Physics and Astronomy
We use data from the GAMA and GALEX surveys to demonstrate that the UV upturn, an unexpected excess of ultraviolet flux from a hot stellar component, seen in the spectra of many early-type galaxies, arises from processes internal to individual galaxies with no measurable influence from the galaxies' larger environment. We first define a clean sample of passive galaxies without a significant contribution to their UV flux from low-level star formation. We confirm that galaxies with the optical colours of red sequence galaxies often have signs of residual star formation, which, without other information, would prevent a convincing demonstration of the presence of UV upturns. However, by including (NUV−u) and WISE (W2-W3) colours, and FUV data where it exists, we can convincingly constrain samples to be composed of non-star-forming objects. Using such a sample, we examine GALEX photometry of low-redshift GAMA galaxies in a range of low-density environments, from groups to the general field, searching for UV upturns. We find a wide range of (NUV−r) colours, entirely consistent with the range seen - and attributed to the UV upturn - in low-redshift red sequence cluster galaxies. The range of colours is independent of group multiplicity or velocity dispersion, with isolated passive galaxies just as likely to have blue UV-to-optical colours, implying significant upturn components, as those in richer groups and in the previous data on clusters. This is supported by equivalent results for (FUV−r) colours which are clear indicators of upturn components.
Phillipps, S.; Ali, S. S.; Bremer, M. N.; de Propris, R.; Sansom, A. E.; Cluver, M. E.; Alpaslan, M.; Brough, S.; Brown, M. J.I.; Davies, L. J.M.; Driver, S. P.; Grootes, M. W.; Holwerda, Benne W.; Hopkins, A. M.; James, P. A.; Pimbblet, K.; Robotham, A. S.G.; Taylor, E. N.; and Wang, L., "Galaxy and Mass Assembly (GAMA): Defining passive galaxy samples and searching for the UV upturn" (2020). Faculty Scholarship. 479.