Evolution of bright star-forming galaxies in the first billion years
Bowler, Rebecca Alison Andrews
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In this thesis, I present the results of a new search for, and study of, luminous galaxies in the first billion years of cosmic time. This work is primarily based on a new selection of bright (L≫L*) Lyman-break galaxies (LBGs) at z ≅6 and z≅7 in the UltraVISTA first and second data releases (DR1, DR2) and the UKIDSS (UKIRT Infrared Deep Survey) UDS DR10 (Ultra Deep Survey). The UltraVISTA survey provides deep Y, J,H andKs near-imaging over 1.5 deg² of the Cosmic Evolution Survey (COSMOS) field and the UKIDSS UDS provides J,H and K band data overlapping with the Subaru XMM-Newton Deep Survey (SXDS), with both fields also containing deep optical and mid-infrared imaging essential for the clean detection of z > 5 galaxies. The fields combined provide an unprecedented 1.65 deg² of deep multiwavelength data with which to securely select LBGs using a photometric redshift fitting technique, which can additionally remove probable low-redshift galaxy interlopers and galactic dwarf stars that can contaminate ground-based samples. At z ≅7, the DR1 of the UltraVISTA survey was used to select a sample of ten high-redshift galaxy candidates, which extended to a 5σ limiting magnitude of Y + J ~ 25 (AB magnitude, 2-arcsec diameter circular aperture) over 1 deg². A stack of the four most robust objects from the sample indicated that they were massive (M*≅5 × 109M ʘ), had blue rest-frame UV slopes (β ≅−2.0±0.2) and were highly star-forming (SFR ≅25–50Mʘ yr−1) when compared to previous, fainter, samples of galaxies at z = 7. The number counts of z≅7 galaxies selected within the UltraVISTA DR1 survey was higher than that expected from extrapolations of the rest-frame UltraViolet (UV) luminosity function (LF) from fainter data, a result that was strongly confirmed with an improved search for z ≅7 galaxies using the UltraVISTA DR2 imaging and the UDS field. A total of 34 galaxies at 6.5 < z < 7.5 were found in the combined fields, which included the previously identified robust galaxies from the DR1 imaging. This expanded sample allowed the first determination of the rest-frame UV LF in the range −23.0 < MUV < −21.5 at z ≅7, and the results reveal a power-law decline to bright magnitudes in contrast to the commonly assumed exponentially declining Schechter function extrapolated from fainter data. The excess of galaxies observed at bright magnitudes cannot be accounted for by gravitational lensing or by significant contamination of the sample by Active Galactic Nuclei (AGN) . The observed LF is well described by a double power law, which at the bright end follows the form of the underlying dark matter halo mass function, suggesting that the physical mechanism that inhibits star formation activity in massive galaxies (e.g. AGN feedback or some other form of ‘mass quenching’) has yet become efficient at z ≅7. The deeper imaging data confirm that the z ≅7 LBGs show blue rest-frame UV slopes (median β = −2.0) and are massive (up to M*≅1010M ʘ). Furthermore, an analysis of the ground-based imaging shows that the majority are resolved consistent with larger sizes (r1/2 ≅1–1.5 kpc) than displayed by less massive galaxies. Finally, a new search for z ≅6 galaxies within the UltraVISTA and UDS datasets was undertaken, resulting in a sample of 266 LBGs (−22.7 < MUV < −20.5) galaxies with which to investigate the rest-frame UV LF. The potential contamination by galactic brown dwarfs was investigated quantitatively using a simple model of the Galaxy, showing that the expected contamination rate of the sample was < 3 per cent, and that the stars can be effectively removed by fitting standard stellar spectra to the observed photometry. The galaxy surface density in the UltraVISTA/COSMOS field exceeds that in the UDS/SXDS by a factor of ≅1.4, indicating strong cosmic variance between the two fields. The number counts of galaxies we find are a factor of 2 lower than predicted by the recent LF determination by Bouwens et al., and the derived rest-frame UV LF at z ≅6 revealed that an under dense UDS field can account for some of the observed differences between previous analyses. An evolution in the characteristic magnitude between z ≅5 and z≅7 of ∆M*~ 0.5 was found in contrast to other smaller area surveys, and a double power law was shown to equally well describe the LF at z = 6 as compared to the commonly assumed Schechter function. The bright-end of the LF at z ≅6 tentatively shows a steeper decline than found at z ≅7, which could indicate the onset of mass quenching of the most massive galaxies or the rise of dust obscuration. Comparison with the predictions of the latest theoretical models and simulations of galaxies reveals that most models require substantial (A1500 ~ 1.5–2) average dust extinction at the bright end to reproduce the shape of the galaxy UV LF at z ≅7.