Mapping UV properties throughout the Cosmic Horseshoe: lessons from VLT-MUSE

We present the first spatially resolved rest-frame ultraviolet (UV) study of the gravitationally lensed galaxy, the `Cosmic Horseshoe' (J1148+1930) at z = 2.38. Our gravitational lens model shows that the system is made up of four star-forming regions, each ∼4-8 kpc<sup>2</sup> in size, from which we extract four spatially exclusive regional spectra. We study the interstellar and wind absorption lines, along with C III] doublet emission lines, in each region to investigate any variation in emission/absorption line properties. The mapped C III] emission shows distinct kinematical structure, with velocity offsets of ∼±50 km s<sup>-1</sup> between regions suggestive of a merging system, and a variation in equivalent width that indicates a change in ionization parameter and/or metallicity between the regions. Absorption line velocities reveal a range of outflow strengths, with gas outflowing in the range -200 ≲ v (km s<sup>-1</sup>) ≲ -50 relative to the systemic velocity of that region. Interestingly, the strongest gas outflow appears to emanate from the most diffuse star-forming region. The star formation rates remain relatively constant (∼8-16 M<sub>⊙</sub> yr<sup>-1</sup>), mostly due to large uncertainties in reddening estimates. As such, the outflows appear to be `global' rather than `locally' sourced. We measure electron densities with a range of log (N_e) = 3.92-4.36 cm^-3, and point out that such high densities may be common when measured using the C III] doublet due to its large critical density. Overall, our observations demonstrate that while it is possible to trace variations in large-scale gas kinematics, detecting inhomogeneities in physical gas properties and their effects on the outflowing gas may be more difficult. This study provides important lessons for the spatially resolved rest-frame UV studies expected with future observatories, such as James Webb Space Telescope.