We investigate the possibility that present-day galactic haloes contain a population of massive black holes (MBHs) that form by hierarchical merging of the black hole remnants of the first stars. Some of the MBHs may be large enough or close enough to the centre of the galactic host that they merge within a Hubble time. We estimate to what extent this process could contribute to the mass of the super-massive black holes (SMBHs) observed in galactic centres today. Many MBHs will not reach the centre of the main halo, however, but continue to orbit within satellite subhaloes. Using a semi-analytical approach that explicitly accounts for dynamical friction, tidal disruption and encounters with the galactic disk, we follow the dynamics of the satellites and their MBHs and determine the abundance and distribution of MBHs in present-day haloes of various masses. Considering two different accretion scenarios we also compute the bolometric luminosity function for the MBHs.Comment: Accepted for publication in MNRAS, 11 pages, 11 figure

We investigate the possibility that present-day galaxies and their dark matter haloes contain a population of massive black holes (MBHs) that form by hierarchical merging of the black hole remnants of the first stars in the Universe. Some of the MBHs may be large enough or close enough to the centre of the galactic host that they merge within a Hubble time. We estimate to what extent this process could contribute to the mass of the supermassive black holes (SMBHs) observed in galactic centres today. The relation between SMBH and galactic bulge mass in our model displays the same slope as that found in observations. Many MBHs will not reach the centre of the host halo, however, but continue to orbit within it. In doing so MBHs may remain associated with remnants of the satellite halo systems of which they were previously a part. Using a semi-analytical approach that explicitly accounts for dynamical friction, tidal disruption and encounters with galactic discs, we follow the hierarchical merging of MBH systems and their subsequent dynamical evolution inside the respective host haloes. We predict the mass and abundance of MBHs in present-day galactic haloes, and also estimate the MBH mass accretion rates as well as bolometric luminosities for two accretion scenarios. MBHs that have not undergone recent merging will retain associated dark matter cusps that were enhanced by black hole accretion growth, and may be possible sources of gamma-rays via neutralino annihilations.

The first stars forming in minihaloes at redshifts z > 20 may have been very massive and could have left behind massive black hole (MBH) remnants. In previous papers we investigated the hierarchical merging of these 'seed' MBHs and their associated haloes, using a semi-analytical approach consisting of a hierarchical merger tree algorithm and explicit prescriptions for the dynamics of merged substructure inside a larger host halo following a merger. We also estimated accretion luminosities for these MBHs and found them to be consistent with observations of ultraluminous X-ray point sources. Here we compute the strength of gravitational wave events as MBHs merge to form the more massive black holes that we predict reside in galaxy haloes today. If MBHs merge efficiently, we predict that as many as 10 4 -10 5 events per year may fall within the sensitivity limits of the proposed Laser Interferometer Space Antenna gravitational wave observatory. The collapse of the first massive stars to form MBHs may also be accompanied by gamma-ray bursts (GRBs). If this is the case and if GRBs are observable out to the redshifts of first star formation, we predict that about 10 5 -10 6 GRBs per year could be detected. As merging MBH binaries reach their last stable orbits before final coalescence, a fraction of the gravitational wave energy may be released as a pulse of gamma-rays (for instance, through interaction with material enveloping a merging MBH binary). This fraction has to be larger than about 10 −2 for MBH mergers to account for some beamed GRBs, and greater than 10 −6 for the gamma-rays to be detectable out to cosmological distances with upcoming GRB detector missions.

The first stars forming in minihaloes at redshifts greater than 20 may have been very massive, and could have left behind massive black hole (MBH) remnants. In a previous paper we investigated the hierarchical merging of these MBHs and their associated haloes, using a semianalytical approach consisting of a hierarchical merger tree algorithm and explicit prescriptions for the dynamics of merged substructure inside a larger host halo following a merger. One of the results was the prediction of a number of MBHs orbiting throughout present-day galactic haloes. In addition, we estimated the mass-accretion rate of these MBHs, assuming that they retained around them a core of material from the original haloes in which they formed. On the basis of these estimates, in this paper we determine the bolometric, optical and X-ray luminosity functions for accreting MBHs, using thin-disc and advection-dominated accretion-flow models. Our predicted MBH X-ray fluxes are then compared with observations of ultraluminous X-ray sources in galaxies. We find that the slope and normalization of the predicted X-ray luminosity functions are similar to those observed, suggesting that MBHs could account for some fraction of these sources.

Abstract. Observations on galactic scales seem to be in contradiction with recent high resolution N -body simulations. This so-called cold dark matter (CDM) crisis has been addressed in several ways, ranging from a change in fundamental physics by introducing self-interacting cold dark matter particles to a tuning of complex astrophysical processes such as global and/or local feedback. All these efforts attempt to soften density profiles and reduce the abundance of satellites in simulated galaxy halos. In this contribution we are exploring the differences between a Warm Dark Matter model and a CDM model where the power on a certain scale is reduced by introducing a narrow negative feature ("dip"). This dip is placed in a way so as to mimic the loss of power in the WDM model: both models have the same integrated power out to the scale where the power of the Dip model rises to the level of the unperturbed CDM spectrum again. Using N -body simulations we show that that the new Dip model appears to be a viable alternative to WDM while being based on different physics: where WDM requires the introduction of a new particle species the Dip stems from a non-standard inflationary period. If we are looking for an alternative to the currently challenged standard ΛCDM structure formation scenario, neither the ΛWDM nor the new Dip model can be ruled out with respect to the analysis presented in this contribution. They both make very similar predictions and the degeneracy between them can only be broken with observations yet to come.