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

Abstract

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 similar to 4-8 kpc(2) 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 similar to +/- 50 km s(-1) 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 less than or similar to v (km s-1) less than or similar to-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 (similar to 8-16 M-circle dot yr(-1)), 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 (Ne)= 3.92-4.36 cm(-3), and point out that such high densities may be common when measured using the CIII] 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.