Abstract
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.
Highlights
The Galactic Center (GC) is host to the Milky Way’s (MW) nuclear wind, powered by the supermassive black hole Sagittarius A∗ and surrounding regions of intense star formation
The highvelocity H2 components are not seen in the spectrum of the foreground star HD 167402, indicating that the H2 detected at −79 and −108 km s−1 in the background star LS 4825 can be bracketed to the range 5 d 10 kpc, which corresponds to a z-distance of −0.6 to −1.2 kpc below the GC
The H2 components detected in the Far-Ultraviolet Spectroscopic Explorer (FUSE) spectrum of LS 4825 (d = 9.9+−10..48 kpc) at −79 and −108 km s−1 are not seen in the spectrum of the foreground star HD 167402 (d = 4.9+−00..87 kpc) and trace gas located between the two stars and most likely within the southern Fermi Bubble
Summary
The Galactic Center (GC) is host to the Milky Way’s (MW) nuclear wind, powered by the supermassive black hole Sagittarius A∗ and surrounding regions of intense star formation. Ultraviolet (UV) absorption-line studies reveal high-velocity absorption in low-ionization (e.g. C I, O I, S II) and high-ionization (e.g. C IV, Si IV) species in sight lines through the Fermi Bubbles. These UV absorbers trace outflowing gas that spans an extremely large range in physical conditions (Keeney et al 2006; Zech et al 2008; Fox et al 2015; Bordoloi et al 2017; Savage et al 2017; Karim et al 2018; Ashley et al 2020). ALMA observations toward J1744-3116 show millimeter-wave molecular absorption (HCO+, HCN, CS) at velocities arising inside the Galactic bulge near the GC (Liszt & Gerin 2018)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
