  The Tycho supernova remnant (SNR), as one of the few historical SNRs, has
been widely studied in various wavebands. Observations show evidence that Tycho
is expanding in a medium with density gradient and possibly interacting with
dense ambient medium toward the northeast direction. From the FCRAO CO survey
of the outer Galaxy, we have identified a patch of molecular clouds in this
area and have conducted a follow-up observation with Nobeyama 45m radio
telescope. The high-resolution (16\arcsec) Nobeyama data shows that a large
molecular cloud surrounds the SNR along the northeastern boundary. We suggest
that the Tycho SNR and the molecular cloud are located in the Perseus arm and
that the dense medium interacting with the SNR is possibly the molecular cloud.
We also discuss the possible connection between the molecular cloud and the
Balmer-dominated optical filaments, and suggest that the preshock gas may be
accelerated within the cosmic ray and/or fast neutral precursor.
