Revealing the Milky Way’s Hidden Circumgalactic Medium with the Cosmic Origins Spectrograph Quasar Database for Galactic Absorption Lines
Abstract
Every quasar (quasi-stellar object; QSO) spectrum contains absorption-line signatures from the interstellar medium, disk-halo interface, and circumgalactic medium (CGM) of the Milky Way (MW). We analyze Hubble Space Telescope/Cosmic Origins Spectrograph (COS) spectra of 132 QSOs to study the significance and origin of Si IV absorption at | {v}LSR}| ≤slant 100 km s-1 in the Galactic halo. The gas in the north predominantly falls in at -50≲ {v}LSR}≲ 0 km s-1, whereas in the south, no such pattern is observed. The Si IV column density has an average and a standard deviation of < {N}Si{{IV}}}> =(3.8+/- 1.4)× {10}13 cm-2. At | b| ≳ 30^\circ , {N}Si{{IV}}} does not significantly correlate with b, which cannot be explained by a commonly adopted flat-slab geometry. We propose a two-component model to reconstruct the {N}Si{{IV}}}-b distribution: a plane-parallel component {N}DH}\perp to account for the MW’s disk-halo interface and a global component {N}{{G}} to reproduce the weak dependence on b. We find {N}DH}\perp ={1.3}-0.7+4.7× {10}12 cm-2 and {N}{{G}}=(3.4+/- 0.3)× {10}13 cm-2 on the basis of Bayesian analyses and block bootstrapping. The global component is most likely to have a Galactic origin, although its exact location is uncertain. If it were associated with the MW’s CGM, we would find {M}gas,{all}}≳ 4.7× {10}9 {M}⊙ ≤ft(\tfrac{{C}f}{1}\right){≤ft(\tfrac{R}{75{kpc}}\right)}2{≤ft(\tfrac{{f}Si{{IV}}}}{0.3}\right)}-1{≤ft(\tfrac{Z}{0.3{Z}⊙ }\right)}-1 for the cool gas at all velocities in the Galactic halo. Our analyses show that there is likely a considerable amount of gas at | {v}LSR}| ≤slant 100 km s-1 hidden in the MW’s CGM. Along with this work, we make our QSO data set publicly available as the COS Quasar Database for Galactic Absorption Lines (COS-GAL).
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2019
- DOI:
- 10.3847/1538-4357/aaf6eb
- arXiv:
- arXiv:1710.10703
- Bibcode:
- 2019ApJ...871...35Z
- Keywords:
-
- Galaxy: halo;
- Galaxy: structure;
- quasars: absorption lines;
- techniques: spectroscopic;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Published on ApJ. https://iopscience.iop.org/article/10.3847/1538-4357/aaf6eb/meta