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 ( \sim 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 , M _ { V, 0 } = -7.85 .
This gives an inferred mass M \sim 1.6 \times 10 ^ { 5 } ~ { } M _ { \odot } , assuming a Kroupa IMF .
An integrated spectrum of Sextans A-GC1 reveals a heliocentric radial velocity v _ { helio } = 305 \pm 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 ] \sim –2.4 , and an age \sim 9 Gyr .
We measure a half light radius , R _ { h } = 7.6 \pm 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 ] \sim –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 .