Galaxy Evolution and Separation of Early- and Late-Type Galaxies at High Redshift

Supervisor: Dr Michael Cowley

Hubble’s tuning fork diagram classifies galaxies into three broad categories—Lenticulars, Ellipticals, and Spirals—based on their visual appearance. However, at high redshift, determining a clear Hubble classification is more challenging due to the lack of resolved imaging. Instead, we rely on colour-based classifications, with younger, star-forming galaxies appearing bluer and older, quiescent galaxies appearing redder.

This project aims to explore galaxy evolution, focusing on the separation of early- and late-type galaxies at high redshift using stellar mass and colour. Data from the FourStar Galaxy Evolution Survey (ZFOURGE) was used, examining the CDFS, UDS, and COSMOS fields. Flux measurements from the Johnson U and V filters, along with the 2MASS J filter, were used to construct UVJ colour diagrams for standard galaxy selection across different redshift ranges. Subsequent analysis was conducted for galaxies with stellar masses above 10¹⁰ M☉, while considering the impact of low signal-to-noise ratios.

The UVJ diagrams reveal distinct populations of quiescent, star-forming, and dusty galaxies, with their relative proportions changing across redshifts. At higher redshifts (z = 2.25–3.5), star-forming galaxies dominate, accounting for 66%, 65%, and 70% of the galaxies in the CDFS, UDS, and COSMOS fields, respectively, while quiescent galaxies represent only 6%, 7%, and 7%. At lower redshifts (z = 0.5–1.5), there is a noticeable transition toward a greater proportion of quiescent galaxies, increasing to 16%, 15%, and 16% in the same fields, while the fraction of star-forming galaxies decreases to 30%, 33%, and 28%, respectively.

This trend illustrates the evolution of young, actively star-forming galaxies in the early universe into a population increasingly composed of quiescent galaxies in the present day. Additionally, these findings support the homogeneity of the universe, providing evidence for the cosmological principle