Physical properties of two low-luminosity z ∼ 1.9 galaxies behind the lensing cluster AC 114

Abstract

We present VLT/ISAAC near-infrared spectroscopy of two gravitationally-lensed z ∼ 1.9 galaxies, A2 and S2, located behind the cluster AC 114. Thanks to large magnification factors, we have been successful in detecting rest-frame optical emission lines (from [O II] λ3727 to Hα+[N II] λ6584) in star-forming galaxies 1 to 2 mag fainter than in previous studies of Lyman break galaxies (LBGs) at z ∼ 3. From the Hα luminosity, we estimate star formation rates (SFRs) of 30 and 15 M⊙ yr-1 for S2 and A2 respectively. These values are 7 to 15 times higher than those inferred from the UV continuum flux at 1500 Å without dust extinction correction. In setting SFRHα ∼ SFRUV, one derives extinction coefficients E(B - V) ∼ 0.3 for S2 and E(B - V) ∼ 0.4 for A2. The behavior of S2 and A2 in terms of O/H and N/O abundance ratios are very different, and they are also different from typical LBGs at z ∼ 3. S2 is a low-metallicity object (Z ∼ 0.03 Z⊙) with a low N/O ratio, similar to those derived in the most metal-poor nearby HII galaxies. In contrast, A2 is a high-metallicity galaxy (Z ∼ 1.3 Z⊙) with a high N/O abundance ratio, similar to those derived in the most metal-rich starburst nucleus galaxies. The line-of-sight velocity dispersions, derived from emission line widths, are 55 and 105 km s-1, yielding a virial mass of 0.5 and 2.4 × 1010 M⊙, for S2 and A2 respectively. Thanks to the gravitational amplification, the line profiles of S2 are spatially resolved, leading to a velocity gradient of ±240 km s-1, which yields a dynamical mass of ∼1.3 × 1010 M⊙ within the inner 1 kpc radius. Combining these new data with the sample of LBGs at z ∼ 3, including the lensed galaxy MS 1512-cB58, which is the only LBG for which physical properties have been determined with similar accuracy, we conclude that these three galaxies exhibit different physical properties in terms of abundance ratios, SFRs, M/LB and reddening. High-redshift galaxies of different luminosities could thus have quite different star formation histories.