A Redshift 4.38 Mg II Absorber Toward BR 1202–0725^*

We have detected a strong (EW_obs = 20 Å) redshift 4.38 Mg II absorber toward the redshift 4.68 quasar BR 1202-0725. The identification of this feature as Mg II λλ2796, 2803 is supported by the identification of a damped Lyα absorber and ultraviolet Si II λ1526 absorption at the same redshift. The strengths of Mg II λλ2796, 2803 and Si II λ1526 are consistent with those of strong low-redshift Mg II systems, damped Lyα absorbers, and diffuse clouds within the Milky Way. C IV λλ1548, 1550 is not detected, indicating a lower ionization state than is seen in lower redshift Mg II absorbers and within the interstellar medium of our Galaxy. The detection of a single strong Mg II absorber at z > 4.2, among the three quasars observed with sufficient signal-to-noise ratio to detect an absorber, is consistent with the statistics and evolution of lower redshift Mg II absorbers. Thus, despite being at nearly twice the redshift of any previously identified Mg II absorber, this system appears quite similar to lower redshift Mg II systems that are identified with normal field galaxies. Because of the saturated nature of the Si II and Mg II lines, the metallicity of the gas is uncertain, but it is clear that some enrichment of Si and Mg, probably amounting to more than 1% of solar, has occurred by a redshift of 4.38.