We present VLT/ISAAC near-infrared spectroscopy of two gravitationally-lensed z \sim 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 ] \lambda 3727 to H { \alpha } + [ N ii ] \lambda 6584 ) in star-forming galaxies 1 to 2 magnitudes fainter than in previous studies of Lyman break galaxies ( LBGs ) at z \sim 3 .
From the H { \alpha } luminosity , we estimate star formation rates ( SFRs ) of 30 and 15 M _ { \odot } yr ^ { -1 } for S2 and A2 respectively .
These values are 7 to 15 times higher than those inferred from the UV continuum flux at 1500Å without dust extinction correction .
In setting SFR _ { H \alpha } \sim SFR _ { UV } , one derives extinction coefficients E ( B-V ) \sim 0.3 for S2 and E ( B-V ) \sim 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 \sim 3 .
S2 is a low-metallicity object ( Z \sim 0.03 Z _ { \odot } ) with a low N/O ratio , similar to those derived in the most metal-poor nearby H ii galaxies .
In contrast , A2 is a high-metallicity galaxy ( Z \sim 1.3 Z _ { \odot } ) 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 \times 10 ^ { 10 } M _ { \odot } , for S2 and A2 respectively .
Thanks to the gravitational amplification , the line profiles of S2 are spatially resolved , leading to a velocity gradient of \pm 240 km s ^ { -1 } , which yields a dynamical mass of \sim 1.3 \times 10 ^ { 10 } M _ { \odot } within the inner 1 kpc radius .
Combining these new data with the sample of LBGs at z \sim 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 / L _ { B } and reddening .
High-redshift galaxies of different luminosities could thus have quite different star formation histories .