A study of interstellar gas and stars in the gravitationally lensed galaxy 'the Cosmic Eye' from rest-frame ultraviolet spectroscopy

Abstract

We report the results of a study of the rest-frame ultraviolet (UV) spectrum of the Cosmic Eye (J213512.73−010143) , a luminous (L∼ 2L*) Lyman break galaxy at z_(sys)= 3.07331 magnified by a factor of ~25 via gravitational lensing by foreground mass concentrations at z= 0.73 and 0.33. The spectrum, recorded at high resolution and signal-to-noise ratio with the Echellette Spectrograph and Imager (ESI) spectrograph on the Keck II telescope, is rich in absorption features from the gas and massive stars in this galaxy. The interstellar absorption lines are resolved into two components of approximately equal strength and each spanning several hundred km s^(−1) in velocity. One component has a net blueshift of −70 km s^(−1) relative to the stars and H II regions and presumably arises in a galaxy-scale outflow similar to those seen in most star-forming galaxies at z= 2–3 . The other is more unusual in showing a mean redshift of +350 km s^(−1) relative to z_(sys); possible interpretations include a merging clump, or material ejected by a previous star formation episode and now falling back on to the galaxy, or more simply a chance alignment with a foreground galaxy. In the metal absorption lines, both components only partially cover the OB stars against which they are being viewed. However, there must also be more pervasive diffuse gas to account for the near-total covering fraction of the strong damped Lyα line, indicative of a column density N(HI) = (3.0 ± 0.8) × 10^(21) cm^(−2) . We tentatively associate this neutral gas with the redshifted component, and propose that it provides the dust 'foreground screen' responsible for the low ratio of far-infrared to UV luminosities of the Cosmic Eye. The C IV P Cygni line in the stellar spectrum is consistent with continuous star formation with a Salpeter initial mass function, stellar masses from 5 to 100 M_⊙ , and a metallicity Z ~ 0.4 Z_⊙. Compared to other well-studied examples of strongly lensed galaxies, we find that the young stellar population of the Cosmic Eye is essentially indistinguishable from those of the Cosmic Horseshoe and MS 1512−cB58 . On the other hand, the interstellar spectra of all three galaxies are markedly different, attesting to the real complexity of the interplay between starbursts and ambient interstellar matter in young galaxies observed during the epoch when cosmic star formation was at its peak.

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