Dwarf galaxies in the MATLAS survey: Hubble Space Telescope observations
  of the globular cluster system in the ultra-diffuse galaxy MATLAS-2019
  Ultra-diffuse galaxies (UDGs) are very low-surface brightness galaxies with
large effective radii. Spectroscopic measurements of a few UDGs have revealed a
low dark matter content, based on the internal motion of stars or globular
clusters (GCs). This is in contrast to the large number of GCs found for these
systems, from which it would be expected to correspond to a large dark matter
halo mass. Here we present HST+ACS observations for the UDG MATLAS-2019 in the
NGC5846 group. Using the F606W and F814W filters, we trace the GC population
two magnitudes below the peak of the GC luminosity function (GCLF). Employing
Bayesian considerations, we identify 26+-6 GCs associated with the dwarf,
yielding a large specific frequency of S_N=58+-14. We use the turnover of the
GCLF to derive a distance of 21+-2 Mpc, which is consistent with the NGC5846
group of galaxies. Due to the superior image quality of the HST, we are able to
resolve the GCs and measure their sizes, which are consistent with the sizes of
GCs around Local Group galaxies. Using the linear relation between the total
mass of galaxies and of GCs, we derive a halo mass of 0.9(+-0.2) *10^11 M_solar
(M_solar/L_solar>1000). The high abundance of GCs, together with the small
uncertainties, make MATLAS-2019 one of the most extreme UDGs, which likely sets
an upper limit of the number of GCs for UDGs.
