2019/04/30 by Michael A. Beasley, Ryan Leaman, Carme Gallart +4 · 1 citation
Physics and Astronomy · #astro-ph.GA
paper · pdf · doi:10.1093/mnras/stz1349
8 pages, 7 figures, 2 tables, accepted for publication in MNRAS after minor revision
arxiv created 2019/05/23 · arxiv updated 2019/05/24
We report the confirmation of an old, metal-poor globular cluster in the nearby dwarf irregular galaxy Sextans A, the first globular cluster known in this galaxy. The cluster, which we designate as Sextans A-GC1, lies some 4.4 arcminutes (∼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\odot, assuming a Kroupa IMF. An integrated spectrum of Sextans A-GC1 reveals a heliocentric radial velocity v\rm helio=305±15~ km/s, 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. Normalising to the galaxy integrated magnitude, we obtain a V-band specific frequency, SN=2.1. We compile a sample of 1,928 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 globular cluster systems which 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.