15min:

TAKESHI OKA, Department of Astronomy and Astrophysics and Department of Chemistry, University of Chicago; THOMAS R. GEBALLE, Gemini Observatory, Hilo, Hawaii; MIWA GOTO, Max Planck Institute for Astronomy, Heidelberg, Germany; TOMONORI USUDA, Subaru Telescope, National Astronomical Observatory of Japan, Hilo, Hawaii; BENJAMIN J. MCCALL, Department of Chemistry and Department of Astronomy, University of Illinois at Urbana-Champaign.

We have observed a vast amount of high temperature (T 250 K) and low density (n 100 cm^-3) gas with a large velocity dispersion in the Central Molecular Zone (CMZ) of the Galactic center. We used H₃⁺ which is a sensitive probe of low density molecular gas. The observed large column density of H₃⁺ 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) (1, 1) spontaneous emission (27 days).

The strongest absorption component observed toward the bright infrared source GCS 3-2 is at velocity of - 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 0 km s^-1 is closer to the Galactic center. The large H₃⁺ 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 H₃⁺ 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 H₃⁺ column density toward GCS 3-2 of 4.3 × 10¹⁵ cm^-2, approximately 3.1 × 10¹⁵ cm^-2 is inferred to be in the CMZ while 1.2 × 10¹⁵ cm^-2 is in the intervening spiral arms. Almost all of H₃⁺ 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.