The ionising continuum from active galactic nuclei ( AGN ) is fundamental for interpreting their broad emission lines and understanding their impact on the surrounding gas .
Furthermore , it provides hints on how matter accretes onto supermassive black holes .
Using HST ’ s Wide Field Camera 3 we have constructed the first stacked ultraviolet ( rest-frame wavelengths 600–2500Å ) spectrum of 53 luminous quasars at z \simeq 2.4 , with a state-of-the-art correction for the intervening Lyman forest and Lyman continuum absorption .
The continuum slope ( f _ { \nu } \propto \nu ^ { \alpha _ { \nu } } ) of the full sample shows a break at \sim 912 Å with spectral index \alpha _ { \nu } = -0.61 \pm 0.01 at \lambda > 912 Å and a softening at shorter wavelengths ( \alpha _ { \nu } = -1.70 \pm 0.61 at \lambda \leq 912 Å ) .
Our analysis proves that a proper intergalactic medium absorption correction is required to establish the intrinsic continuum emission of quasars .
We interpret our average ultraviolet spectrum in the context of photoionisation , accretion disk models , and quasar contribution to the ultraviolet background .
We find that observed broad line ratios are consistent with those predicted assuming an ionising slope of \alpha _ { \mathrm { ion } } = -2.0 , similar to the observed ionising spectrum in the same wavelength range .
The continuum break and softening are consistent with accretion disk plus X–ray corona models when black hole spin is taken into account .
Our spectral energy distribution yields a 30 % increase to previous estimates of the specific quasar emissivity , such that quasars may contribute significantly to the total specific Lyman limit emissivity estimated from the Ly \alpha forest at z < 3.2 .