The SAURON project - XVIII. The integrated UV-line-strength relations of early-type galaxies

Abstract

Using far‐ultraviolet (FUV) and near‐ultraviolet (NUV) photometry from guest investigator programmes on the Galaxy Evolution Explorer (GALEX) satellite, optical photometry from the MDM Observatory and optical integral‐field spectroscopy from SAURON, we explore the UV–line‐strength relations of the 48 nearby early‐type galaxies in the SAURON sample. Identical apertures are used for all quantities, avoiding aperture mismatch. We show that galaxies with purely old stellar populations show well‐defined correlations of the integrated FUV −V and FUV − NUV colours with the integrated Mg b and Hβ absorption line‐strength indices, strongest for FUV − NUV. Correlations with the NUV −V colour, Fe5015 index and stellar velocity dispersion σ are much weaker. These correlations put stringent constraints on the origin of the UV‐upturn phenomenon in early‐type galaxies and highlight its dependence on age and metallicity. In particular, despite recent debate, we recover the negative correlation between FUV −V colour and Mg line strength originally publicized by Burstein et al., which we refer to as the ‘Burstein relation’, suggesting a positive dependence of the UV upturn on metallicity. We argue that the scatter in the correlations is real and present mild evidence that a strong UV excess is preferentially present in slow‐rotating galaxies. We also demonstrate that most outliers in the correlations are galaxies with current or recent star formation, some at very low levels. We believe that this sensitivity to weak star formation, afforded by the deep and varied data available for the SAURON sample, explains why our results are occasionally at odds with other recent but shallower surveys. This is supported by the analysis of a large, carefully crafted sample of more distant early‐type galaxies from the Sloan Digital Sky Survey (SDSS), more easily comparable with current and future large surveys.

Modelling the UV spectrum of SDSS-III/BOSS galaxies: hints towards the detection of the UV upturn at high-z

“…The correlations of Burstein et al (1988) were weakly recovered by Boselli et al (2005) for early-type members of the Virgo cluster, however it is unclear how these correlations extend to low-luminosity systems. Bureau et al (2011) recover the negative correlation between FUV -V colour and Mg2 line strength using 48 nearby early-type galaxies in the SAURON sample, suggesting a positive dependence of the UV upturn on metallicity.…”

“…We cannot single out the exact reason for this discrepancy, but we can envisage a number of factors contributing to a different result. First, we use a sample of really massive galaxies, whereas locally one has access to much fewer truly massive objects simply due to volume effects and sample selection (Bureau et al (2011)). Then, we use spectral indices whereas local studies are mostly based on broad-band photometry.…”

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS)

“…The FUV − NUV vs. M star relation plotted in Fig. 11, which is sensitive to the presence of young populations in star-forming systems and to very old stars in evolved objects (the UV upturn, O'Connell 1999; Boselli et al 2005) suggests that these most massive galaxies are also those with the oldest stellar populations (Bureau et al 2011). This evidence suggests that the major merging events happened at very early epochs.…”

Far-UV to mid-IR properties of nearby radio galaxies

“…Zirbel & Baum (1995), Rawling & Saunders (1989, Hine & Longair (1979. In this figure we have also plotted SAURON sources (Bureau et al 2011) that have been detected in the radio. Here we also see a clear separation in the properties of high-and low-luminosity sources.…”