Surveying the Giant HII Regions of the Milky Way with SOFIA: VII. Galactic Center Regions Sgr B1, Sgr B2, and Sgr C

TL;DR

This paper uses SOFIA infrared imaging to analyze the properties and star formation activity of giant HII regions near the Galactic Center, revealing environmental differences and potential new classifications of such regions.

Contribution

It provides new mid-infrared observations of Sgr B1, Sgr B2, and Sgr C, identifying MYSO candidates and highlighting environmental and evolutionary differences from typical GHII regions.

Findings

Sgr B2 hosts the youngest and most obscured MYSOs.

Sgr B1 and Sgr C show signs of evolved interloper stars ionizing the regions.

CMZ GHII regions have less prolific star formation compared to Galactic plane regions.

Abstract

This study examines the mid-infrared properties of Giant HII (GHII) regions in the Milky Way's Central Molecular Zone (CMZ) -- Sgr B1, Sgr B2, and Sgr C -- using SOFIA-FORCAST imaging at 25 and 37 microns. It compares these mid-infrared data with previous multi-wavelength observations to explore their present star formation activity and global properties. The study identifies 77 massive young stellar object (MYSO) candidates in and around the three regions. Sgr B2 appears to host the youngest MYSOs and have much higher extinction than the other regions, containing several radio sources not detected in the mid-infrared even at 37 microns. Meanwhile, cm radio continuum regions of Sgr B1 shows remarkable correspondence to its mid-infrared emission. Sgr C has fewer confirmed MYSOs, and seems to have a higher fraction of low-mass young stellar objects and contamination from more evolved interloper/foreground stars. Derived MYSO densities are consistent with GHII regions elsewhere in the Galactic plane, though the CMZ GHII regions appear to have less prolific present star formation overall. Unlike Sgr B2, the cm continuum emission in Sgr B1 and Sgr C GHII regions appears to be absent cold dust and molecular gas, suggesting environmental differences, possibly driven by turbulence and rapid dynamical changes near the Galactic Center. Furthermore, unlike typical GHII regions, Sgr B1 and Sgr C are significantly ionized by evolved interloper stars, which likely did not form within these regions. In these ways, Sgr B1 and Sgr C deviate from classical GHII region behavior, thus potentially representing a new category of GHII region or challenging their classification as GHII regions.