Galaxy And Mass Assembly (GAMA): Defining passive galaxy samples and searching for the UV upturn
: S Phillipps, S S Ali, M N Bremer, R De Propris, A E Sansom, M E Cluver, M Alpaslan, S Brough, M J I Brown, L J M Davies, S P Driver, M W Grootes, B W Holwerda, A M Hopkins, P A James, K Pimbblet, A S G Robotham, E N Taylor, L Wang
Publisher: OXFORD UNIV PRESS
: 2020
Monthly Notices of the Royal Astronomical Society
: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
: MON NOT R ASTRON SOC
: 492
: 2
: 2128
: 2139
: 12
: 0035-8711
: 1365-2966
DOI: https://doi.org/10.1093/mnras/stz3552
: https://research.utu.fi/converis/portal/detail/Publication/46084625
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.
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