Hot and diffuse clouds near the galactic center probed by metastable H ₃^{+ 1,2,3}

Using an absorption line from the metastable (J, K) = (3, 3) level of H₃⁺ together with other lines of H₃ ⁺ and CO observed along several sight lines, we have discovered 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 Galaxy, i.e., within 200 pc of the center. Approximately three-fourths of the H₃⁺ along the line of sight to the brightest source we observed, the Quintuplet object GCS 3-2, is inferred to be in the CMZ, with the remaining H₃⁺ located in intervening spiral arms. About half of the H₃⁺ in the CMZ has velocities near ∼-100 km s^-1, indicating that it is associated with the 180 pc radius expanding molecular ring, which approximately forms the outer boundary of the CMZ. The other half, with velocities of ∼-50 and ∼0 km s^-1, is probably closer to the center. CO is not very abundant in these clouds. Hot and diffuse gas in which the (3, 3) level is populated was not detected toward several dense clouds and diffuse clouds in the Galactic disk where large column densities of colder H₃ ⁺ have been reported previously. Thus, the newly discovered environment appears to be unique to the CMZ. The large observed H ₃⁺ column densities in the CMZ suggest an ionization rate much higher than in the diffuse interstellar medium in the Galactic disk. Our finding that the H₃⁺ in the CMZ is almost entirely in diffuse clouds indicates that the reported volume filling factor (f ≥ 0.1) for n ≥ 10⁴ cm^-3 clouds in the CMZ is an overestimate by at least an order of magnitude.