(Proposal ID) S15A-121

(PI) Oka, Takeshi

(Proposal Title)
 Dynamics of the Gas in the Central Molecular Zone of the GC probed by H_3^+ 

(Abstract)
We propose to observe infrared spectra of L-band H_3^+ and K-band CO toward 8 
stars in the Central Molecular Zone (CMZ) of the Galactic center using IRCS. 
The observations will increase the extent and continuity of the longitude-
velocity diagram which is beginning to indicate uniformly expanding gas with a 
radius of ~140pc whose front is moving with the speed of ~140km s^{-1}. This 
gas dynamic is incompatible with the current consensus of gas motion in the 
barred gravitational potential proposed by Binney et al. in 1991 and revives 
the idea of the Expanding Molecular Ring proposed by Kaifu et al. and Scoville 
in 1972. It suggests violent explosions in the last few million years. The 
observation will also provide information on the location, dimension, and the 
volume filling factor of the warm and diffuse gas which is now believed to be 
the dominant in the CMZ  and define the extent of the high H_2 ionization rate.