HOT AND DIFFUSE CLOUDS NEAR THE GALACTIC CENTER PROBED BY METASTABLE H$_3^+$
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Date
2005
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Ohio State University
Abstract
We have observed a vast amount of high temperature (T $\sim$ 250 K) and low density (n $\sim$ 100 cm$^{-3}$) gas with a large velocity dispersion in the Central Molecular Zone (CMZ) of the Galactic center. We used H$_3^+$ which is a sensitive probe of low density molecular gas. The observed large column density of H$_3^+$ in the (3, 3) metastable rotational level gives evidence for high temperature, and absence in the (2, 2) level indicates low density. This remarkable non-thermal rotational distribution is caused by metastability of the (3, 3) level and the fast (2, 2) $\rightarrow$ (1, 1) spontaneous emission (27 days)}. The strongest absorption component observed toward the bright infrared source GCS 3-2 is at velocity of $\sim$ - 100 km s$^{-1}$, indicating that about a half of the hot and diffuse gas is associated with the 180 pc Expanding Molecular Ring. The other half with lower velocities of - 50 km s$^{-1}$ and $\sim$ 0 km s$^{-1}$ is closer to the Galactic center. The large H$_3^+$ column density indicates high ionization rate on the order of 10$^{-14}$ s$^{-1}$ in the CMZ if the C/H ratio is indeed as high as reported. With the hot X-rays and high magnetohydrodynamic activities, such a high value may be reasonable. The non-thermal rotational distribution of H$_3^+$ has also been observed toward 7 other infrared sources within 30 pc of the Galactic center indicating that the hot and diffuse gas is ubiquitous in the CMZ. The spectrum toward GC IRS 3 near Sgr A* shows presence of the hot and diffuse gas in the ``50 km s$^{-1}$ cloud", the complex of giant molecular clouds which plays a central role in the discussion of Sgr A* and its environment. Of the observed total H$_3^+$ column density toward GCS 3-2 of 4.3 $\times$ 10$^{15}$ cm$^{-2}$, approximately 3.1 $\times$ 10$^{15}$ cm$^{-2}$ is inferred to be in the CMZ while 1.2 $\times$ 10$^{15}$ cm$^{-2}$ is in the intervening spiral arms. Almost all of H$_3^+$ in the CMZ is in diffuse clouds. This suggests that the previously reported volume filling factor (f $\geq$ 0.1) of dense clouds is an overestimate by at least an order of magnitude.
Description
{T. Oka and E. Epp, ApJ, 613, 349 (2004)
Author Institution: Department of Astronomy and Astrophysics and Department of Chemistry, University of Chicago; Gemini Observatory, Hilo, Hawaii; Max Planck Institute for Astronomy, Heidelberg, Germany; Subaru Telescope, National Astronomical Observatory of Japan, Hilo, Hawaii; Department of Chemistry and Department of Astronomy, University of Illinois at Urbana-Champaign
Author Institution: Department of Astronomy and Astrophysics and Department of Chemistry, University of Chicago; Gemini Observatory, Hilo, Hawaii; Max Planck Institute for Astronomy, Heidelberg, Germany; Subaru Telescope, National Astronomical Observatory of Japan, Hilo, Hawaii; Department of Chemistry and Department of Astronomy, University of Illinois at Urbana-Champaign