The ultraviolet luminosity function of star-forming galaxies between redshifts of 0.4 and 0.6

Abstract: We combine ultraviolet imaging of the 13H survey field, taken with the XMM-Newton Optical Monitor telescope (XMM-OM) and the Neil Gehrels Swift Observatory Ultraviolet and Optical Telescope (UVOT) in the UVM2 band, to measure rest-frame ultraviolet 1500A luminosity functions of star-forming galaxies with redshifts between 0.4 and 0.6. In total the UVM2 imaging covers a sky area of 641 square arcmin, and we detect 273 galaxies in the UVM2 image with 0.4<z<0.6. The luminosity function is fit by a Schechter function with best-fit values for the faint end slope alpha = -1.8 +0.4 -0.3 and characteristic absolute magnitude M* = -19.1 +0.3 -0.4. In common with XMM-OM based studies at higher redshifts, our best-fitting value for M* is fainter than previous measurements. We argue that the purging of active galactic nuclei from the sample, facilitated by the co-spatial X-ray survey carried out with XMM-Newton is important for the determination of M*. At the brightest absolute magnitudes (M1500<-18.5) the average UV colour of our galaxies is consistent with that of minimal-extinction local analogues, but the average UV colour is redder for galaxies at fainter absolute magnitudes, suggesting that higher levels of dust attenuation enter the sample at absolute magnitudes somewhat fainter than M*.