The Supernova Remnant Population of NGC6946 as Observed in [Fe II] 1.644
  $\mu$m with HST
  NGC6946 is a high star formation rate face-on spiral galaxy that has hosted
ten supernovae since 1917. Not surprisingly, a large number of supernova
remnants and candidates have been identified either as optical nebulae with
high [S II]:H$\alpha$ line ratios (147) or as compact non-thermal radio sources
(35). However, there are only seven overlaps between these two samples. Here,
we apply [Fe II] 1.644 $\mu$m emission as a new diagnostic to search for
supernova remnants in an attempt to resolve this discrepancy. [Fe II] is
expected to be relatively strong in the radiative shocks of supernova remnants
and almost absent in HII regions. It is less susceptible to the effects of
absorption along the line of sight than the optical lines normally used to
identify remnants. Using data from the WFC3 camera on HST}, we identify 132 [Fe
II] emission nebulae in NGC6946 as likely supernova remnants. Of these, 54
align with previously known optical supernova remnants. The remaining 78
objects are new; of these 44 are visible in new HST imagery in H$\alpha$ and [S
II]. This brings the total number of supernova remnant candidates (from optical
and/or IR data) in NGC6946 to 225. A total of 14 coincidences with radio
supernova remnant candidates (out of 30 in our search area) are found in this
expanded list. The identification of so many new remnant candidates validates
the use of [Fe II] imagery for finding remnants, and suggests that previous
remnant searches in other galaxies may be far from complete.
