Mainly on the Plane: Solving the Milky Way CGM Anomaly with Low-Galactic-Latitude QSOs
Abstract
Either the Milky Way's ionized CGM is an outlier compared to the CGM of other L* galaxies, or our primary method used to measure its content - hundreds of QSO sightlines that pierce its halo - is fundamentally biased. The latter possibility cannot yet be convincingly ruled out because 95% of existing UV-bright QSO sightlines with archival high-resolution UV spectra lie at Galactic latitudes |b| > 25 degrees. We now have a unique opportunity to address this huge low-latitude gap in UV-bright QSO lines of sight with our proposed sample of newly discovered, rare QSOs at |b| < 25 degrees. We argue that a key requirement to determining the structure, mass, and dynamics of the CGM is understanding its distribution along the disk axis. For the first time, we will be able to use these novel Milky Way sightlines to test the latest hydrodynamical, cosmological simulations that suggest extended, ionized disk-like structures in galaxies may persist out to ~100 kpc. Our study, which will combine high-resolution HST/COS G160M spectra of these new low-|b| QSOs with > 100 high-quality archival G160M QSO spectra, is designed to address two primary questions: (1) With an unbiased view of the halo, is the Milky Way truly an anomaly? (2) Does the MWCGM show evidence for an extended, outer, ionized disk morphology, and if so, what are its properties? Finally, if we find evidence for an extended disk, it would represent one of the largest structures on the sky ever discovered, covering nearly half of the sky.
- Publication:
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HST Proposal
- Pub Date:
- June 2021
- Bibcode:
- 2021hst..prop16679W