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An old, metal-poor globular cluster in Sextans A and the metallicity floor of globular cluster systems

MPS-Authors

Beasley,  Michael A.
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

Leaman,  Ryan
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

Gallart,  Carme
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

Larsen,  Søren S.
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

Battaglia,  Giuseppina
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

Monelli,  Matteo
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

Pedreros,  Mario H.
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

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Citation

Beasley, M. A., Leaman, R., Gallart, C., Larsen, S. S., Battaglia, G., Monelli, M., et al. (2019). An old, metal-poor globular cluster in Sextans A and the metallicity floor of globular cluster systems. Monthly Notices of the Royal Astronomical Society, 487, 1986-1993.


Cite as: http://hdl.handle.net/21.11116/0000-0005-D1DA-9
Abstract
We report the confirmation of an old, metal-poor globular cluster (GC) in the nearby dwarf irregular galaxy Sextans A, the first GC known in this galaxy. The cluster, which we designate as Sextans A-GC1, lies some 4.4 arcmin (~1.8 kpc) to the SW of the galaxy centre and clearly resolves into stars in sub-arcsecond seeing ground-based imaging. We measure an integrated magnitude V = 18.04, corresponding to an absolute magnitude, MV,0 = -7.85. This gives an inferred mass M ~ 1.6 105 M☉, assuming a Kroupa IMF. An integrated spectrum of Sextans A-GC1 reveals a heliocentric radial velocity vhelio = 305 ± 15 km s-1, consistent with the systemic velocity of Sextans A. The location of candidate red giant branch stars in the cluster, and stellar population analyses of the cluster's integrated optical spectrum, suggests a metallicity [Fe/H] ~ -2.4, and an age ~9 Gyr. We measure a half-light radius, Rh = 7.6 ± 0.2 pc. Normalizing to the galaxy integrated magnitude, we obtain a V-band specific frequency, SN = 2.1. We compile a sample of 1928 GCs in 28 galaxies with spectroscopic metallicities and find that the low metallicity of Sextans A-GC1 is close to a `metallicity floor' at [Fe/H] ~ -2.5 seen in these GC systems that include the Milky Way, M31, M87, and the Large Magellanic Cloud. This metallicity floor appears to hold across 6 dex in host galaxy stellar mass and is seen in galaxies with and without accreted GC subpopulations.