The first galaxies in the Hubble Frontier Fields
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Abstract
In this thesis we have exploited the power of gravitational lensing of massive clusters to probe galaxy evolution with the galaxy stellar mass functions (GSMF) and UV luminosity functions (UV LF) at z=6β9. Throughout the thesis the data utilized is from the Hubble Frontier Fields (HFF) program.
We present new measurements of the evolution of the GSMF and UV LF for galaxies from z=6β9 within the HFF cluster MACSJ0416.1-2403 and its parallel field. To obtain these results, we have developed a novel method to subtract the massive foreground galaxies that lie close to the critical line from the MACSJ0416.1-2403 cluster, allowing for a deeper and cleaner detection of the faintest systems at zβ₯6.
We derive the stellar masses of our sample by fitting synthetic stellar population models to their observed spectral energy distribution (SED) with the inclusion of nebular emission lines. This is the deepest and most distant mass function measured to date and probes down to a level of Mββ=106.8Mββ. The main result of this study is that the low-mass end of our stellar mass functions to these limits and redshifts are measured to be Ξ±=β1.98β0.07+0.07β at z=6 and Ξ±=β2.38β0.88+0.72β at z=9 and we find no evidence of any turnover in the mass range probed. The faint end slope of the UV LF for these system are also measured to be Ξ±=β2.03β0.10+0.12β at z=6 and Ξ±=β2.20β0.47+0.51β at z=9, without any evidence of a turnover in the luminosity range probed. Our MUVββMββ relation exhibit shallower slopes than previously observed and are in accordance with a constant mass-to-light ratio.
Integrating our GSMF, we find that the stellar mass density increases from log10βΟββ=5.61β0.90+0.92β MββMpcβ3 at z=9 to log10βΟββ=6.79β0.12+0.13β MββMpcβ3 at z=6. We also find that there is a surprisingly high amount of stellar mass density for galaxies in the early universe up to zβΌ9. We estimate the dust-corrected star formation rates (SFRs) to calculate the specific star formation rates (sSFR=SFR/Mββ) of our sample, and find that for a fixed stellar mass of 5Γ109Mββ, sSFR β(1+z)2.01Β±0.16. From our new measurements, we also estimate the UV luminosity density (ΟUVβ) and find that our results support a smooth decline of ΟUVβ towards high redshifts.
Finally, we use the same dataset to investigate the evolution of the galaxy rest-frame UV colours (UV spectral slope Ξ²) for our sample of high redshift galaxies at z=6β9. We measure the UV spectral slope Ξ² by fitting the observed spectral energy distribution to a set of synthetic stellar population models and estimate the value of Ξ² from the best-fit model spectrum. With this method, we find no correlation between Ξ² and rest-frame UV magnitude M1500β at all redshifts probed in this work. However, a possible weak evolution of the median Ξ² values (from Ξ²=β2.24 at zβΌ6 to Ξ²=β2.52 at zβΌ9) for galaxies at all luminosities from z=6β9 is observed, likely due to increased dust extinction.
Furthermore, we find that at z=7, the bluest value of our sample is Ξ²=β2.31Β±0.31, which is redder than previously reported values at this redshift in the literature. Similarly, with the help of our SED fitting method, we determine the UV slopes for the first time at zβΌ9 and find that our bluest data point has a value of Ξ²=β2.63Β±0.21, indicating no evidence as yet for extreme stellar populations at z>6.
Examining the Ξ² to stellar mass relation, we find a strong correlation between Ξ² with stellar mass, in that lower mass galaxies exhibit bluer UV slopes. We also find that low mass galaxies at logM/Mββ9 appear to exhibit a nearly constant Ξ² at each redshift. We also investigate, for the first time, the correlation between Ξ² and SFR and find that there is a strong correlation between Ξ² and SFR, in that galaxies with low SFRs exhibit bluer slopes, and they also appear to get bluer with increasing redshift