Massive black hole remnants of the first stars - II. Optical and X-ray signatures in present-day galactic haloes

Abstract

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.

GRAVITATIONAL WAVES AND STALLED SATELLITES FROM MASSIVE GALAXY MERGERS ATz⩽ 1

“…However, the possibility of "stalled" mergers, where a smaller satellite black hole resides in the outer regions of a massive host galaxy, is an interesting candidate as a model for ULXs. Indeed, several authors have studied the dynamics of stalled mergers and their viability as ULX sources (Islam et al 2003(Islam et al , 2004a(Islam et al , 2004b(Islam et al , 2004cVolonteri & Perna 2005). Given the rarity of very massive galaxies, the probability of a merger with a comparable-mass galaxy is small.…”

X-ray Source Populations in Galaxies

“…In particular, the slope and normalization of the high-luminosity XLFs of star-forming galaxies have been reproduced in both IMBH (Islam, Taylor & Silk 2004;Krolik 2004) and jet models (Koerding, Colbert & Falke 2004). Zezas & Fabbiano (2002) discuss the effect of either a beamed population of ULXs or a population of IMBH in the context of the XLF of the Antennae.…”

“…IMBHs may be remnants of collapse in the early universe (e.g., Van der Marel 2004;Islam, Taylor & Silk 2004), or may result from the collapse of dense stellar clusters (e.g., Gurcan, Freitag & Rasio 2004;). In the cosmological remnant options, one would expect IMBHs to be particularly abundant in the more massive elliptical galaxies, contrary to the observed association with star-forming galaxies (Zezas & Fabbiano 2002).…”

Ultraluminous X‐Ray Sources: Evidence for Very Efficient Formation of Population III Stars Contributing to the Cosmic Near‐Infrared Background Excess?

“…The mass function of IMBHs in the local universe may have been altered from the original one at the formation epoch, by merging or gas accretion in the hierarchical structure formation. Such calculations have been performed by Islam et al (2004) and Volonteri & Perna (2005). According to these calculations, the typical mass of free-floating IMBHs may be increased up to $10 4 M depending on model parameters, even if the initial mass of Pop.…”

“…However, it is not easy to explain the observed number of ULXs by this scenario. Islam et al (2004) considered a possibility that free-floating IMBHs accrete from a residual core of gas associated with the IMBHs from their formation epoch. It is highly uncertain whether such a gas core can survive until the present from the early universe, and this scenario cannot explain the majority of ULXs that are associated with star-forming regions.…”