The comoving mass density of MgII from z ~ 2 to 5.5

We present the results of a survey for intervening MgII absorbers in the redshift range z ≃ 2-6 in the foreground of four high-redshift quasar spectra, 5.79 ≤ zem ≤ 6.133, obtained with the European Southern Observatory Very Large Telescope X-Shooter.We visually identify 52 MgII absorption systems and perform a systematic completeness and false positive analysis. We find 24 absorbers at ≥ 5σ significance in the equivalent width range 0.117 ≤ W2796 ≤ 3.655Åwith the highest redshift absorber at z=4.89031±4×10-5. For weak (W2796 < 0.3Å) systems, we measure an incidence rate dN/dz = 1.35 ± 0.58 at 〈z〉 = 2.34 and find that it the number expected from an exponential fit to stronger systems (W2796 > 0.3 Å).We find that there must be significant evolution in the absorption halo properties of MgII absorbers with W2796 > 0.1 Å by 〈z〉 = 4.77 and/or that they are associated with galaxies with luminosities beyond the limits of the current luminosity function at z ~ 5. We find that the incidence rate of strong MgII absorbers (W2796 > 1.0 Å) can be explained if they are associated with galaxies with L ≥ 0.29L* and/or their covering fraction increases. If they continue to only be associated with galaxies with L ≥ 0.50L* then their physical cross-section (σphys) increases from 0.015 Mpc2 at z = 2.3-0.041 Mpc2 at 〈z〉 = 4.77. We measure ΩMg II = 2.1-0.6+6.3× 10-8, 1.9-0.2+2.9 × 10-8 and 3.9-2.4+7.1 × 10-7 at 〈z〉 = 2.48, 3.41 and 4.77, respectively. At 〈z〉 = 4.77, ΩMg II exceeds the value expected from ΩH I estimated from the global metallicity of damped Ly α systems (DLAs) at z ≃ 4.85 by a factor of ~44 suggesting that either MgII absorbers trace both ionized and neutral gas and/or are more metal rich than the average DLA at this redshift.