Limits On The OH Molecule In The Smith High Velocity Cloud

By Keith Cowing
Status Report
March 14, 2024
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Limits On The OH Molecule In The Smith High Velocity Cloud
Hi column density in contours plotted atop WISE 22µm image that shows emission from the dust along the line of sight. White crosses show the locations of the individual OH pointings described in Tables 2 and 3. HI contours are drawn at 0.5, 1.4, 1.8, 2.9, and 3.8 × 1020 cm−2 . The WISE 22 µm image has arbitrary units as obtained from NASA’s HEASARC SkyView. — astro-ph.GA

We have used the Green Bank Telescope (GBT) to search for the OH molecule at several locations in the Smith Cloud, one of the most prominent of the high-velocity clouds that surround the Milky Way.

Five positions with a high HI column density were selected as targets for individual pointings, along with a square degree around a molecular cloud detected with the Planck telescope near the tip of the Smith Cloud. Gas in the Galactic disk with similar values of NHI has detectable OH emission.

Although we found OH at velocities consistent with the foreground Aquila molecular cloud, nothing was found at the velocity of the Smith Cloud to an rms level of 0.7 mK (Tb) in a 1 km s1 channel. The three positions that give the strictest limits on OH are analyzed in detail. Their combined data imply a 5σ limit on N(H2)/NHI≤0.03 scaled by a factor dependent on the OH excitation temperature and background continuum Tex/(Tex−Tbg).

There is no evidence for far-infrared emission from dust within the Smith Cloud. These results are consistent with expectations for a low-metallicity diffuse cloud exposed to the radiation field of the Galactic halo rather than a product of a galactic fountain.

Anthony H. Minter, Felix J. Lockman, S. A. Balashev, H. Alyson Ford

Comments: Accepted for The Astrophysical Journal 21 pages, 4 figures, 5 tables
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2403.08704 [astro-ph.GA] (or arXiv:2403.08704v1 [astro-ph.GA] for this version)
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Submission history
From: Anthony Minter
[v1] Wed, 13 Mar 2024 17:08:56 UTC (7,027 KB)
Astrobiology, Astrochemistry

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