A spectroscopic study of MATLAS-2019 with MUSE: an ultra-diffuse galaxy
  with an excess of old globular clusters
  The MATLAS deep imaging survey has uncovered a plethora of dwarf galaxies in
the low density environment it has mapped. A fraction of them are unusually
extended and have a low-surface brightness. Among these so-called ultra-diffuse
galaxies, a few seem to host an excess of globular clusters. With the
integral-field unit spectrograph MUSE we have observed one of these galaxies -
MATLAS J15052031+0148447 (MATLAS-2019) - located towards the nearby group NGC
5846 and measured its systemic velocity,age, and metallicity, and that of its
globular clusters candidates. For the stellar body of MATLAS-2019 we derive a
metallicity of -1.33+0.19-0.01 dex and an age of 11.2+1.8-0.8 Gyr. For some of
the individual GCs and the stacked GC population, we derive consistent ages and
metallicities. From the 11 confirmed globular clusters and using a Markov Chain
Monte Carlo approach we derived a dynamical mass-to-light ratio of
4.2+8.6-3.4M/L. This is at the lower end of the luminosity-mass scaling
relation defined by the Local Group dwarf galaxies. Furthermore, we couldn't
confirm nor reject the possibility of a rotational component of the GC system.
If present, this would further modify the inferred mass. Follow-up observations
of the globular cluster population and of the stellar body of the galaxy are
needed to assess whether this galaxy is lacking dark matter like it was
suggested for the pair of dwarf galaxies in the field of NGC 1052, or if this
is a miss-interpretation arising from systematic uncertainties of the method
commonly used for these systems and the large uncertainties of the individual
globular cluster velocities.
