Hot and Diffuse Clouds near the Galactic Center Probed by Metastable \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\mathrm{H}\,^{+}_{3}$ \end{document}

Abstract

Using an absorption line from the metastable (J, K) = (3, 3) level of H3+\ntogether with other lines of H3+ and CO observed along several sightlines, we\nhave discovered a vast amount of high temperature (T ~ 250 K) and low density\n(n ~ 100 cm-3) gas with a large velocity dispersion in the Central Molecular\nZone (CMZ) of the Galaxy, i.e., within 200 pc of the center. Approximately\nthree fourths of the H3+ along the line of sight to the brightest source we\nobserved, the Quintuplet object GCS 3-2, is inferred to be in the CMZ, with the\nremaining H3+ located in intervening spiral arms. About half of H3+ in the CMZ\nhas velocities near ~ - 100 km s-1 indicating that it is associated with the\n180 pc radius Expanding Molecular Ring which approximately forms outer boundary\nof the CMZ. The other half, with velocities of ~ - 50 km s-1 and ~ 0 km s-1, is\nprobably closer to the center. CO is not very abundant in those clouds. Hot and\ndiffuse gas in which the (3, 3) level is populated was not detected toward\nseveral dense clouds and diffuse clouds in the Galactic disk where large column\ndensities of colder H3+ have been reported previously. Thus the newly\ndiscovered environment appears to be unique to the CMZ. The large observed H3+\ncolumn densities in the CMZ suggests an ionization rate much higher than in the\ndiffuse interstellar medium in the Galactic disk. Our finding that the H3+ in\nthe CMZ is almost entirely in diffuse clouds indicates that the reported volume\nfilling factor (f ≥ 0.1) for n ≥ 104 cm-3 clouds in the CMZ is an\noverestimate by at least an order of magnitude.\n