TY - JOUR
T1 - The H i Column Density Distribution of the Galactic Disk and Halo
AU - French, David M.
AU - Fox, Andrew J.
AU - Wakker, Bart P.
AU - Lehner, Nicolas
AU - Howk, J. Christopher
AU - Savage, Blair D.
AU - Richter, Philipp
AU - O'Meara, John
AU - Borthakur, Sanchayeeta
AU - Heckman, Timothy
AU - Norman, Colin
N1 - Publisher Copyright:
© 2021. The American Astronomical Society. All rights reserved.
PY - 2021/12/10
Y1 - 2021/12/10
N2 - We present a census of neutral gas in the Milky Way disk and halo down to limiting column densities of N(H i) ∼ 1014 cm-2 using measurements of H i Lyman series absorption from the Far Ultraviolet Spectroscopic Explorer. Our results are drawn from an analysis of 25 AGN sight lines spread evenly across the sky with Galactic latitude |b| ≳ 20°. By simultaneously fitting multi-component Voigt profiles to 11 Lyman series absorption transitions covered by FUSE (Lyβ-Lyμ) plus HST measurements of Lyα, we derive the kinematics and column densities of a sample of 152 H i absorption components. While saturation prevents accurate measurements of many components with column densities 17 ≲ log N(H i) ≲ 19, we derive robust measurements at log N(H i) ≲ 17 and log N(H i) ≳ 19. We derive the first ultraviolet H i column density distribution function (CDDF) of the Milky Way, both globally and for low-velocity (ISM), intermediate-velocity clouds (IVCs), and high-velocity clouds (HVCs). We find that IVCs and HVCs show statistically indistinguishable CDDF slopes, with β IVC = -1.01-0.14+0.15 and β HVC = -1.05-0.06+0.07. Overall, the CDDF of the Galactic disk and halo appears shallower than that found by comparable extragalactic surveys, suggesting a relative abundance of high column density gas in the Galactic halo. We derive the sky-covering fractions as a function of H i column density, finding an enhancement of IVC gas in the northern hemisphere compared to the south. We also find evidence for an excess of inflowing H i over outflowing H i, with -0.88 ± 0.40 M o˙ yr-1 of HVC inflow versus ≈0.20 ± 0.10 M o˙ yr-1 of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.
AB - We present a census of neutral gas in the Milky Way disk and halo down to limiting column densities of N(H i) ∼ 1014 cm-2 using measurements of H i Lyman series absorption from the Far Ultraviolet Spectroscopic Explorer. Our results are drawn from an analysis of 25 AGN sight lines spread evenly across the sky with Galactic latitude |b| ≳ 20°. By simultaneously fitting multi-component Voigt profiles to 11 Lyman series absorption transitions covered by FUSE (Lyβ-Lyμ) plus HST measurements of Lyα, we derive the kinematics and column densities of a sample of 152 H i absorption components. While saturation prevents accurate measurements of many components with column densities 17 ≲ log N(H i) ≲ 19, we derive robust measurements at log N(H i) ≲ 17 and log N(H i) ≳ 19. We derive the first ultraviolet H i column density distribution function (CDDF) of the Milky Way, both globally and for low-velocity (ISM), intermediate-velocity clouds (IVCs), and high-velocity clouds (HVCs). We find that IVCs and HVCs show statistically indistinguishable CDDF slopes, with β IVC = -1.01-0.14+0.15 and β HVC = -1.05-0.06+0.07. Overall, the CDDF of the Galactic disk and halo appears shallower than that found by comparable extragalactic surveys, suggesting a relative abundance of high column density gas in the Galactic halo. We derive the sky-covering fractions as a function of H i column density, finding an enhancement of IVC gas in the northern hemisphere compared to the south. We also find evidence for an excess of inflowing H i over outflowing H i, with -0.88 ± 0.40 M o˙ yr-1 of HVC inflow versus ≈0.20 ± 0.10 M o˙ yr-1 of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.
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U2 - 10.3847/1538-4357/ac1b9f
DO - 10.3847/1538-4357/ac1b9f
M3 - Article
AN - SCOPUS:85122882774
SN - 0004-637X
VL - 923
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
M1 - 50
ER -