Accretion radiation from nearby isolated black holes

Recent work attempting to establish the presence of dark matter in the solar neighbourhood has led to renewed interest in the search for the nature of this matter. Bahcall et al. attempt to exclude large |$(\geq2\,{M}_{\odot})$| objects by considering their tidal effect on wide binaries. Here we provide independent constraints on such dark massive objects, if they are black holes, by the requirement that their radiation due to accretion from the ISM should not make the nearest ones directly observable as optical objects. We also predict the expected infrared brightness. Ipser & Price have studied the observability of very massive |$({10}^{5}\,{M}_{\odot})$| pregalactic black holes in our galactic halo. Their more recent accretion model calculations allow such constraints to be evaluated for lower mass holes. We show that halo holes must be less massive than about |${10}^{3}\,{M}_{\odot}$|⁠, and that the dark matter in the galactic disc cannot be made up of black holes of mass more than 10 M⊙. Even if black holes do not make up the dark matter,theyare expected to be present in the disc as remnants of massive stars. The failure to detect such holes so far is not in conflict with their presence provided the minimum mass MBH of their progenitors is more than 10 M⊙. More detailed analysis and improved observations could set more useful constraints on MBH, and if the holes do indeed exist some of them should be bright enough to be discovered and observed.

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