Spectroscopic Constraints on the Build-up of the Intracluster Light in
  the Coma Cluster
  The stellar content of the intracluster light (ICL) provides unique insight
into the hierarchical assembly process of galaxy clusters.However, the ICL is
difficult to study due to its low surface brightness and large physical extent.
We present optical spectra of three ICL regions in the Coma cluster, located
between 100-180kpc from their nearest BCGs: NGC4889 and NGC4874. The mean
surface brightness of the three ICL regions are {\mu}$_g$~25.3-26.2mag
arcsec$^{-2}$. IFU spectroscopy with 13.5 hr on-source integration time were
acquired as part of an ancillary program within the SDSS-IV MaNGA survey. We
stacked the 127 individual fiber spectra in each IFU in order to achieve a
1{\sigma} limiting surface brightness of 27.9mag arcsec$^{-2}$, corresponding
to a mean S/N in the optical of 21.6,9.6,and 11.6\AA$^{-1}$. We apply stellar
population models to the stacked spectra, and measure the recession velocities,
velocity dispersions ($\sigma$), stellar ages, and [Fe/H]. Our results show
that the $\sigma$ of ICL regions are very high, indicating the stars are
tracing the gravitational potential of the cluster, instead of any individual
galaxy. The line-of-sight velocities of the three ICL regions are different
from each other by ~700km/s, while the velocity of each region is similar to
the closest BCG. This suggests that the ICL regions are associated with two
distinct subclusters centered on NGC4889 and NGC4874.The stellar populations of
these regions are old and metal poor, with ages of 7-12Gyr, and [Fe/H] of -0.8
to -0.6 dex. From the derived age and [Fe/H], the build-up of ICL in Coma is
likely to be through the accretion of low mass galaxies or the tidal stripping
of the outskirts of massive galaxies that have ended their star formation early
on, instead of directly from major mergers of massive galaxies.
