Galaxy And Mass Assembly (GAMA): Defining passive galaxy samples and searching for the UV upturn

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 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.

Probing the Stellar Populations and Star Formation History of Early-type Galaxies at 0 < z < 1.1 in the Rest-frame Ultraviolet

“…In general both the cluster and group galaxies show the same trend with regard to the evolution in their UV color, in that there is an excess in the UV emission below z = 0.6, which largely disappears beyond z = 0.8. This suggests, as with previous studies in local galaxy clusters (Ali et al 2019) and groups (Phillipps et al 2020) that the UV upturn-driven by a subpopulation of blue HB stars-is an intrinsic feature of all ETGs, or old stellar populations at large, which develops given sufficient time irrespective of the external environment. This would also explain the observation of blue HB stars in similarly Figure 9.…”

The Ultraviolet Upturn in field Luminous Red Galaxies at $0.3 < z < 0.7$

“…These observations indicate that the UV upturn is a fundamental property of the stellar populations of ETGs across all environments (Ali et al 2019;Phillipps et al 2020) from field to cluster, irrespective of environment at least below 𝑧 = 0.7. Since field LRGs are likely to be less quenched than cluster ETGs it is appropriate to consider the level of residual star formation within them.…”

“…Indeed, the UV upturn and the multiple populations in globular clusters could be manifestations of the same phenomenon albeit in different environments, or the He rich stars may be produced within metal-rich globular clusters as well (Goudfrooij 2018;Chung et al 2020) Each of these explanations imply different predictions as to the evolution and environmental dependence of the UV upturn, and these can be used to discriminate between them. Ali et al (2019) and Phillipps et al (2020) carried out the first comparison of UV upturn properties across a wide range of environments at low redshifts, ranging from clusters to groups and the more isolated field, and they found no evidence of any strong environmental dependence. This suggests that the UV upturn is an intrinsic property of ETGs originating at early times and argues against a component from residual star formation as the UV upturn is independent of local galaxy density, which otherwise affects star formation properties elsewhere, including residual star formation in ETGs - Crossett et al 2014.…”

Measuring the Total Ultraviolet Light from Galaxy Clusters at z=0.5-1.6: The Balance of Obscured and Unobscured Star-Formation

“…At this redshift, the GALEX N U V band corresponds roughly to the rest frame F U V (∼ 1100-1800 Å at z = 0.57). We assume a rest-frame F U V − r color of 5.5 for UV upturn galaxies (Yi et al 2011;Ali et al 2018;Phillipps et al 2020). The colors of UV upturn galaxies are not expected to vary significantly with redshift or environment (Boissier et al 2018).…”