Globular clusters in the far-ultraviolet: evidence for He-enriched second populations in extra-galactic globular clusters?

Abstract

We investigate the integrated far-ultraviolet (FUV) emission from globular clusters. We present new FUV photometry of M~87's clusters based on archival HST WFPC2 F170W observations. We use these data to test the reliability of published photometry based on HST STIS FUV-MAMA observations, which are now known to suffer from significant red-leak. We generally confirm these previous FUV detections, but suggest they may be somewhat fainter. We compare the FUV emission from bright ($M_V<-9.0$) clusters in the Milky Way, M~31, M~81 and M~87 to each other and to the predictions from stellar populations models. Metal-rich globular clusters show a large spread in FUV-V, with some clusters in M~31, M~81 and M~87 being much bluer than standard predictions. This requires that some metal-rich clusters host a significant population of blue/extreme horizontal branch (HB) stars. These hot HB stars are not traditionally expected in metal-rich environments, but are a natural consequence of multiple populations in clusters - since the enriched population is observed to be He-enhanced and will therefore produce bluer HB stars, even at high metallicity. We conclude that the observed FUV emission from metal-rich clusters in M~31, M~81 and M~87 provides evidence that He-enhanced second populations, similar to those observed directly in the Milky Way, may be a ubiquitous feature of globular clusters in the local universe. Future HST FUV photometry is required to both confirm our interpretation of these archival data and provide constraints on He-enriched second populations of stars in extra-galactic globular clusters.

9 pages, 5 figures, accepted for publication in MNRAS

References in corpus (12)

• The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies
• The fraction of second generation stars in Globular Clusters from the analysis of the Horizontal Branch
• NGC 6441: another indication for a very high helium content in Globular Cluster stars
• The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441: new insights from UV observations
• A model for the Globular Cluster extreme anomalies
• Helium and Multiple Populations in the Massive Globular Cluster NGC6266 (M62)
• Reddening, Colour and Metallicity of the M31 Globular Cluster System
• The Dearth of UV-Bright Stars in M32: Implications for Stellar Evolution Theory
• GALEX Ultraviolet Photometry of Globular Clusters in M31: Three Year Results and a Catalog
• UV Bright Globular Clusters in M87: More Evidence for Super-helium-rich Stellar Populations?
• UV Insights into the Complex Populations of M 87 Globular Clusters
• Probing the Intermediate-Age Globular Clusters in NGC 5128 from Ultraviolet Observations

Cited by in corpus (1)

• Environmental Effects on the UV Upturn in Local Clusters of Galaxies