Astrophys. J., 871, 35-35 (2019/January-3)
Revealing the Milky Way's hidden circumgalactic medium with the Cosmic Origins Spectrograph Quasar Database for Galactic Absorption Lines.
ZHENG Y., PEEK J.E.G., PUTMAN M.E. and WERK J.K.
Abstract (from CDS):
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 | vLSR| ≤ 100 km s–1 in the Galactic halo. The gas in the north predominantly falls in at -50≲vLSR≲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 <NSiIV>=(3.8±1.4)×1013 cm–2. At | b| ≳30○, NSiIV 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 NSiIV-b distribution: a plane-parallel component NDH^⊥^ to account for the MW's disk-halo interface and a global component NG to reproduce the weak dependence on b. We find NDH^⊥=1.3-0.7_+4.7×1012 cm–2 and NG=(3.4±0.3)×1013 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 Mgas,all≳4.7×109 M☉(Cf/1)(R/75kpc)2(f_Si IV/0.3)-1(Z/0.3Z☉_)–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 | vLSR| ≤ 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).
© 2019. The American Astronomical Society. All rights reserved.
Galaxy: halo - Galaxy: structure - quasars: absorption lines - techniques: spectroscopic
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<Available at CDS (J/ApJ/871/35): table1.dat sp/*>
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