Optical absorption lines in the high redshift BL Lac object 0215 + 015

We present results from our study of 0215 + 015 (PKS 0215 + 015), the highest redshift and most luminous BL Lac object so far detected. Optical continuum measurements show that the object had brightened to 15.2 mag on 1980 September 10, thus confirming an overall variability range of more than 4 mag. Variations on a time-scale of ⩽ 2 months may also be present. A review of the published radio data indicates that 0215 + 015 has a corehalo or multi-component structure, similar to that found for the BL Lac 0735 + 178. Four absorption redshift systems occur with z_a = 1.254, 1.345, 1.549 and 1.649. The z_a = 1.345 system exhibits a rich, mixed-ionization spectrum whilst the other three are predominantly C IV systems. All four show complex structure when observed at high resolution ( ~ 0.65 Å FWHM). Most remarkable are the z_a = 1.549 system which splits into three C IV components with average separation, $$\bar{\Delta\upsilon} \sim 130 \,\text{km} \,\text{s}^{-1} \,\text{and the} \,z_{a} = 1.649$$ system to which we fit seven C IV components with $$\bar{\Delta \upsilon} \sim 110 \,\text{km} \,\text{s}^{-1}.$$ Individual C IV clouds are narrow, having internal velocity dispersions (b values) in the range 15–55 km s^–1, the lower values being similar to that produced by thermal broadening in gas at 10⁵ K. Column densities are in the range N(C IV) = $$2-20 \times 10^{13} \,\text{cm}^{-2}.$$ Possible origins of the absorption systems are discussed. Column densities of the z_a = 1.345 system are very similar to those found for our Galaxy in sight-lines towards the Magellanic Clouds, and an origin in an intervening galaxy seems almost certain for this system. The velocity structure and linewidths of the 1.649 system are less plausibly due to intervening galaxies, but are consistent with the intrinsic absorption model of Dyson, Falle & Perry (1980).