Temperature, Density, Ionization Rate, And Morphology Of Diffuse Gas Near The Galactic Center Probed By H3+

Since last year, infrared spectra of H$_3^+$ and CO have been obtained toward nine stars (designated by us $\alpha$+, $\beta$, $\gamma$, $\gamma$$-$, $\delta$, $\theta$, $\kappa$, $\lambda$, and $\lambda$$-$) along the Galactic plane from 138 pc to the west of Sgr A* to 115 pc east, using IRCS of the Subaru Telescope and GNIRS of the Gemini North Observatory. All of the objects lie within the Central Molecular Zone (CMZ), a region of radius $\sim$150~pc at the center of the Galaxy. All sightlines except that toward $\lambda$ (a red giant not suitable for H$_3^+$ spectroscopy) have high H$_3^+$ column densities on the order of a few times 10$^{15}$~cm$^{-2}$. The metastable $R$(3,3)$^l$ absorption line was sought on seven sightlines ($\alpha$+, $\beta$, $\gamma$, $\gamma$$-$, $\delta$, $\theta$, $\kappa$), each of which showed significant signal except $\kappa$ for which detection of this line was inconclusive. These results indicate that the long (at least several tens of parsecs) columns of warm ($T$~$\sim$~250~K) and diffuse ($n$~$\leq$~100~cm$^{-3}$) gas in which a high ionization rate of $\zeta$ of a few times 10$^{-15}$~s$^{-1}$ exists, found earlier by us on sightlines passing through the central 30 pc of the CMZ\footnote{Oka, T., Geballe, T. R., Goto, M., Usuda, T., and McCall, B. J. 2005, ApJ, \textbf{632} 882}\footnote{Goto, M., Usuda, T., Nagata, T. et al. 2008, ApJ, \textbf{688} 306} are present over nearly the entire CMZ. The velocity profiles of the H$_3^+$ absorption lines provide information on the morphology of the diffuse gas in the CMZ. The velocity profile toward star $\lambda$$-$ (2MASS J17482472-2824313) observed by GNIRS is particularly noteworthy. The sightline toward this star, located 115 pc to the east of Sgr~A*, shows the presence of warm diffuse gas near 0 radial velocity and complements an identical result at the west end (on sightlines toward $\alpha$+ and previously observed sources $\alpha$ and $\beta$). Stars nearer to the center of the CMZ show the warm diffuse gas at negative velocities only. Although many more stars need to be observed, the results to date suggest the existence of an expanding molecular ring of diffuse gas which is, unlike previously reported, not rotating but purely expanding.\footnote{Oka, T. 2013, Chem. Rev. \textbf{113} 8738