Two New Low-Redshift 21 Centimeter Absorbers

As part of a larger program to identify low-redshift radio analogs of the damped Lyα absorbers seen in the spectra of high-redshift quasars, Westerbork Synthesis Radio Telescope observations have discovered two new H I 21 cm absorption lines at z = 0.394 and z = 0.437 in the spectra of the radio sources B0248+430 and B1243-072, respectively. These sight lines and redshifts were selected for study on the basis of the previously known low-ionization absorption lines of Mg II, and neither has been observed in the Lyα line. The 21 cm line observations provide information on the column densities, temperatures, and kinematics of the thickest cold neutral clouds in the absorbers.