ULTRALUMINOUS X-RAY SOURCES: EVIDENCE FOR VERY EFFICIENT FORMATION OF POPULATION III STARS CONTRIBUTING TO THE COSMIC NEAR-INFRARED BACKGROUND EXCESS?

摘要

Accumulating evidence indicates that some ultraluminous X-ray sources (ULXs) are intermediate-mass black holes (IMBHs), but the formation process of IMBHs is unknown. One possibility is that they were formed as remnants of Population III (Pop. III) stars, but it has been thought that the probability of being a ULX is too low for IMBHs distributed in galactic halos to account for the observed number of ULXs. Here we show that the number of ULXs can be explained by such halo IMBHs passing through a dense molecular cloud if Pop. III star formation is very efficient as recently suggested by the excess of the cosmic near-infrared background radiation that cannot be accounted for by normal galaxy populations. We calculate the luminosity function of X-ray sources in our scenario and find that it is consistent with observed data. Our scenario can explain the fact that ULXs are preferentially found at the outskirts of large gas concentrations in star-forming regions. A few important physical effects are pointed out and discussed, including gas dynamical friction, radiative efficiency of the accretion flow, and radiative feedback to the ambient medium. ULXs could last for ～10~5—10~6 yr to emit a total energy of ～10~(53) ergs, which is sufficient to power the ionized expanding nebulae found by optical observations.