Detection of unidentified infrared bands in a Halpha filament in the
  dwarf galaxy NGC1569 with AKARI
  We performed observations of NGC1569 for 6 infrared bands (3.2, 4.1, 7, 11,
15, and 24 micron) with the Infrared Camera (IRC) onboard AKARI. Near- to
mid-infrared (2--13 micron) spectroscopy of a Halpha filament was also carried
out with the IRC.
  The extended structure associated with a Halpha filament appears bright at 7
micron, suggesting that the filament is bright at the UIR band emission.
Follow-up spectroscopic observations with the IRC confirm the presence of 6.2,
7.7, and 11.3 micron emission in the filament. The filament spectrum exhibits
strong 11.3 micron UIR band emission relative to the 7.7 micron band compared
to the galaxy disk observed with the Infrared Spectrograph on Spitzer. The
near-infrared spectrum also suggests the presence of excess continuum emission
in 2.5--5 micron in the filament.
  The Halpha filament is thought to have been formed by the galactic outflow
originating from the star-formation activity in the disk of NGC1569. The
destruction timescale of the UIR band carriers in the outflow is estimated to
be much shorter (~ 1.3 x 10^3 yr) than the timescale of the outflow (~ 5.3
Myr). Thus it is unlikely that the band carriers survive the outflow
environment. Alternatively, we suggest that the band carriers in the filaments
may be produced by the fragmentation of large carbonaceous grains in shocks,
which produces the Halpha emission. The NIR excess continuum emission cannot be
accounted for by free-free emission alone and a hot dust contribution may be
needed, although the free-free emission intensity estimated from HI
recombination lines has a large uncertainty.
