2021 journal article

Molecular Gas within the Milky Way's Nuclear Wind

ASTROPHYSICAL JOURNAL LETTERS, 923(1).

By: F. Cashman*, A. Fox*, B. Savage*, B. Wakker*, D. Krishnarao*, R. Benjamin*, P. Richter*, T. Ashley* ...

co-author countries: Germany 🇩🇪 United States of America 🇺🇸
Source: Web Of Science
Added: December 20, 2021

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 <?CDATA ${}_{-0.8}^{+1.4}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow /> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> kpc from the Sun, ∼1 kpc below the Galactic plane near the Galactic center, shows two high-velocity H 2 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 <?CDATA ${}_{-0.7}^{+0.8}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow /> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.8</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> kpc reveals no H 2 absorption at these velocities. Over 60 lines of H 2 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 (H 2 ) ≥ 16.75 cm −2 , molecular fraction <?CDATA ${f}_{{{\rm{H}}}_{2}}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> ≥ 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 (H 2 ) = 16.13 ± 0.10 cm −2 , <?CDATA ${f}_{{{\rm{H}}}_{2}}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> ≥ 0.5%, and T 01 = 77 <?CDATA ${}_{-18}^{+34}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow /> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>18</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>34</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and T 25 = 1092 <?CDATA ${}_{-117}^{+149}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow /> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>117</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>149</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> K, for which the excited-state ortho- to para-H 2 is <?CDATA ${1.0}_{-0.1}^{+0.3}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>1.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , 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.