Unravelling the Origin of the Counter-Rotating Core in IC 1459 with KMOS
  and MUSE
  The massive early-type galaxy (ETG) IC 1459 is a slowly rotating galaxy that
exhibits a rapidly counter-rotating kinematically decoupled core (KDC, $R_{\rm
KDC}\approx 5^{\prime\prime}\approx 0.1 R_{\rm e}$). To investigate the origin
of its KDC, we coupled large data mosaics from the near-infrared (NIR)/optical
integral field unit (IFU) instruments K-band Multi-Object Spectrograph (KMOS)
and Multi Unit Spectroscopic Explorer (MUSE). We studied IC 1459's stellar
populations and, for the first time for a KDC, the spatially resolved initial
mass function (IMF). We used full-spectral-fitting to fit the stellar
populations and IMF simultaneously, and an alternative spectral-fitting method
that does not assume a star-formation history (SFH; although does not constrain
the IMF) for comparison. When no SFH is assumed, we derived a negative
metallicity gradient for IC 1459 that could be driven by a distinct metal-poor
population in the outer regions of the galaxy, and a radially constant old
stellar age. We found a radially constant bottom-heavy IMF out to $\sim
\frac{1}{3} R_{\rm e}$. The radially flat IMF and age extend beyond the
counter-rotating core. We detected high velocity dispersion along the galaxy's
major axis. Our results potentially add weight to findings from orbital
modelling of other KDCs that the core is not a distinct population of stars but
in fact two smooth co-spatial counter-rotating populations. No clear picture of
formation explains the observational results of IC 1459, but we propose it
could have included a gas-rich intense period of star formation at early times,
perhaps with counter-rotating accreting cold streams, followed by dry and
gas-rich mergers through to the present day.
