Rest-frame ultraviolet spectrum of the gravitationally lensed galaxy “the 8 o'clock arc”: stellar and interstellar medium properties

We present the first detailed analysis of the rest-frame ultraviolet spectrum of the gravitationally lensed Lyman break galaxy (LBG), the “8 o'clock arc”, obtained with the intermediate-resolution X-shooter spectrograph recently commissioned on the ESO Very Large Telescope. Besides MS 1512–cB58, the Cosmic Horseshoe, and the Cosmic Eye, three other lensed LBGs at comparable redshifts, this is the fourth of such a study, usually unfeasible at high redshifts. The spectrum of the 8 o'clock arc is rich in stellar and interstellar features, and presents several similarities to the well-known MS 1512–cB58 LBG. The stellar photospheric absorption lines allowed us to constrain the systemic redshift, $z_{\rm sys} = 2.7350$ ± 0.0003, of the galaxy, and derive its stellar metallicity, $Z=0.82~Z_{\odot}$, which is in excellent agreement with the metallicity determined from nebular emission lines. With a total stellar mass of ${\sim}4.2$ $\times$ $10^{11}~M_{\odot}$, the 8 o'clock arc agrees with the mass-metallicity relation found for z>2 star-forming galaxies, although being located near the upper end of the distribution given its high mass and high metallicity. Broad HeI