Abstract
Observations with the Owens Valley Radio Observatory interferometer show that BL Lac has a 21-cm hydrogen absorption line, produced by a local intervening cloud. The line is centered on zero velocity (LSR), has a width of about 5 km s−1, and shows an optical depth of 0.5. BL Lac is at galactic latitude b= −10°. No firm distance limit is possible, but such clouds are, typically, 100 pc apart. This observation is consistent with BL Lac being an extragalactic object. Other observations leading to a similar conclusion are of rotation measure (MacLeod and Andrew, 1968) and of reddening (Bertaud et al., 1969).
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