Deciphering the Origin of Ionized Gas in IC 1459 with VLT/MUSE
  IC 1459 is an early-type galaxy (ETG) with a rapidly counter-rotating stellar
core, and is the central galaxy in a gas-rich group of spirals. In this work,
we investigate the abundant ionized gas in IC 1459 and present new stellar
orbital models to connect its complex array of observed properties and build a
more complete picture of its evolution. Using the Multi-Unit Spectroscopic
Explorer (MUSE), the optical integral field unit (IFU) on the Very Large
Telescope (VLT), we examine the gas and stellar properties of IC 1459 to
decipher the origin and powering mechanism of the galaxy's ionized gas. We
detect ionized gas in a non-disk-like structure rotating in the opposite sense
to the central stars. Using emission-line flux ratios and velocity dispersion
from full-spectral fitting, we find two kinematically distinct regions of
shocked emission-line gas in IC 1459, which we distinguished using narrow
($\sigma$ $\leq$ 155 km s$^{-1}$) and broad ($\sigma$ $>$ 155 km s$^{-1}$)
profiles. Our results imply that the emission-line gas in IC 1459 has a
different origin than that of its counter-rotating stellar component. We
propose that the ionized gas is from late-stage accretion of gas from the group
environment, which occurred long after the formation of the central stellar
component. We find that shock heating and AGN activity are both ionizing
mechanisms in IC 1459 but that the dominant excitation mechanism is by
post-asymptotic giant branch stars from its old stellar population.
