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

Abstract

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, <i>±<i/> <i>0.0003<i/>, of the galaxy, and derive its stellar metallicity, , which is in excellent agreement with the metallicity determined from nebular emission lines. With a total stellar mass of , the 8 o'clock arc agrees with the mass-metallicity relation found for <i>z>2<i/> star-forming galaxies, although being located near the upper end of the distribution given its high mass and high metallicity. Broad HeII <i>λ<i/>1640 emission is found, indicative of the presence of Wolf-Rayet stars formed in an intense period of star formation. The 31 interstellar absorption lines detected led to the abundance measurements of 9 elements. The metallicity of the interstellar medium (ISM), (Si), is very comparable to the metallicity of stars and ionized gas, and suggests that the ISM of the 8 o'clock arc has been rapidly polluted and enriched by ejecta of OB stars. The ISM lines extend over a very large velocity range, km s<sup>-1<sup/>, from about -800 to <i>+300<i/> km s<sup>-1<sup/> relative to the systemic redshift, and have their peak optical depth blueshifted relative to the stars, implying gas outflows of km s<sup>-1<sup/>. The zero residual intensity in the strongest lines indicates a nearly complete coverage of the UV continuum by the ISM. The Ly<i>α<i/> line is dominated by a damped absorption profile on top of which is superposed a weak emission, redshifted relative to the ISM lines by about +690 km s<sup>-1<sup/> and resulting from multiply backscattered Ly<i>α<i/> photons emitted in the HII region surrounded by the cold, expanding ISM shell. A homogeneous spherical shell model with a constant outflow velocity, determined by the observations, is able to reproduce the observed Ly<i>α<i/> line profile. Furthermore, the required dust content, , is in good agreement with the attenuation measured from the Balmer decrement. These results obtained from the radiation transfer modeling of the Ly<i>α<i/> line in the 8 o'clock arc fully support the scenario proposed earlier, where the diversity of Ly<i>α<i/> line profiles in Lyman break galaxies and Ly<i>α<i/> emitters, from absorption to emission, is mostly due to variations of HI column density and dust content.