VLA observations of 21 centimeter H I absorption toward cassiopeia A

We have used the Very Large Array to map the 21 cm neutral hydrogen absorption in the Perseus spiral arm feature toward the supernova remnant Cassiopeia A. The velocity resolution of the data is 0.6 km s^-1, and the final angular resolution displayed is 7″ (HPBW), which corresponds to a linear size of 0.10 pc at the adopted distance of Cas A (3 kpc). We present optical depth images as gray scale with contours superposed, for 41 spectral channels showing H I absorption. We also show representative optical depth spectra as a function of position over Cas A. The images represent a factor of 11 improvement in angular resolution over the earlier aperture synthesis work of Greisen, and a factor of 4 over the H I results reported by Goss, Kalberia, and Dickel. Our H I images show a great deal of structure in the absorbing gas, with the morphology dominated by elongated structures, appearing as loops, arcs, and filaments. We briefly discuss the implications of our results in terms of the structure of the interstellar medium, and of the relationship between the atomic and molecular gas in the Perseus arm features toward Cas A.