Million solar mass black holes at high redshift

The existence of quasars at redshift z > 5 indicates that supermassive black holes have been present since very early times. If they grew by accretion, the seeds of mass ≳10⁵ M_solar must have formed at z~9. These seed black holes may result from the collapse and dissipation of primordial gas during the early stages of galaxy formation. I discuss the effects of magnetic fields on the fragmentation of cold gas clouds embedded in a hot diffuse phase and a virialized dark matter halo. The field of 10^-4 G ejected by supernova remnants can halt cloud break-up at 10⁴ M_solar. High star formation rates keep the clouds partially ionized, making ambipolar diffusion inefficient. The magnetically supported clouds collapse into black holes, which later spiral via dynamical friction into a central cluster with total mass M_bh~6×10⁶ M_solar. As the cluster collapses, the black holes merge, emitting gravitational radiation that should be detectable by LISA.