LBT/LUCIFER OBSERVATIONS OF THE z ∼ 2 LENSED GALAXY J0900+2234

We present rest-frame optical images and spectra of the gravitationally lensed, star-forming galaxy J0900+2234 (z = 2.03). The observations were performed with the newly commissioned LUCIFER1 near-infrared (NIR) instrument mounted on the Large Binocular Telescope. We fitted lens models to the rest-frame optical images and found that the galaxy has an intrinsic effective radius of 7.4 +/- 0.8 kpc with a lens magnification factor of about 5 for the A and B components. We also discovered a new arc belonging to another lensed high-z source galaxy, which makes this lens system a potential double Einstein ring system. Using the high signal-to-noise ratio rest-frame spectra covered by the H+K band, we detected H beta, [O III], H alpha, [N II], and [S II] emission lines. Detailed physical properties of this high-z galaxy were derived. The extinction toward the ionized H II regions (E-g (B - V)) was computed from the flux ratio of H alpha and H beta and appears to be much higher than that toward the stellar continuum (E-s (B - V)), derived from the optical and NIR broadband photometry fitting. The metallicity was estimated using N2 and O3N2 indices. It is in the range of 1/5-1/3 solar abundance, which is much lower than for typical z similar to 2 star-forming galaxies. From the flux ratio of [S II]lambda 6717 and [S II]lambda 6732, we found that the electron number density of the H II regions in the high-z galaxy was similar or equal to 1000 cm(-3), consistent with other z similar to 2 galaxies butmuch higher than that in local H II regions. The star formation rate was estimated via the Ha luminosity, after correction for the lens magnification, to be about 365 +/- 69 M-circle dot yr(-1). Combining the FWHM of Ha emission lines and the half-light radius, we found that the dynamical mass of the lensed galaxy is (5.8 +/- 0.9) x 10(10) M-circle dot. The gas mass is (5.1 +/- 1.1) x 10(10) M-circle dot from the Ha flux surface density using global Kennicutt-Schmidt law, indicating a very high gas fraction of 0.79 +/- 0.19 in J0900+2234.