On the Origin of Diffuse Ionized Gas in the Antennae Galaxy
  The "Antennae Galaxy" (NGC 4038/39) is the closest major interacting galaxy
system and therefore often taken as merger prototype. We present the first
comprehensive integral field spectroscopic dataset of this system, observed
with the MUSE instrument at the ESO VLT. We cover the two regions in this
system which exhibit recent star-formation: the central galaxy interaction and
a region near the tip of the southern tidal tail. In these fields, we detect
HII regions and diffuse ionized gas to unprecedented depth. About 15% of the
ionized gas was undetected by previous observing campaigns. This newly detected
faint ionized gas is visible everywhere around the central merger, and shows
filamentary structure. We estimate diffuse gas fractions of about 60% in the
central field and 10% in the southern region. We are able to show that the
southern region contains a significantly different population of HII regions,
showing fainter luminosities. By comparing HII region luminosities with the HST
catalog of young star clusters in the central field, we estimate that there is
enough Lyman-continuum leakage in the merger to explain the amount of diffuse
ionized gas that we detect. We compare the Lyman-continuum escape fraction of
each HII region against ionization-parameter sensitive emission line ratios.
While we find no systematic trend between these properties, the most extreme
line ratios seem to be strong indicators of density bounded ionization.
Extrapolating the Lyman-continuum escape fractions to the southern region, we
conclude that just from the comparison of the young stellar populations to the
ionized gas there is no need to invoke other ionization mechanisms than
Lyman-continuum leaking HII regions for the diffuse ionized gas in the
Antennae.
