Finding Distant Galactic HII Regions
  The WISE Catalog of Galactic HII Regions contains $\sim2000$ HII region
candidates lacking ionized gas spectroscopic observations. All candidates have
the characteristic HII region mid-infrared morphology of WISE $12\,\,\mu\,m$
emission surrounding $22\,\mu\,m$ emission, and additionally have detected
radio continuum emission. We here report Green Bank Telescope (GBT) hydrogen
radio recombination line (RRL) and radio continuum detections at X-band (9GHz;
3cm) of 302 WISE HII region candidates (out of 324 targets observed) in the
zone $225^{\circ} > l > -20^{\circ}$, $|b| \le 6^{\circ}$. Here we extend the
sky coverage of our HII region Discovery Survey (HRDS), which now contains
nearly 800 HII regions distributed across the entire northern sky. We provide
LSR velocities for the 302 detections and kinematic distances for 131 of these.
Of the 302 new detections, five have ($l, b, v$) coordinates consistent with
the Outer Scutum-Centaurus Arm (OSC), the most distant molecular spiral arm of
the Milky Way. Due to the Galactic warp, these nebulae are found at Galactic
latitudes $>1^{\circ}$ in the first Galactic quadrant, and therefore were
missed in previous surveys of the Galactic plane. One additional region has a
longitude and velocity consistent with the OSC but lies at a negative Galactic
latitude (G039.183$-$01.422; $-$54.9 kms). With Heliocentric distances >22 kpc
and Galactocentric distances >16 kpc, the OSC HII regions are the most distant
known in the Galaxy. We detect an additional three HII regions near $l \simeq
150^{\circ}$ whose LSR velocities place them at Galactocentric radii >19 kpc.
If their distances are correct, these nebulae may represent the limit to
Galactic massive star formation.
