The Gemini Deep Deep Survey. II. Metals in star-forming galaxies at redshift 1.3 z 2

The goal of the Gemini Deep Deep Survey (GDDS) is to study an unbiased sample of K < 20.6 galaxies in the redshift range 0.8 < z < 2.0. Here we determine the statistical properties of the heavy-element enrichment in the interstellar medium (ISM) of a subsample of 13 galaxies with 1.34 < z < 1.97 and UV absolute magnitude M2000 < -19.65. The sample contains 38% of the total number of identified galaxies in the first two fields of the survey with z > 1.3. The selected objects have colors typical of irregular and Sbc galaxies. Strong [O II] emission indicates high star formation activity in the H II regions (star formation rate ∼13-106 M⊙ yr -1). The high signal-to-noise ratio composite spectrum shows strong ISM Mg II and Fe II absorption, together with weak Mn II and Mg I lines. The Fe II column density, derived using the curve of growth analysis, is log N Fe II = 15.540.13+0.23. This is considerably larger than typical values found in damped Lyα systems (DLAs) along QSO sight lines, where only 10 of 87 (∼11%) have log NFe II ≥ 15.2. High Fe II column densities are observed in the z = 2.72 Lyman break galaxy cB58 (log NFe II ≃ 15.25) and in gamma-ray burst host galaxies (log NFe II ∼ 14.8 - 15.9). Given our measured Fe II column density and assuming a moderate iron dust depletion (δFe ∼ 1 dex), we derive an optical dust extinction of AV ∼ 0.6. If the H I column density is log NHI < 21.7 (as in 98% of DLAs), then the mean metallicity is Z/Z⊙ > 0.2. The high completeness of the GDDS sample implies that these results are typical of star-forming galaxies in the 1 < z < 2 redshift range, an epoch that has heretofore been particularly challenging for observational programs.