Neutral Hydrogen 21 Centimeter Absorption at Redshift 0.673 toward 1504+377

We detect the 21 cm line of neutral hydrogen in absorption at a redshift of 0.673 toward the 1 Jy radio source 1504+377. The 1504+377 radio source is located toward the center of what appears to be an inclined disk galaxy at z = 0.674. The 21 cm absorption line shows multiple velocity components over a velocity range of about 100 km s^-1, with a total H I column density of N(H I) = 3.8 × 10¹⁹(T_s/f) cm^-2. The velocity-integrated optical depth of this system is the largest yet seen for redshifted H I 21 cm absorption-line systems. The 21 cm absorption line is coincident in redshift with a previously detected broad molecular absorption line system. We do not detect H I 21 cm absorption associated with the narrow molecular absorption line system at z = 0.67150, nor do we detect absorption at these redshifts by the 18 cm lines of OH, nor by the 2 cm transition of H₂CO.

There is no evidence for a bright optical active galactic nucleus (AGN) in 1504+377, suggesting significant obscuration through the disk--a hypothesis supported by the strong absorption observed. The 1504+377 system resembles the ``red quasar'' PKS 1413+135, which has been modeled as an optically obscured AGN with a very young radio jet in the center of a gas-rich disk galaxy. The presence of very bright radio jets at the centers of these two disk galaxies presents a challenge to unification schemes for extragalactic radio sources and to models for the formation of radio-loud AGNs.