  The youngest known Galactic supernova remnant (SNR) G1.9+0.3 has
high-velocity supernova shock beyond 10000 km s-1, and it is considered to be
one of the major candidates of a PeVatron. Despite these outstanding
properties, the surrounding interstellar matter of this object is poorly
understood. We investigated the interstellar gas toward G1.9+0.3 using the
12CO(J=3-2) data with the angular resolution of 15" obtained by the CHIMPS2
survey by the James Clerk Maxwell Telescope, and discovered three individual
clouds at -1, 7, and 45 km s-1. From its morphological and velocity structures,
the -1 km s-1 cloud, having the largest velocity width >20 km s-1 and located
at the distance of the Galactic Center, is possibly associated with the SNR.
The associated cloud shows a cavity structure both in space and velocity and
coincides well with the SNR. We found that the associated cloud has higher
column densities toward three bright, radio synchrotron-emitted rims where the
radial expansion velocity of the supernova shock is decelerated, and the cloud
is faint in the other parts of the SNR. This is the first direct evidence
indicating that the highly anisotropic expansion of G1.9+0.3 observed by
previous studies results from the deceleration by the interaction between the
supernova shock and surrounding dense interstellar medium.
