Evidence for a Stellar Disruption by an IMBH in an Extragalactic Globular Cluster

Abstract

We report [O III] 5007 Angstrom and [N II] 6583 Angstrom emission from a globular cluster harboring the ultraluminous X-ray source (ULX) CXOJ033831.8-352604 in the Fornax elliptical galaxy NGC1399. No accompanying Balmer emission lines are present in the spectrum. One possibility is that the forbidden lines emanate from X-ray illuminated debris of a star that has been tidally-disrupted by an intermediate-mass black hole (IMBH), with this debris also feeding the black hole leading to the observed X-ray emission. The line strengths indicate that the minimum size of the emitting region is ~10^15 cm, and if the 70 km/s half-widths of the emission lines represent rotation around the black hole, a minimum black hole mass of 1000 solar mass is implied. The non-detection of H-alpha and H-beta emission lines suggests a white dwarf star was disrupted, although the presence of strong nitrogen emission is somewhat of a mystery.