Intergalactic stars in the Fornax Cluster

Abstract

We have identified ten candidate intergalactic planetary nebulae in the Fornax galaxy cluster. These objects were found during observations in 1992 and 1993 in three fields chosen well away from any Fornax galaxy at 15 arcmin, 30 arcmin, and 45 arcmin from the centre of Fornax. We used the usual method of blinking images taken in a narrow OIII filter, with images taken in an adjacent broad filter. The measured fluxes in the narrow, broad, and I bands are consistent with these unresolved objects being planetary nebulae immersed in an intergalactic population of stars. Such a population is expected to arise as a consequence of tidal encounters between galaxies, and our findings strengthen the case for the existence of such tidal debris. The confirmation of some or all of these ten candidates as planetary nebulae would imply that intergalactic stars constitute a substantial fraction of all the stars in Fornax, up to an estimated 40 per cent. Intergalactic planetary nebulae could prove useful in probing the underlying cluster potential, since they would be far more abundant than galaxies. We discuss possible contamination of the sample by emission-line galaxies, but conclude that planetary nebulae is the most likely identification for the detected objects. keywords: intergalactic medium-- galaxies: interactions -- planetary nebulae: general -- clusters: individual: Fornax