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 signalto-noise ratio rest-frame spectra covered by the H+ K band, we detected Hβ, [O iii], Hα, [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 Hα and Hβ 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 ∼ 2 star-forming galaxies. From the flux ratio of [S ii]λ6717 and [S ii]λ6732, we found that the electron number density of the H ii regions in the high-z galaxy was ≃1000 cm^-3, consistent with other z ∼ 2 galaxies butmuch higher than that in local H ii regions. The star formation rate was estimated via the Hα luminosity, after correction for the lens magnification, to be about 365±69M_⊙ yr^-1. Combining the FWHM of Hα emission lines and the half-light radius, we found that the dynamical mass of the lensed galaxy is (5.8 ± 0.9) × 10¹⁰M _⊙. The gas mass is (5.1 ± 1.1) × 10 ¹⁰M_⊙ from the Hα flux surface density using global Kennicutt-Schmidt law, indicating a very high gas fraction of 0.79 ± 0.19 in J0900+2234.