A ULX in NGC 4559: a ``mini-cartwheel'' scenario?

Abstract

We have studied the peculiar environment around a ULX in NGC 4559 (with L[x] ≈ 2×10⁴⁰ erg s-1 and M[BH] ≳ 50M_⊙). The X-ray source is located near the rim of a young (age < 30 Myr), large (diameter ≈ 700 pc) ring-like star forming complex possibly triggered by the impact of a dwarf satellite galaxy through the gas-rich outer disk of NGC 4559. We speculate that galaxy interactions (including the infall of high-velocity clouds and satellites on a galactic disk) and low-metallicity environments offer favourable conditions for the formation of compact remnants more massive than ``standard'' X-ray binaries, and accreting from a massive Roche-lobe filling companion.