Using an absorption line from the metastable ( J , K ) = ( 3 , 3 ) level of H _ { 3 } ^ { + } together with other lines of H _ { 3 } ^ { + } and CO observed along several sightlines , we have discovered 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 Galaxy , i.e. , within 200 pc of the center .
Approximately three-fourths of the H _ { 3 } ^ { + } 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 H _ { 3 } ^ { + } located in intervening spiral arms .
About half of H _ { 3 } ^ { + } in the CMZ has velocities near \sim - 100 km s ^ { -1 } indicating that it is associated with the 180 pc radius Expanding Molecular Ring which approximately forms outer boundary of the CMZ .
The other half , with velocities of \sim - 50 km s ^ { -1 } and \sim 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 H _ { 3 } ^ { + } 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 suggests an ionization rate much higher than in the diffuse interstellar medium in the Galactic disk .
Our finding that the H _ { 3 } ^ { + } in the CMZ is almost entirely in diffuse clouds indicates that the reported volume filling factor ( f \geq 0.1 ) for n \geq 10 ^ { 4 } cm ^ { -3 } clouds in the CMZ is an overestimate by at least an order of magnitude .