The environs of Tycho's supernova remnant explored through the H I 21 centimeter line

The detection of high-density H I that is very likely to be interacting with Tycho's supernova remnant (SNR) is reported. The interstellar medium around the remnant was surveyed using high-resolution (similar to 13 ") VLA archive observations in the 21 cm line of neutral hydrogen. We have found cold, dense gas at kinematic velocities compatible with the SNR, in excellent agreement with the locations where, based on earlier proper-motion expansion studies of the synchrotron emission, the shock front is expanding more slowly than most of the SNR shell. The site of lowest expansion velocity along the whole shell is coincident with a small H I clump observed as an absorption feature at upsilon = -51.5 km s(-1), with a density in the range 160-325 cm(-3). The clump seems to have been intercepted by the shock front in the recent past and has not yet undergone significant evaporation. Toward the western side of the SNR shell, which morphologically preserves spherical symmetry and expands at a higher rate, no H I features have been observed. We conclude that the presence of dense concentrations toward the east is currently slowing down the expansion of the shock front locally. These observations demonstrate that the properties of the local interstellar medium can affect the dynamical evolution of SNRs even at early stages.