UV SEDs of early-type cluster galaxies: a new look at the UV upturn

Sadman Shariar Ali*, Malcolm Bremer, Steven Phillipps, R. De Propris

*Corresponding author for this work

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

8 Citations (Scopus)
171 Downloads (Pure)

Abstract

Using GALEX, Ultraviolet Optical Telescope (UVOT), and optical photometry, we explore the prevalence and strength of the Ultraviolet (UV) upturn in the spectra 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 3000 Å, we generate for the first time comparatively detailed UV spectral energy distributions for Coma cluster galaxies. Fitting the UV upturn component with a blackbody, 26 of these show a range of characteristic temperatures (10 000-21 000K) for the UV upturn population. Assuming a single temperature to explain GALEX-optical colours could underestimate the fraction of galaxies withUVupturns and mis-classify some as systems with residual star formation. The UV upturn phenomenon is not an exclusive feature found only in giant galaxies; we 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 theUVupturn are blue horizontal branch stars, themost likely mechanism for this is the presence of a substantial (between 4 per cent and 20 per cent) Helium-rich (Y > 0.3) population of stars in these galaxies, potentially formed at z ~ 4 and certainly at z > 2; this plausibly sets a lower limit of ~0.3-0.8 × 1010 M⊙ to the in situ stellar mass of ~L* galaxies at this redshift.

Original languageEnglish
Pages (from-to)1010-1020
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Volume476
Issue number1
Early online date29 Jan 2018
DOIs
Publication statusPublished - 1 May 2018

Keywords

  • Galaxies: clusters: individual: Coma
  • Galaxies: evolution
  • Galaxies: formation
  • Stars: horizontal branch

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