A peculiar planetary nebula candidate in a globular cluster in the Fornax dwarf spheroidal galaxy

Abstract

Context: Until now, only one planetary nebula (PN) has been known in the Fornax dwarf spheroidal galaxy.
Aims: The discovery of a second PN candidate, associated with one of the 5 globular clusters in the Fornax dwarf, is reported.
Methods: Spectra of the globular cluster H5, obtained with the UVES echelle spectrograph on the ESO Very Large Telescope, show [O III] line emission at a radial velocity consistent with membership of the Fornax dwarf. A possible counterpart of the [O III] emission is identified in archival images from the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The source of the emission is located about 1.5 arcsec (less than one core radius) southwest of the centre of the cluster.
Results: The emission line source is identified as a likely PN, albeit with several peculiar properties. No Hβ, He I, or He II line emission is detected and the [O III]/Hβ ratio is >25 (2σ). The expansion velocity inferred from the [O III] 5007 Å line is V_e ≈ 55 km s^-1, which is large for a PN. The diameter measured on the HST images is about 0.23 arcsec or 0.15 pc at the distance of the Fornax dSph.
Conclusions: This object doubles the number of known PNe in Fornax, and is only the 5th PN associated with an old GC for which direct imaging is available. It may be a member of the rare class of extremely H-deficient PNe, the second such case found in a GC.

Based on observations collected at the European Southern Observatory, Chile under programme 078.B-0631(A), and with the NASA/ESA Hubble Space Telescope.