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

TL;DR

This paper studies the unique properties and origins of the Sgr E star formation complex near the Galactic center, revealing its unusual HII regions, associated molecular cloud, and dynamic orbit within the Galactic bar.

Contribution

It provides new radio observations identifying over 60 HII regions in Sgr E and links their properties to their orbital history in the Galactic bar.

Findings

Sgr E contains over 60 HII regions, many detected for the first time.

A massive molecular cloud is associated with Sgr E, but shows few PDRs.

Sgr E likely formed upstream in the Galactic bar's dust lane and is moving towards the CMZ.

Abstract

Sgr E is a massive 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. Compared with other Galactic star formation complexes, the Sgr E complex is unusual because its HII regions all have similar radio luminosities and angular extents, and they are deficient in ~10micron emission from their photodissociation regions (PDRs). Our Green Bank Telescope (GBT) radio recombination line observations increase the known membership of Sgr E to 19 HII regions. There are 43 additional HII region candidates in the direction of Sgr E, 26 of which are detected for the first time here using MeerKAT 1.28GHz data. Therefore, the true HII region population of Sgr E may number >60. Using APEX SEDIGISM 13CO 2-1 data we discover a 3.0x10^5 Solar mass molecular cloud associated with Sgr E, but find few molecular or far-infrared concentrations at the locations of the Sgr E HII regions. Comparison with simulations and an analysis of its radio continuum properties indicate that Sgr E formed upstream in the far dust lane of the Galactic bar a few Myr ago and will overshoot the CMZ, crashing into the near dust lane. We propose that the unusual infrared properties of the Sgr E HII regions are caused by their orbit about the Galactic center, which have possibly stripped their PDRs.