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The SLUGGS survey: globular cluster stellar population trends from weak absorption lines in stacked spectra
Monthly Notices of the Royal Astronomical Society (2015)
  • Christopher Usher, Swinburne University of Technology
  • Duncan A. Forbes, Swinburne University of Technology
  • Jean P. Brodie, University of California Observatories
  • Aaron J. Romanowsky, San Jose State University
  • Jay Strader, Michigan State University
  • Charlie Conroy, University of California Observatories
  • Caroline Foster, Australian Astronomical Observatory
  • Nicola Pastorello, Swinburne University of Technology
  • Vincenzo Pota, Swinburne University of Technology
  • Jacob A. Arnold, University of California Observatories
As part of the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, we stack 1137 Keck DEIMOS (Deep Imaging Multi-Object Spectrograph) spectra of globular clusters from 10 galaxies to study their stellar populations in detail. The stacked spectra have median signal-to-noise ratios of ∼90 Å−1. Besides the calcium triplet, we study weaker sodium, magnesium, titanium and iron lines as well as the Hα and higher order Paschen hydrogen lines. In general, the stacked spectra are consistent with old ages and a Milky Way-like initial mass function. However, we see different metal line index strengths at fixed colour and magnitude, and differences in the calcium triplet–colour relation from galaxy to galaxy. We interpret this as strong evidence for variations in the globular cluster colour–metallicity relation between galaxies. Two possible explanations for the colour–metallicity relation variations are that the average ages of globular clusters vary from galaxy to galaxy or that the average abundances of light elements (i.e. He, C, N and O) differ between galaxies. Stacking spectra by magnitude, we see that the colours become redder and metal line indices stronger with brighter magnitudes. These trends are consistent with the previously reported ‘blue tilts’ being mass–metallicity relations.
  • globular clusters: general,
  • galaxies: abundances,
  • galaxies: star clusters: general,
  • galaxies: stellar content
Publication Date
January, 2015
Publisher Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
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Citation Information
Christopher Usher, Duncan A. Forbes, Jean P. Brodie, Aaron J. Romanowsky, et al.. "The SLUGGS survey: globular cluster stellar population trends from weak absorption lines in stacked spectra" Monthly Notices of the Royal Astronomical Society Vol. 446 Iss. 1 (2015) p. 369 - 390 ISSN: 1365-2966
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