SOFIA FIFI-LS Observations of Sgr B1: Ionization Structure and Sources of Excitation

TL;DR

This study uses SOFIA FIFI-LS observations to analyze the ionization structure of Sgr B1, revealing multiple ionizing sources likely from an older star formation episode, distinct from the nearby Sgr B2 region.

Contribution

It provides new far-infrared spectroscopic data of Sgr B1, identifying multiple ionizing sub-regions and characterizing their stellar sources, suggesting a previous star formation event.

Findings

Multiple ionizing sub-regions identified in Sgr B1.

Ionizing sources are late O stars with X-ray augmentation.

Stars ionizing Sgr B1 are from an older star formation episode.

Abstract

The current paradigm of Galactic Center (GC) gas motions and star formation envisions sequential star formation in streams of gas as they pass near the supermassive black hole, Sgr A*. This is based on the relative positions of dense molecular clouds, the very young star-forming region Sgr B2, the much older region Sgr C, and the several Myr old Arches and Quintuplet Clusters. Because Sgr B1 is found with Sgr B2 in a common envelope of molecular gas and far-infrared emission, the two sources are thought to be physically related, even though there are indicators of a significantly greater age for Sgr B1. To clarify the status of Sgr B1, we have mapped it with the FIFI-LS spectrometer on SOFIA in the far-infrared lines of [O III] 52 and 88 micron. From the ratios of these lines and lines measured with the Spitzer Infrared Spectrograph, we find that there are at least eight separate sub-regions that must contain the stars that excite the gas. We infer spectral energy distributions (SEDs) of the ionizing sources from models and find they are in agreement only with SEDs of late O stars augmented at the highest frequencies with interstellar X-rays from fast shocks. We suggest that although the gas, from its velocity structure, must be part of the very young Sgr B2 complex, the stars that are ionizing the gas were not formed there but are the remnants of a previous generation of star formation in the GC.