Three low-redshift quasars with absorption systems: IUE and optical observations

We have discovered the first two cases of narrow absorption systems with an absorption redshift equal to the emission redshift (Δv ≲ 500 km s⁻¹) in the low-redshift quasars PKS 1912–54 (z_em = 0.401) and PKS 2135–14 (z_em = 0.200). We also confirm the existence of a sharp absorption system (z_ab ≪ z_em) in the spectrum of PKS 2128 – 12 (z_em = 0.4996). The three quasars have been observed both in the far-UV and in the optical. In the sharp line metal-containing system (z_ab ≪ z_em) the level of excitation is fairly low and a strong Lyman discontinuity is present at the absorption redshift. We may also have found the first Lyα system at low redshift in PKS 2128 – 12. If confirmed, this would imply an absence of any cut-off in the redshift distribution of these metal-deficient systems, and also suggests the possibility of detecting them directly by low-dispersion techniques. In the $$z_\text{ab}\simeq z_\text{em}$$ systems the excitation level is very high and comparable to that in the quasar broad line emission region. A strong Lyman discontinuity is also present in PKS 1912 – 54. The absorption lines in PKS 2135 – 14 are found to vary, showing some resemblance to NGC 4151. This quasar has a very close companion galaxy with high-excitation emission lines, and the interpretation of the absorption line variations requires further monitoring of the quasar. The UV continuum in PKS 2135 – 14 can be described by a steep spectrum component longward of 1950 Å, and a variable and much harder component at shorter wavelengths, similar to the situation in NGC 4151. In the two other quasars the continuum spectrum shows a smooth steepening with decreasing wavelengths. In all three quasars, Lyα is narrow when compared to C IV, Mg II and Hβ.