The nature and evolution of Lyman-α emitters over large co-moving volumes

Da Graça Santos, Sergio and Sobral, David (2021) The nature and evolution of Lyman-α emitters over large co-moving volumes. PhD thesis, Lancaster University.

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Abstract

How do young star-forming galaxies evolve with cosmic time? Extensive work has been done over the past years on the discovery and follow-up of numerous galaxies at the highest redshifts (look-back times). However, most surveys aim for deep observations in small volumes which often are not enough to detect the brightest (but also rarer) sources. Comparing multiple studies/samples in an unbiased way can also be challenging due to different selection methods. A large volume program, which probes multiple look-back times using the same method, would greatly contribute to our understanding of how galaxies evolve in the distant Universe. In this thesis, we use 12(+4) medium(+narrow) band images to identify ~4000 z~2-6 Lyman-α emitters (LAEs) over 2 deg2 in the COSMOS field, producing the SC4K (Slicing COSMOS with 4k LAEs) sample. Lyman-α (Lyα) emission is typically associated with young star-forming galaxies (but also active galactic nuclei, AGNs). We use these ~4000 LAEs to produce a 3D map of the early Universe, in 16 individual redshift slices, providing a unique sample to explore galaxy evolution, with all sources being selected using the same self-consistent selection method. We construct Lyα luminosity functions (LFs) with our LAEs selected over a very wide co-moving volume (108 Mpc3), complementing ultra-deep surveys. The Schechter component of the Lyα LF reveals a ~5x rise in L*Lyα and a ~7x decline in Φ*Lyα from z~2 to 6. At z~2-3 we find an excess in number densities at high luminosities LLyα>1043.3 erg s-1 which is consistent with a higher AGN fraction at those luminosities. This excess is not detected or falls below our detection limits at z>4. We measure a Lyα luminosity density increase by a factor of ~2 from z~2 to 3, which then remains constant to z~6, which contrasts the ultraviolet (UV) luminosity density decrease at the same redshift ranges. The Lyα/UV luminosity density ratio rises from 4% to 30% from z~2.2 to 6. We conduct aperture photometry for individual SC4K LAEs, using 34 bands of deep multi-wavelength data in the COSMOS field from rest-frame UV to far-infrared (FIR), to measure their individual spectral energy distributions (SEDs). We find typical stellar masses 109.3±0.6 M⊙ and star formation rates (SFR) of SFRSED=4.4+10.5-2.4 M⊙ yr-1 and SFRLyα=5.9+6.3-2.6 M⊙ yr-1, combined with very blue UV slopes of β=-2.1+0.5-0.4, but with significant variations within the population. Overall, we measure little to no evolution of the Lyα EW0 and scale length parameter (w0) which are consistently high (EW0=140+280-70 Å, w0=129+11-11 Å) from z~6 to z~2, although w0 is anti-correlated with rest-frame UV luminosity (MUV) and stellar mass. Our results imply that sources selected as LAEs have a high Lyα escape fraction (fesc, Lyα) irrespective of cosmic time, but fesc, Lyα is still higher for UV-fainter and lower mass LAEs. We also find that the least massive LAEs are typically above the star formation "Main Sequence" and thus undergoing intense star formation, which could be explained by a bursty nature. Furthermore, we measure the evolution from z~2 to z~6 of the rest-frame UV luminosity function (LF) and the stellar mass function (SMF) of the SC4K sample. We explore a range of 6 dex in MUV and 5 dex in M*, which is unprecedented for such a large sample of LAEs, covering such a redshift range. For both the LFs and SMFs, we find that the Lyα luminosity limit significantly affects the shape and Schechter parameters of the distributions. As such, to probe for evolution in an unbiased way, we study a subset of the SC4K sample, with LLyα>1043.0 erg s-1, which is a luminosity regime probed at all redshift ranges. For the UV LF of these LAEs, we find a characteristic number density (Φ*) increase from log10(Φ*/Mpc-3)~-5.2 at z=2.5 to ~-4.6 at z~3, remaining constant up to z~5-6 and a characteristic UV luminosity (MUV*) brightening from -21.1 at z~3 to -22.0 at z~5-6. We find no significant evolution of the SMF of these LAEs with redshift, with log10(Φ*/Mpc-3) staying constant at ~-5.5 from z~2.5 to z~6 and the characteristic stellar mass staying constant at log(M**/M⊙)~10.7 for the same redshift range. We measure that the UV luminosity density (ρUV) changes from 1024.2 to 1025.0 erg s-1 Hz-1 Mpc-3 and the stellar mass density (ρM) remains constant at ~105.5 M⊙ Mpc-3, with both always being smaller than literature measurements from continuum-selected galaxies. Both ρUV and ρM of LAEs converge to the measurements of continuum-selected galaxies at z>6, which suggests a key role of LAEs in the epoch of reionisation. Overall, our results show that LAEs are a unique subset of the star-forming population, and that as we move to higher redshifts, LAEs become more and more representative of the full population of galaxies. The SC4K sample is made fully public, together with derived physical properties, so the community can fully benefit from this work.