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 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 (Eg(B − V)) was computed from the flux ratio of Hα and Hβ and appears to be much higher than that toward the stellar continuum (Es(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 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 but much 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 ±  69 M☉ 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) × 1010M☉ 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.