have redder colours than z≈ 7 galaxies. One of our z'-drops is probably a T-dwarf star. The z≈ 7-z'-drops appear to have much bluer spectral slopes than Lyman-break galaxies at lower redshift. Our brightest z'-drop is not present in the NICMOS J-band image of the same field taken 5 years before, and is a possible transient object. From the 10 remaining z≈ 7 candidates we determine a lower limit on the star formation rate density of 0.0017-M--yr-1-Mpc-3 for a Salpeter initial mass function, which rises to 0.0025-0.004-M--yr-1-Mpc-3 after correction for luminosity bias. The star formation rate density is a factor of ≈10 less than that of Lyman-break galaxies at z= 3-4, and is about half the value at z≈ 6. We also present the discovery of seven Y-drop objects with (Y-J)AB > 1.0 and JAB < 28.5 which are candidate star-forming galaxies at higher redshifts (z≈ 8-9). We find no robust J-drop candidates at z≈ 10. While based on a single deep field, our results suggest that this star formation rate density would produce insufficient Lyman continuum photons to reionize the Universe unless the escape fraction of these photons is extremely high (fesc > 0.5), and the clumping factor of the Universe is low. Even then, we need to invoke a large contribution from galaxies below our detection limit (a steep faint-end slope). The apparent shortfall in ionizing photons might be alleviated if stellar populations at high redshift are of low metallicity or have a top-heavy initial mass function. © 2010 The Authors. Journal compilation © 2010 RAS.