Stellar populations of the first galaxies

Abstract

The stellar populations harboured by some of the Universe’s earliest galaxies are within observational reach. Determining the details of these stellar populations and their formation histories within the first billion years after the Big Bang is crucial for both understanding the earliest stages of galaxy evolution and for assessing the contribution of early star-forming galaxies to cosmic reionization. This thesis presents observational measurements of the rest-frame UV and optical colours of star-forming Lyman Break galaxies (LBGs) at redshifts 4 < z < 9, and their inferred stellar population parameters. By combining ground-based ~1 deg² surveys with deeper, narrower space-based deep-field surveys, we have constrained the rest-frame UV spectral slope of galaxies over a wide-range of cosmic time (4 < z < 9) and luminosity (−23 < MUV < −17) in a self-consistent way. To do so, we developed simulations to allow the inference of intrinsic colours from noisy, potentially biased observations. With these simulations, a robust UV colour measurement method was devised in preparation for the Hubble Ultra Deep Field 2012 (UDF12) survey. Then, after delivery of the UDF12 data, our technique and simulations were applied to yield the first bias-free measurements of the UV spectral slope of galaxies at z ≈ 7 and 8. We found no support for the previously claimed dominant sub-population of exotically blue, faint galaxies at z ≈ 7. In fact with careful consideration of their errors and selection biases, even the most extreme galaxies we observed can have their colours explained by stellar population synthesis models of unremarkable parameters. Expanding this study to brighter, rarer, galaxies required the inclusion of wide-area ground-based survey data, and consequently a more focused examination of galaxies at z ≈ 5. We selected high signal-to-noise galaxies from four fields, with absolute magnitudes spanning MUV = −22.5 to −17.5, and measured their rest-frame UV spectral slopes. Coupling these measurements with our simulated observations, we were able to determine the width of the intrinsic colour distribution of galaxies at z ≈ 5. We found that brighter galaxies are not only on average redder than their fainter counterparts, but they are also less self-similar in their colours. The redder average UV colours of brighter galaxies can be attributed to those galaxies being either older, or more dust reddened. By pairing these measurements, which are primarily a probe only of the presently forming portion of the stellar population, with those of LBG’s Balmer Breaks, which are more sensitive to bygone star formation, we were able to break this age–dust degeneracy and conclude that, at z ≈ 5, brighter galaxies are more heavily reddened than fainter galaxies even though their stars are no older.