2018MNRAS.480.2236A

We have analysed the strength of the ultraviolet (UV) upturn in red sequence galaxies with luminosities reaching to below the L* point within four clusters at z = 0.3, 0.55, and 0.7. We 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 ∼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 sub-population 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 our results with the prediction made by the YEPS Helium-enhanced spectrophotometic models, we 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 Y >= 0.45, if formed at z_f ∼ 4. 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.