Dusty Superwind from a Galaxy with a Compact Obscured Nucleus: Optical
  Spectroscopic Study of NGC 4418
  We report our optical spectroscopic study of the nucleus and its surrounding
region of a nearby luminous infrared galaxy NGC 4418. This galaxy has been
known to host a compact obscured nucleus, showing distinct characteristics such
as a very compact ($\sim 20$ pc) sub-mm and mid-infrared core and dusty
circumnuclear region with massive molecular gas concentration. We detected
dusty superwind outflow at $\gtrsim 1$ kpc scale along the disk semiminor axis
in both shock-heated emission lines and enhanced interstellar Na D absorption.
This superwind shows basic characteristics similar to those of the prototypical
superwind in the starburst galaxy M82, such as a kpc-scale extended structure
of gas and dust along the disk minor axis, outflowing components (multiphase
gas and dust), physical conditions of the ionized gas, and monotonically
blueshifting radial velocity field with increasing distance from the nucleus on
the front side of the superwind. We also detected a moderately extinct
starburst population in the SDSS nuclear spectrum with the burst age of $\simeq
10$ Myr and stellar mass of $\simeq 1\times 10^7\ M_\mathrm{\odot}$. It is
powerful enough to drive the superwind within the dynamical age of the
superwind ($\simeq 10$ Myr). On the basis of comparison between this
starburst--superwind scenario and the observations in terms of the burst age,
stellar mass, infrared luminosity, and obscuration in the optical bands, we
argue that this superwind-driving starburst is separate from the sub-mm core
even if the core is a very young star cluster. Therefore, this galaxy hosts
both the enshrouded compact core and the superwind-driving circumnuclear
starburst.
