Intermediate-mass black holes and ultraluminous X-ray sources in the Cartwheel ring galaxy

TL;DR

This study investigates whether intermediate-mass black holes can explain the ultraluminous X-ray sources in the Cartwheel galaxy, finding that most sources are unlikely to be powered by IMBHs based on simulations and required IMBH numbers.

Contribution

The paper combines galaxy interaction simulations with IMBH population analysis to assess their role in ULX phenomena in the Cartwheel galaxy.

Findings

Halo IMBHs do not account for observed X-ray sources.

Disc IMBHs are pulled into the outer ring during galaxy collision.

An unrealistically large number of IMBHs would be needed to explain all sources.

Abstract

Chandra and XMM-Newton observations of the Cartwheel galaxy show ~17 bright X-ray sources (>~5x10^38 erg s^-1), all within the gas-rich outer ring. We explore the hypothesis that these X-ray sources are powered by intermediate-mass black holes (IMBHs) accreting gas or undergoing mass transfer from a stellar companion. To this purpose, we run N-body/SPH simulations of the galaxy interaction which might have led to the formation of Cartwheel, tracking the dynamical evolution of two different IMBH populations: halo and disc IMBHs. Halo IMBHs cannot account for the observed X-ray sources, as only a few of them cross the outer ring. Instead, more than half of the disc IMBHs are pulled in the outer ring as a consequence of the galaxy collision. However, also in the case of disc IMBHs, accretion from surrounding gas clouds cannot account for the high luminosities of the observed sources. Finally, more than 500 disc IMBHs are required to produce <~15 X-ray sources via mass transfer from very young stellar companions. Such number of IMBHs is very large and implies extreme assumptions. Thus, the hypothesis that all the observed X-ray sources in Cartwheel are associated with IMBHs is hardly consistent with our simulations, even if it is still possible that IMBHs account for the few (<~1-5) brightest ultraluminous X-ray sources (ULXs).