Strong neutral hydrogen 21 cm absorption has been discovered at z = 0.24671 +/- 0.00001 in the radio spectrum of the BL Lac object PKS 1413 + 135. The implied H I column density is N(H I) = 1.3 x 10^19^ x (T_s_/f) cm^-2^, where T_s_ is the spin temperature of the gas and f is the H I covering factor. Given that the source is well resolved on VLBI scales, it is likely that f <= 0.1. This observation supports the conclusions of Stocke et al. (1992) that the extremely red spectrum of this BL Lac object in the near-infrared is due to a large amount of extinction and that the "host" galaxy is a spiral of type later than S0.
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Carilli et al. (1992) studied this question.