2 mm GISMO Observations of the Galactic Center. II. A Nonthermal
  Filament in the Radio Arc and Compact Sources
  We have used the Goddard IRAM 2-Millimeter Observer (GISMO) with the 30 m
IRAM telescope to carry out a 2 mm survey of the Galaxy's central molecular
zone (CMZ). These observations detect thermal emission from cold ISM dust,
thermal free-free emission from ionized gas, and nonthermal synchrotron
emission from relatively flat-spectrum sources. Archival data sets spanning
$3.6 \mu$m to 90 cm are used to distinguish different emission mechanisms.
After the thermal emission of dust is modeled and subtracted, the remaining 2
mm emission is dominated by free-free emission, with the exception of the
brightest nonthermal filament (NTF) that runs though the middle of the bundle
of filaments known as the Radio Arc. This is the shortest wavelength at which
any NTF has been detected. The GISMO observations clearly trace this NTF over a
length of ~0.2$^\circ$, with a mean 2 mm spectral index which is steeper than
at longer wavelengths. The 2 mm to 6 cm (or 20 cm) spectral index steepens from
$\alpha \approx -0.2$ to $-0.7$ as a function distance from the Sickle H II
region, suggesting that this region is directly related to the NTF. A number of
unresolved (at $21''$) 2 mm sources are found nearby. One appears to be thermal
dust emission from a molecular cloud that is associated with an enigmatic radio
point source whose connection to the Radio Arc is still debated. The morphology
and colors at shorter IR wavelengths indicate other 2 mm unresolved sources are
likely to be compact H II regions.
