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Description
The rest-frame far to extreme ultraviolet (UV) colour-redshift relationship has been constructed from data on over 480,000 quasars carefully cross-matched between SDSS Data Release 14 and the final GALEX photometric catalog. UV matching and detection probabilities are given for all the quasars, including dependencies on separation, optical brightness, and redshift. Detection limits in the UV bands are also provided for all objects. The UV colour distributions are skewed redward at virtually all redshifts, especially when detection limits are accounted for. The median GALEX far-UV minus near-UV (FUV-NUV) colour-redshift relation is reliably determined up to z~2.8, corresponding to rest-frame wavelengths as short as 400{AA}. Extreme-UV (EUV) colours are substantially redder than found previously, when detection limits are properly accounted for. Quasar template spectra were forward modeled through the GALEX bandpasses, accounting for intergalactic opacity, intrinsic reddening, and continuum slope variations. Intergalactic absorption by itself cannot account for the very red EUV colours. The colour-redshift relation is consistent with no intrinsic reddening, at least for SMC-like extinction. The best model fit has a far-UV continuum power-law slope of -0.34+/-0.03 consistent with previous results, but an EUV slope of -2.90+/-0.04 that is much redder and inconsistent with any previous composite value (all >-2.0). The EUV slope difference can be attributed in part to the tendency of previous studies to preferentially select UV brighter and bluer objects. The weak EUV flux suggests quasar accretion disc models that include outflows such as disc winds.
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