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The Ultraviolet Spectrum of the Gravitationally Lensed Galaxy `The Cosmic Horseshoe': A Close-up of a Star-forming Galaxy at z = 2

Taking advantage of strong gravitational lensing, we have recorded the rest-frame UV spectrum of the z = 2.38115 galaxy `The Cosmic Horseshoe' (J1148+1930) at higher resolution and S/N than is currently feasible for unlensed galaxies at z = 2 -3. From the analysis of stellar spectral features, we conclude that a continuous mode of star formation with a Salpeter slope gives a good representation of the UV spectrum, ruling out significant departures from a `standard' IMF. Generally, we find good agreement between the values of metallicity deduced from stellar and nebular tracers. Interstellar absorption is present over a velocity range of 1000 km/s, from -800$ to +250 km/s relative to the stars and their H II regions, and there is evidence that the outflowing interstellar gas may be patchy, covering only 60% of the UV stellar continuum. The Lya line shares many of the characteristics of the so-called Lya emitters. Its double-peaked profile can be reproduced by models of Lya photons resonantly scattered by an expanding shell of gas and dust, with 10-15% of the photons escaping the galaxy. Many of the physical properties of the Cosmic Horseshoe are similar to those of the only other galaxy at z = 2-3 studied in comparable detail up to now: MS 1512-cB58. The fact that these two galaxies have drastically different Lya lines may be due simply to orientation effects, or differences in the covering factor of outflowing gas, and cautions against classifying high-z galaxies only on the basis of spectral features, such as Lya, whose appearance can be affected by a variety of different parameters.