Abstract
Interarm star formation contributes significantly to a galaxy's star
formation budget, and provides an opportunity to study stellar birthplaces
unperturbed by spiral arm dynamics. Using optical integral field spectroscopy
of the nearby galaxy NGC 628 with VLT/MUSE, we identify 391 HII regions at 35pc
resolution over 12 kpc^2. Using tracers sensitive to the underlying
gravitational potential, we associate HII regions with either arm (271) or
interarm (120) environments. We find that most HII region physical properties
(luminosity, size, metallicity, ionization parameter) are independent of
environment. We calculate the fraction of Halpha luminosity due to the diffuse
ionized gas (DIG) background contaminating each HII region, and find the DIG
surface brightness to be higher within HII regions compared to the
surroundings, and slightly higher within arm HII regions. Use of the
temperature sensitive SII/Halpha line ratio map instead of the Halpha surface
brightness to identify HII region boundaries does not change this result. Using
the dust attenuation as a tracer of the gas, we find relatively short depletion
times (6 x 10^8 yr) with no differences between the arm and interarm, however
this is very sensitive to the DIG correction. Unlike molecular clouds, which
can be dynamically affected by the galactic environment, we see fairly
consistent HII region properties in both arm and interarm environments. This
suggests either a difference in arm star formation and feedback, or a
decoupling of dense star forming clumps from the more extended surrounding
molecular gas.
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