Strong [O III] and [N II] Optical Emission Lines from a Suspected BlackHole In A Globular Cluster In the Elliptical Galaxy NGC1399: Evidence ForStellar Disruption?

We report [O III] lambda-5007 and [N II] lambda-6584 emission from the globular cluster harboring the ultraluminous X-ray source (ULX) CXOJ033831.8-352604 in the Fornax elliptical galaxy NGC1399. The odds of finding either optical emission lines or ULXs separately within a globular cluster are small enough that the optical emission lines are undoubtedly associated with the ULX. The high (10 times Eddington), steady X-ray luminosity of the source strongly argues that it is an accreting black hole rather than a neutron star. We investigate scenarios for which the ULX could produce forbidden line emission lines without the benefit of significant interstellar gas surrounding it in its gas-poor globular cluster environment. One intriguing 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, with this debris also feeding the the black hole leading to the observed X-ray emission. The lack of accompaning H-alpha and H-beta emission lines in the optical spectrum could imply that the disrupted star lacked hydrogen, such as a white dwarf.