Spatially resolved spectroscopy of the globular cluster RZ 2109 and the nature of its black hole*

Abstract

We present optical Hubble Space Telescope/Space Telescope Imaging Spectrograph ($HST/S\text{TIS}$) spectroscopy of RZ 2109, a globular cluster (GC) in the elliptical galaxy NGC 4472. This GC is notable for hosting an ultraluminous X-ray source as well as associated strong and broad [O III] $\lambda\lambda4959$, 5007 emission. We show that the $HST/S\text{TIS}$ spectroscopy spatially resolves the [O III] emission in RZ 2109. While we are unable to make a precise determination of the morphology of the emission-line nebula, the best-fitting models all require that the [O III] $\lambda5007$ emission has a half-light radius in the range 3-7 pc. The extended nature of the [O III] $\lambda5007$ emission is inconsistent with published models that invoke an intermediate-mass black hole origin. It is also inconsistent with the ionization of ejecta from a nova in the cluster. The spatial scale of the nebula could be produced via the photoionization of a strong wind driven from a stellar mass black hole accreting at roughly its Eddington rate.