# Galaxy and Mass Asseembly (GAMA): Defining Passive Galaxy Samples and Searching for the UV Upturn

Steven Phillipps, S.S. Ali, Malcolm 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

Research output: Contribution to journalArticle (Academic Journal)peer-review

1 Citation (Scopus)

## Abstract

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 {\it 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.
Original language English 13 Monthly Notices of the Royal Astronomical Society 19 Dec 2019 https://doi.org/10.1093/mnras/stz3552 E-pub ahead of print - 19 Dec 2019

## Keywords

• galaxies: evolution
• galaxies: stellar content
• galaxies: star formation

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