Evolution of the UV Upturn in Early-type Cluster Galaxies

Abstract

The UV upturn is a rise in the spectra of early-type galaxies below 2500AA, down to the Lyman limit. The objective of this thesis is to analyse the stellar population giving rise to this phenomenon and to understand how it evolves with redshift in cluster early-type galaxies. In chapter 2, using GALEX, UVOT and optical photometry, I explore the prevalence and strength of the UV upturn in the SEDs of quiescent early-type galaxies in several nearby clusters. Even for galaxies with completely passive optical colours, there is a large spread in vacuum UV colour consistent with almost all having some UV upturn component. Combining GALEX and UVOT data below 3000AA, I generate for the first time comparatively detailed UV SEDs for Coma cluster galaxies. Fitting the UV upturn component with a blackbody, twenty six of these show a range of characteristic temperatures (10000--21000K) for the UV upturn population. Assuming a single temperature to explain GALEX-optical colours could underestimate the fraction of galaxies with UV upturns and mis-classify some as systems with residual star formation. The UV upturn phenomenon is not an exclusive feature found only in giant galaxies; I identify galaxies with similar (or even bluer) $FUV-V$ colours to the giants with upturns over a range of fainter luminosities. The temperature and strength of the UV upturn are correlated with galaxy mass. Under the plausible hypothesis that the sources of the UV upturn are blue horizontal branch stars, the most likely mechanism for this is the presence of a substantial (between 4% and 20%) Helium rich ($Ygeqslant 0.38$) population of stars in these galaxies, potentially formed at $zsim 4$ and certainly at $z>2$. This plausibly sets a lower limit of $sim {rm 0.3 - 0.8} times 10^{10}$ $M_{odot}$ to the {it in situ} stellar mass of $sim L^*$ galaxies at this redshift. In chapter 3, I test the effect of the cluster environment on the UV upturn of member galaxies. To do so, I measure the $FUV-r$ and $NUV-r$ colours of early-type galaxies in 20 2dF clusters of a wide variety of size and richness. These colours are then plotted against the velocity dispersion and X-ray luminosity of the host cluster, as well as the clustercentric distances and line-of-sight velocities of the galaxies, all parameters that are defined by the cluster environment. In all cases, no clear correlation is found, indicating that the strength of the upturn is unaffected by the cluster environment. Furthermore, the $FUV-r$ and $NUV-r$ colours of the 2dF clusters are consistent with those of Coma and Perseus, which suggests that the UV upturn is a ubiquitous feature in all cluster early-type galaxies at low redshift. In chapter 4, I measure the strength of the UV upturn for red sequence galaxies in the Abell~1689 cluster at $z=0.18$, reaching to or below the $L^*$ level and therefore probing the general evolution of the upturn phenomenon. The range of UV upturn strengths in the population as a whole has not declined over the past 2.2 Gyrs. This is consistent with a model where hot horizontal branch stars, produced by a Helium-enriched population, provide the required UV flux. Based on local counterparts, this interpretation of the result implies Helium abundances of at least 1.5 times the primordial value for this HB population, along with high formation and assembly redshifts for the galaxies and at least a subset of their stellar populations. In chapter 5, I analyse the strength of the UV upturn in red sequence galaxies with luminosities reaching to below the $L^*$ point within four clusters at $z$ = 0.3, 0.55 & 0.7. I find that the incidence and strength of the upturn remains constant up to $z=0.55$. In comparison, the prevalence and strength of the UV upturn is significantly diminished in the $z=0.7$ cluster, implying that the stellar population responsible for the upturn in a typical red sequence galaxy is only just developing at this redshift and is essentially fully-developed by $sim 1$ Gyr later. Of all the mainstream models that seek to explain the UV upturn phenomenon, it is those that generate the upturn through the presence of a Helium-enhanced stellar subpopulation on the (hot) horizontal branch that are most consistent with this behaviour. The epoch ($z=0.7$) where the stars responsible for the upturn first evolve from the red giant branch places constraints on their age and chemical abundances. By comparing the results with the prediction made by the YEPS Helium-enhanced spectrophotometic models, I find that a solar metallicity sub-population that displays a consistent upturn between $0<z<0.55$ but then fades by $z=0.7$ would require a Helium abundance of $Ygeqslant0.45$, if formed at $z_fsim4$. Later formation redshifts and/or higher metallicity would further increase the Helium enhancement required to produce the observed upturn in these clusters and vice versa.

Date of Award	23 Jan 2019
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Malcolm N Bremer (Supervisor) & Steven Phillipps (Supervisor)