The evolution of the UV luminosity function of globular clusters in the E-MOSAICS simulations

We present the evolution of the rest-frame ultraviolet (UV) properties of the globular cluster (GC) populations and their host galaxies formed in the E-MOSAICS suite of cosmological hydrodynamical simulations. We compute the luminosities of all clusters associated with 25 simulated Milky-Way-mass galaxies, discussed in previous works, in the rest-frame UV and optical bands by combining instantaneous cluster properties (age, mass, metallicity) with simple stellar population models, from redshifts z = 0 to 10. Due to the rapid fading of young stellar populations in the UV, most of the simulated galaxies do not host GCs bright enough to be individually identified in deep Hubble Space Telescope observations, even in highly magnified systems. The median age of the most UV-luminous GCs is <10 Myr (assuming no extinction), increasing to ≳100 Myr for red optical filters. We estimate that these GCs typically only contribute a few per cent of the total UV luminosity of their host galaxies at any epoch. We predict that the number density of UV-bright proto-GCs (or cluster clumps) will peak between redshifts z = 1 and 3. In the main progenitors of Milky-Way-mass galaxies, 10-20 per cent of the galaxies at redshifts 1 ≲ z ≲ 3 have clusters brighter than M_UV < -15, and less than 10 per cent at other epochs. The brightest cluster in the galaxy sample at z > 2 is typically M_UV ̃ -16, consistent with the luminosities of observed compact high-redshift sources....