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

Abstract

We present rest-frame optical images and spectra of the gravitationally\nlensed, star-forming galaxy J0900+2234 (z=2.03). The observations were\nperformed with the newly commissioned LUCIFER1 near-infrared instrument mounted\non the Large Binocular Telescope (LBT). We fit lens models to the rest-frame\noptical images and find the galaxy has an intrinsic effective radius of 7.4 kpc\nwith a lens magnification factor of about 5 for the A and B components. We also\ndiscovered a new arc belonging to another lensed high-z source galaxy, which\nmakes this lens system a potential double Einstein ring system. Using the high\nS/N rest-frame optical spectra covering H+K band, we detected Hbeta, OIII,\nHalpha, NII and SII emission lines. Detailed physical properties of this high-z\ngalaxy were derived. The extinction towards the ionized HII regions (E_g(B-V))\nis computed from the flux ratio of Halpha and Hbeta and appears to be much\nhigher than that towards stellar continuum (E_s(B-V)), derived from the optical\nand NIR broad band photometry fitting. The metallicity was estimated using N2\nand O3N2 indices. It is in the range of 1/5-1/3 solar abundance, which is much\nlower than the typical z~2 star-forming galaxies. From the flux ratio of SII\n6717 and 6732, we found that the electron number density of the HII regions in\nthe high-z galaxy were &gt;1000 cm^-3, consistent with other z~2 galaxies but much\nhigher than that in local HII regions. The star-formation rate was estimated\nvia the Halpha luminosity, after correction for the lens magnification, to be\nabout 365\\pm69 Msun/yr. Combining the FWHM of Halpha emission lines and the\nhalf-light radius, we found the dynamical mass of the lensed galaxy is\n5.8\\pm0.9x10^10 Msun. The gas mass is 5.1\\pm1.1x10^10~Msun from the H\\alpha\nflux surface density by using global Kennicutt-Schmidt Law, indicating a very\nhigh gas fraction of 0.79\\pm0.19 in J0900+2234.\n