Ionization state of the absorption systems in the BL Lac object 0215 + 015 and properties of low-excitation absorbers

Spectra of the BL Lac object 0215 + 015 have been obtained with the ESO 3.6-m telescope at a resolution of 18 and 29 km s⁻¹ from 5650 to 6850 Å. Absorption lines of Mg I, Mg II, Mn II and Fe II are detected in the system at z = 1.345 known from previous observations. These lines break up into four components spread over 180 km s⁻¹ with observed equivalent widths of isolated features ranging from 35 to 2800 mÅ. Accurate estimates of the velocity structure and column densities of the singly ionized and neutral elements mentioned above are derived for the multiple system at z = 1.345 using a line-profile fitting procedure. No Mg II or Fe II absorption was found in the six other metal-rich systems known by their Lyα and C IV or Sim absorptions even though weak C II absorption has been reported for two of them. In the Lyαm-C IV range we find that the following are good indicators for the presence of Mg II and Fe II absorptions (i) strong absorption lines from O I and Si II, and if only Si II is detected the Si IIλλ 1304, 1526 lines must be present, (ii) equivalent width ratios W_r(C IIλ 1334)/W_r(C IVλ 1548) and W_r(Si IIλ 1260)/ Wr(Si ni/l 1206) of order unity or larger. We have estimated the occurrence of Mg II absorption in homogeneous surveys at high redshift. We find a density per unit redshift for Mg II systems of dN/dz = 0.29 at $$\langle z\rangle = 1.7$$, which is five times smaller than that derived for C IV systems. The comparison of our results with those obtained at lower redshift, $$\langle z\rangle = 0.5$$, suggests an absence of cosmological evolution for the Mg II systems.