Unusual Galactic HII Regions at the Intersection of the Central Molecular Zone and the Far Dust Lane

Sgr E is a star formation complex found toward the Galactic center that consists of numerous discrete, compact HII regions. It is located at the intersection between the Central Molecular Zone (CMZ) and the far dust lane of the Galactic bar, similar to “hot spots” seen in external Galaxies. Sgr E is unusual in that: 1) the individual Sgr E HII regions all share similar radio luminosities and angular extents; 2) it has the largest absolute radial velocity of any known Galactic HII region; and 3) the individual Sgr E HII regions are deficient in ∼10μm emission from polycyclic aromatic hydrocarbons. Green Bank Telescope (GBT) radio recombination line observations allowed us to increase the known membership of Sgr E to 20 HII regions, although the many mid-infrared sources in the area indicates that there may be as many as 100 HII regions in Sgr E. Using APEX SEDIGISM ¹³CO 3-2 data, we discover a 3.0×10⁵ Solar mass molecular cloud associated with Sgr E, but find no molecular or far-infrared concentrations at the locations of the Sgr E HII regions. Far-infrared data and new GBT NH₃ observations both show that the dust and gas have elevated temperatures relative to the dust and gas temperatures of other known HII regions. Comparison with simulations indicates that the Sgr E HII regions formed in the dust lane of the Galactic bar a couple Myr ago and will overshoot the CMZ, crashing into the near dust lane. We propose that the PDRs of the Sgr E HII regions were stripped during its orbit about that Galactic center.