Strong Mg II and Fe II Absorbers at 2.2 < z < 6.0

Abstract

We present a study of strong intervening absorption systems in the near-IR spectra of 31 luminous quasars at z > 5.7. The quasar spectra were obtained with Gemini GNIRS that provide continuous wavelength coverage from ∼0.9 to ∼2.5 μm. We detect 32 strong Mg II doublet absorbers with rest-frame equivalent width W_r(λ2796) > 1.0 Å at 2.2 < z < 6.0. Each Mg II absorber is confirmed by at least two associated Fe II absorption lines in the rest-frame wavelength range of ∼1600-2600 Å. We find that the comoving line density (dN/dX) of the strong Fe II-bearing Mg II absorbers decreases toward higher redshift at z > 3, consistent with previous studies. Compared with strong Mg II absorbers detected in damped Lyα systems at 2 < z < 4, our absorbers are potentially less saturated and show much larger rest-frame velocity widths. This suggests that the gas traced by our absorbers are potentially affected by galactic superwinds. We analyze the Hubble Space Telescope near-IR images of the quasars and identify possible associated galaxies for our strong absorbers. There are a maximum of two galaxy candidates found within 5″ radius of each absorber. The median F105W-band magnitude of these galaxy candidates is 24.8 mag, which is fainter than the L* galaxy luminosity at z ∼ 4. By using our observed dN/dX of strong Mg II absorbers and galaxy candidates median luminosity, we suggest that at high redshift, strong Mg II absorbers tend to have a more disturbed environment but smaller halo size than that at z < 1.