Bhatawdekar, Rachana
(2019)
The first galaxies in the Hubble Frontier Fields.
PhD thesis, University of Nottingham.
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 $z=6-9$. 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 $z=6-9$ 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 $z\geq6$.
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 M$_{*} = 10^{6.8}M_{\odot}$. 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 $\alpha=-1.98_{-0.07}^{+0.07}$ at $z=6$ and $\alpha=-2.38_{-0.88}^{+0.72}$ 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 $\alpha=-2.03_{-0.10}^{+0.12}$ at $z=6$ and $\alpha=-2.20_{-0.47}^{+0.51}$ at $z=9$, without any evidence of a turnover in the luminosity range probed. Our $M_{\mathrm{UV}}-M_{*}$ 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 log$_{10}\rho_{*}=5.61_{-0.90}^{+0.92}$ M$_{\odot}$Mpc$^{-3}$ at $z=9$ to log$_{10}\rho_{*}=6.79_{-0.12}^{+0.13}$ M$_{\odot}$Mpc$^{-3}$ 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 $z \sim 9$. We estimate the dust-corrected star formation rates (SFRs) to calculate the specific star formation rates ($\mathrm{sSFR}=\mathrm{SFR/M_{*}}$) of our sample, and find that for a fixed stellar mass of $5\times10^{9}M_{\odot}$, sSFR $\propto(1+z)^{2.01\pm0.16}$. From our new measurements, we also estimate the UV luminosity density ($\rho_{\textrm{UV}}$) and find that our results support a smooth decline of $\rho_{\textrm{UV}}$ towards high redshifts.
Finally, we use the same dataset to investigate the evolution of the galaxy rest-frame UV colours (UV spectral slope $\beta$) for our sample of high redshift galaxies at $z=6-9$. We measure the UV spectral slope $\beta$ by fitting the observed spectral energy distribution to a set of synthetic stellar population models and estimate the value of $\beta$ from the best-fit model spectrum. With this method, we find no correlation between $\beta$ and rest-frame UV magnitude $M_{1500}$ at all redshifts probed in this work. However, a possible weak evolution of the median $\beta$ values (from $\beta=-2.24$ at $z\sim6$ to $\beta=-2.52$ at $z\sim9$) for galaxies at all luminosities from $z=6-9$ is observed, likely due to increased dust extinction.
Furthermore, we find that at $z=7$, the bluest value of our sample is $\beta=-2.31\pm0.31$, 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 $z\sim9$ and find that our bluest data point has a value of $\beta=-2.63\pm0.21$, indicating no evidence as yet for extreme stellar populations at $z>6$.
Examining the $\beta$ to stellar mass relation, we find a strong correlation between $\beta$ with stellar mass, in that lower mass galaxies exhibit bluer UV slopes. We also find that low mass galaxies at $\log M/M_{\odot}<9$ appear to become bluer with increasing redshift, whereas the massive galaxies at $\log M/M_{\odot}>9$ appear to exhibit a nearly constant $\beta$ at each redshift. We also investigate, for the first time, the correlation between $\beta$ and SFR and find that there is a strong correlation between $\beta$ and SFR, in that galaxies with low SFRs exhibit bluer slopes, and they also appear to get bluer with increasing redshift.
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