Number density evolution of strong, wr(2796) > 1.0 Å (top panel), and weak, 0.3 < wr(2796) < 1.0 Å (bottom panel), Mg ii systems towards blazars (blue squares), GRBs (red small triangles) as reported by Vergani et al. (2009) for their overall sample (with a downwards correction for the absolute values of strong systems: Vergani, priv. comm.) and SDSS QSOs (solid line) as given by Prochter et al. (2006) for strong systems and Nestor (priv. comm.) for weaker systems.
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