Galactic H i Column Densities toward Quasars and Active Galactic Nuclei

Abstract

We have determined accurate values of the Galactic neutral hydrogen column density, N_H_, toward 220 quasars and active galactic nuclei from 21 cm H I measurements made on the 140 Foot Telescope (42.7 m). Accurate values of N_H_ have now been obtained for the whole PG bright quasar sample and most quasars that have been observed by ROSAT and the Hubble Space Telescope through mid-1993. The spectra were corrected for stray 21 cm radiation yielding values of NH with a typical uncertainty of 1 x 1O^19^ cm^-2^ for high Galactic latitude directions. The H I column densities will be useful for correcting for interstellar opacity at UV and soft X-ray wavelengths, and for estimating the reddening and extinction toward these objects.