An old, metal-poor globular cluster in Sextans A and the metallicity floor of globular cluster systems. (arXiv:1904.01084v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Beasley_M/0/1/0/all/0/1">Michael A. Beasley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leaman_R/0/1/0/all/0/1">Ryan Leaman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gallart_C/0/1/0/all/0/1">Carme Gallart</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Larsen_S/0/1/0/all/0/1">Soeren Larsen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Battaglia_G/0/1/0/all/0/1">Giuseppina Battaglia</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Monelli_M/0/1/0/all/0/1">Matteo Monelli</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pedreros_M/0/1/0/all/0/1">Mario H. Pedreros</a>
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 ($sim1.8$ 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 $Msim$1.6$times10^5$ solar
masses, assuming a Kroupa IMF. An integrated spectrum of Sextans A-GC1 reveals
a heliocentric radial velocity $v_{rm helio}=305pm15$ km/s, consistent with
the systemic velocity of Sextans A. Stellar population analyses from the upper
giant branch of the cluster, and its integrated optical spectrum, suggests a
metallicity [Fe/H] $sim$-2.4, and an age $sim9$ Gyr. We measure a half light
radius, $R_h = 7.6pm0.2$ 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 out accreted GC subpopulations.
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 ($sim1.8$ 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 $Msim$1.6$times10^5$ solar
masses, assuming a Kroupa IMF. An integrated spectrum of Sextans A-GC1 reveals
a heliocentric radial velocity $v_{rm helio}=305pm15$ km/s, consistent with
the systemic velocity of Sextans A. Stellar population analyses from the upper
giant branch of the cluster, and its integrated optical spectrum, suggests a
metallicity [Fe/H] $sim$-2.4, and an age $sim9$ Gyr. We measure a half light
radius, $R_h = 7.6pm0.2$ 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 out accreted GC subpopulations.
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