We consider the observational properties of a static black hole space-time immersed in a dark matter envelope. We thus investigate how the modifications to geometry, induced by the presence of dark matter affect the luminosity of the black holes accretion disk. We show that the same disks luminosity produced by a black hole in vacuum may be produced by a smaller black hole if surrounded by dark matter under certain conditions. In particular, we demonstrate that the luminosity of the disk is markedly altered by dark matters presence, suggesting that mass estimation of distant super-massive black holes may be changed if they are immersed in dark matter. We argue that a similar effect holds in more realistic scenarios and we discuss about the refractive index related to dark matter lensing. Hence we show how this may help explain the observed luminosity of super-massive black holes in the early universe.
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