A deep view into the nucleus of the Sagittarius Dwarf Spheroidal Galaxy
  with MUSE. I. Data and stellar population characterization
  The center of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) hosts a
nuclear star cluster, M54, which is the only galaxy nucleus that can be
resolved into individual stars at optical wavelengths. It is thus a key target
for understanding the formation of nuclear star clusters and their relation to
globular clusters. We present a large Multi-Unit Spectroscopic Explorer (MUSE)
data set that covers M54 out to $\sim$2.5 half-light radius, from which we
extracted the spectra of $\sim$6600 cluster member stars. We use these data in
combination with HST photometry to derive age and metallicity for each star.
The stellar populations show a well defined age-metallicity relation, implying
an extended formation history for the central region of Sgr dSph. We classify
these populations into three groups, all with the same systemic velocity: young
metal-rich (YMR; 2.2\,Gyr, \mbox{[Fe/H]$=-0.04$}); intermediate-age metal-rich
(IMR; 4.3\,Gyr, \mbox{[Fe/H]$=-0.29$}); and old metal-poor (OMP; 12.2\,Gyr,
\mbox{[Fe/H]$=-1.41$}). The YMR and OMP populations are more centrally
concentrated than the IMR population, which are likely stars of the Sgr dSph.
We suggest the OMP population is the result of accretion and merging of two or
more old and metal-poor globular clusters dragged to the center by dynamical
friction. The YMR is consistent with being formed by in situ star formation in
the nucleus. The ages of the YMR population suggest that it may have been
triggered into forming when the Sgr dSph began losing its gas during the most
recent interaction with the Milky Way, $\sim$3\,Gyr ago.
