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

Abstract

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 groundbased imaging. We measure an integrated magnitude V = 18.04, corresponding to an absolute magnitude, M V,0 = −7.85. This gives an inferred mass M ∼1.6×10 5 M , assuming a Kroupa IMF. An integrated spectrum of Sextans A-GC1 reveals a heliocentric radial velocity v helio = 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, R h = 7.6 ± 0.2 pc. Normalising to the galaxy integrated magnitude, we obtain a V-band specific frequency, S N = 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.

The accreted nuclear clusters of the Milky Way

“…However, no such population is evident in the study of Husser et al (2020), despite the larger sample size of their study. If confirmed, this population would be near the observed floor in GC metallicity in the Milky Way and other Local Group galaxies (Beasley et al 2019;Kruijssen 2019).…”

The tidal remnant of an unusually metal-poor globular cluster

“…The tidal remnant of an unusually metal-poor globular cluster Zhen Wan 1 , Geraint F. Lewis 1 ✉ , Ting S. Li 2,3,4,5 , Jeffrey D. Simpson 6 , Sarah L. Martell 6,7 , Daniel B. Zucker 8,9 , Jeremy R. Mould 10 , Denis Erkal 11 , Andrew B. Pace 12 , Dougal Mackey 13 , Alexander P. Ji 2 , Sergey E. Koposov 12,14 , Kyler Kuehn 15,16 , Nora Shipp 4,5,17 , Eduardo Balbinot 18 , Joss Bland-Hawthorn 1,7 , Andrew R. Casey 19 , Gary S. Da Costa 13 , Prajwal Kafle 20 , Sanjib Sharma 1,7 & Gayandhi M. De Silva 7,16 Globular clusters are some of the oldest bound stellar structures observed in the Universe 1 .…”

“…They are ubiquitous in large galaxies and are believed to trace intense star-formation events and the hierarchical build-up of structure 2,3 . Observations of globular clusters in the Milky Way, and a wide variety of other galaxies, have found evidence for a 'metallicity floor', whereby no globular clusters are found with chemical (metal) abundances below approximately 0.3 to 0.4 per cent of that of the Sun [4][5][6] . The existence of this metallicity floor may reflect a minimum mass and a maximum redshift for surviving globular clusters to form-both critical components for understanding the build-up of mass in the Universe 7 .…”

Discovery of new globular clusters in the Sagittarius dwarf galaxy

“…It is very important in this context to have a well characterized sample of the GC systems of local dwarf galaxies that could in principle be used as comparisons. There is, however, evidence that the GC systems of the nearby dwarf galaxies may still be incomplete, as shown by the recent discoveries of faint additional GC members of the Fornax, Eridanus II, and Sextans A dwarf spheroidal galaxies (Crnojevic et al 2016;Wang et al 2019;Beasley et al 2019). It is also possible that a large fraction of Local Group dwarf galaxies may have hosted GCs, now in the form of disrupted remnants (Fernández-Trincado et al 2020a).…”