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.
