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 ultraviolet (UV) spectrum of the z= 2.38115 galaxy ‘the Cosmic Horseshoe’ (J1148+1930) at higher resolution and signal-to-noise ratio than is currently feasible for unlensed galaxies at z= 2–3. With a star formation rate of ∼100 M_⊙ yr⁻¹, dynamical mass M_vir≃ 1 × 10¹⁰ M_⊙, half-solar metallicity and moderate reddening E(B−V) = 0.15, the Cosmic Horseshoe is a good example of the population of galaxies responsible for most of the star formation activity at these redshifts.

From the analysis of stellar spectral features we conclude that a continuous mode of star formation with a Salpeter slope for stars in the mass range 5–100 M_⊙ gives a good representation of the UV spectrum, ruling out significant departures from a ‘standard’ initial mass function. Generally, we find good agreement between the values of metallicity deduced from stellar and nebular tracers. Interstellar absorption is present over a velocity range Δv≃ 1000 km s⁻¹, from −800 to +250 km s⁻¹ relative to the stars and their H ii regions, but we still lack a model relating the kinematic structure of the gas to its location within the galaxy. There is evidence to suggest that the outflowing interstellar gas is patchy, covering only ∼60 per cent of the UV stellar continuum.

The Lyα line shares many of the characteristics of the so-called Lyα emitters; its double-peaked profile can be reproduced by models of Lyα photons resonantly scattered by an expanding shell of gas and dust, with ∼10–15 per cent 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 Lyα 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 Lyα, whose appearance can be affected by a variety of different parameters.