MgII absorption systems in Sloan Digital Sky Survey QSO spectra

We present the results of a Mg II lambda lambda 2796, 2803 absorption-line survey using QSO spectra from the Sloan Digital Sky Survey (SDSS) Early Data Release. Over 1300 doublets with rest equivalent widths greater than 0.3 angstrom and redshifts 0.366 <= z <= 2.269 were identified and measured. We find that the lambda 2796 rest equivalent width (W-0(lambda 2796)) distribution is described very well by an exponential function partial derivative N/partial derivative W-9(lambda 2796) = (N*/W*) e(-W0/W*), with N* = 1.187 +/- 0.052 and W* = 0.702 +/- 0.017 angstrom. Previously reported power-law fits drastically overpredict the number of strong lines. Extrapolating our exponential fit underpredicts the number of W-0(lambda 2796) <= 0.3 angstrom systems, indicating a transition in partial derivative N/partial derivative W-0(lambda 2796) near W-0(lambda 2796) similar or equal to 0.3 angstrom. A combination of two exponentials reproduces the observed distribution well, suggesting that Mg II absorbers are the superposition of at least two physically distinct populations of absorbing clouds. We also derive a new redshift parameterization for the number density of W-0(lambda 2796) >= 0.3 angstrom lines: N* = 1.001 +/- 0.132 (1 + z)(0.226 +/- 0.170) and W* = 0.443 +/- 0.032 (1 + z)(0.634 +/- 0.097) angstrom. We find that the distribution steepens with decreasing redshift, with W* decreasing from 0.80 +/- 0.04 angstrom at z 1.6 to 0.59 +/- 0.02 angstrom at z = 0.7. The incidence of moderately strong (0.4 angstrom less than or similar to W-0(lambda 2796) less than or similar to 2 angstrom) Mg II lambda 2796 lines does not show evidence for evolution with redshift. However, lines stronger than approximate to 2 angstrom show a decrease relative to the no-evolution prediction with decreasing redshift for z less than or similar to 1. The evolution is stronger for increasingly stronger lines. Since W-0 in saturated absorption lines is an indicator of the velocity spread of the absorbing clouds, we interpret this as an evolution in the kinematic properties of galaxies from moderate to low redshift. Monte Carlo simulations do not reveal any strong systematic effects or biases in our results. While more recent SDSS QSO spectra offer the opportunity to increase the Mg II absorber sample by another order of magnitude, systematic errors in line identification and measurement will begin to dominate in the determination of absorber property statistics.