Y-NBS : surveying star formation from the epoch of reionisation to the local universe

Wade, Heather and Wardlow, Julie and Sobral, David (2024) Y-NBS : surveying star formation from the epoch of reionisation to the local universe. PhD thesis, Lancaster University.

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Abstract

The first galaxies, inside the epoch of reionisation, are very different to those locally (13.7 billion years later) and understanding this evolution is one of the most fundamental areas of research in observational astrophysics. The star formation history of the Universe probes the growth of galaxies and shows that the star formation rate densities (SFRD) of galaxies increase from the early Universe, during the epoch of reionisation, until it peaks at z = 2 − 3, at cosmic noon. After cosmic noon, the star formation rate density of galaxies decreases to the present day. To understand this evolution and its causes requires measuring the evolving SFRD for different types of galaxies. One method to probe the Universe and its star formation history is to select samples of line emitting galaxies in a self-consistent way. In this work, we present a wide-area narrowband survey, Y-NBS, conducted on VLT/HAWK-I with the NB1060 filter in the COSMOS field. We complement this with a deeper archival VLT/HAWK-I pointing covering GOODS-S. We present the full data reduction pipeline for the COSMOS data and then combine it with complementary broadband and redshift data. Using spectroscopic redshifts, photometric redshifts and colour-colour selection, we select samples of Hα, [Oiii] and [Oii] emitting galaxies at z = 0.62, z = 1.12 and z = 1.85, respectively. Luminosity functions of these samples are plotted and Schechter functions are fitted with all three parameters free, which is only possible due to our simultaneously wide survey with deep regions. Our results for the faint-end slope (α) are α = −1.48+0.17−0.17, −1.95+0.18−0.15 and −2.43+0.19−0.16 for Hα, [Oiii] and [Oii] respectively. Our measurements are consistent with previous studies of these luminosity functions at similar redshifts but we probe deeper or over a wider range of luminosities than many other surveys. Star formation rate densities calculated from our luminosity functions are consistent with others, but our [Oii] result is ∼ 0.3 dex higher than expected, likely because we observed a steeper faint-end slope than other surveys. Y-NBS also targets z = 7.7 Lyα emitters, probing into the epoch of reionisation. Although we do not find any, we place robust constraints on the z = 7.7 Lyα luminosity function, which are in agreement with spectroscopically confirmed sources at this redshift. We constrain the neutral fraction of hydrogen to be χHI > 0.4 at z = 7.7, which is consistent with previous studies at z = 7.0 − 7.3. Overall, our work highlights the need for future narrowband surveys to be wide and deep, specifically in the same field, to help overcome cosmic variance and to simultaneously constrain both the faint and bright ends of the luminosities functions. If the deep and shallow regions were in the same field, it would help to rule out the possibility that any observed differences in the density between the deep and shallow regions were the result of cosmic variance. Such work would further our understanding of the star formation history of the Universe from the epoch of reionisation, through cosmic noon, to the local Universe.