Molecular Gas within the Milky Way's Nuclear Wind

Abstract

We report the first direct detection of molecular hydrogen associated with the Galactic nuclear wind. The Far-Ultraviolet Spectroscopic Explorer spectrum of LS 4825, a B1 Ib–II star at l, b = 1.67°,−6.63° lying d = 9.9 − 0.8 + 1.4 kpc from the Sun, ∼1 kpc below the Galactic plane near the Galactic center, shows two high-velocity H2 components at v LSR = −79 and −108 km s−1. In contrast, the FUSE spectrum of the nearby (∼0.6° away) foreground star HD 167402 at d = 4.9 − 0.7 + 0.8 kpc reveals no H2 absorption at these velocities. Over 60 lines of H2 from rotational levels J = 0 to 5 are identified in the high-velocity clouds. For the v LSR = −79 km s−1 cloud we measure total log N(H2) ≥ 16.75 cm−2, molecular fraction f H 2 ≥ 0.8%, and T 01 ≥ 97 and T 25 ≤ 439 K for the ground- and excited-state rotational excitation temperatures. At v LSR = −108 km s−1, we measure log N(H2) = 16.13 ± 0.10 cm−2, f H 2 ≥ 0.5%, and T 01 = 77 − 18 + 34 and T 25 = 1092 − 117 + 149 K, for which the excited-state ortho- to para-H2 is 1.0 − 0.1 + 0.3 , much less than the equilibrium value of 3 expected for gas at this temperature. This nonequilibrium ratio suggests that the −108 km s−1 cloud has been recently excited and has not yet had time to equilibrate. As the LS 4825 sight line passes close by a tilted section of the Galactic disk, we propose that we are probing a boundary region where the nuclear wind is removing gas from the disk.

CO Emission Delineating the Interface between the Milky Way Nuclear Wind Cavity and the Gaseous Disk

“…The H I voids, which are physically associated with the Fermi bubbles, are thus surrounded by enhanced CO emission at low latitudes of |b|  2°-5°and R GC  2.6-3.1 kpc. This scenario also agrees well with the recent observation that the Milky Way nuclear wind can remove gas from the disk to the halo (e.g., Cashman et al 2021) and can accelerate MC fragments into the high-z regions (e.g., Di .…”

A Detection of H2 in a High-velocity Cloud toward the Large Magellanic Cloud

“…Three of the clear and wellcharacterized detections were found in the Magellanic system: the Leading Arm and main body of the Magellanic Stream (Richter et al 2001;Sembach et al 2001) and the Magellanic Bridge (Lehner 2002). The fourth detection was found in the direction of the Galactic center and may be an example of gas ejection from the Galactic disk by a nuclear wind (Cashman et al 2021).…”

“…Molecular fractions f (H 2 ) as a function of total hydrogen column N H for different populations of molecular absorbers. The populations are found in the Magellanic Clouds (small black circles;Welty et al 2012), the Milky Way disk at low and high Galactic latitudes (small and large gray circles;Savage et al 1977;Gillmon et al 2006;Rachford et al 2009;Shull et al 2021), intermediate-velocity clouds (large black circles;Richter et al 2003a;Wakker 2006), high-velocity clouds including the Sk-70°32 absorber (green circles;Richter et al 2001;Sembach et al 2001;Lehner 2002;Wakker 2006;Cashman et al 2021), and extragalactic Damped and sub-Damped Lyman α absorbers at z < 0.6 (black diamonds;Muzahid et al 2015Muzahid et al , 2016 and 1.8 < z (black squares;Noterdaeme et al 2008;Balashev et al 2014;Ledoux et al 2015;Noterdaeme et al 2018;Balashev et al 2019;Ranjan et al 2020). The two connected Sk-70°32 data points represent the uncertainty on the total column density of the absorber.…”

A detection of H$_2$ in a high velocity cloud toward the Large Magellanic Cloud

“…Three of the clear and well-characterized detections were found in the Magellanic system: the Leading Arm and main body of the Magellanic Stream (Sembach et al 2001;Richter et al 2001) and the Magellanic Bridge (Lehner 2002). The fourth detection was found in the direction of the Galactic center and may be an example of gas ejection from the Galactic disk by a nuclear wind (Cashman et al 2021).…”

“…The cloud metallicity is set to 0.3 times solar, the nominal metallicity determined by Lehner et al (2009). The dust-to-gas ra- Richter et al 2001;Sembach et al 2001;Lehner 2002;Wakker 2006;Cashman et al 2021), and extragalactic Damped and sub-Damped Lyman α absorbers at z < 0.6 (black diamonds; Muzahid et al 2015Muzahid et al , 2016 and 1.8 < z (black squares; Noterdaeme et al 2008;Balashev et al 2014;Ledoux et al 2015;Noterdaeme et al 2018;Balashev et al 2019;Ranjan et al 2020. The two connected Sk-70 • 32 datapoints represent the uncertainty on the total column density of the absorber.…”