The light element abundance distribution in NGC 5128 from planetary nebulae^⋆,⋆⋆

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
e-mail: jwalsh@eso.org
² Giant Magellan Telescope Organization/Carnegie Observatories, Pasadena, USA
e-mail: gjacoby@gmto.org
³ Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
e-mail: r.f.peletier@astro.rug.nl
⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
e-mail: naw@ast.cam.ac.uk

Received: 5 December 2011
Accepted: 30 April 2012

Abstract

Context. Planetary nebulae in the nearest large elliptical galaxy provide light element abundances difficult or impossible to measure by other means in a stellar system very different from the galaxies in the Local Group.

Aims. The light element abundance pattern from many planetary nebulae (PNe) at a range of radial distances was measured from optical spectroscopy in the elliptical galaxy NGC 5128, which hosts the radio source Centaurus A. The PN abundances, in particular for oxygen, and the PN progenitor properties are related to the galaxy stellar properties.

Methods. PNe in NGC 5128 covering the upper 4 mag of the luminosity function were selected from a catalogue. VLT FORS1 multi-slit spectra in blue and red ranges were obtained over three fields at 3, 9 and 15′ projected radii (4, 8 and 17 kpc, for an adopted distance of 3.8 Mpc) and spectra were extracted for 51 PNe.

Accurate electron temperature and density diagnostics are usually required for abundance determination, but were not available for most of the PNe. Cloudy photoionization models were run to match the spectra by a spherical, constant density nebula ionized by a black body central star. He, N, O and Ne abundances with respect to H were determined and, for brighter PN, S and Ar; central star luminosities and temperatures are also derived.

Results. Emission line ratios for the 51 PNe are entirely typical of PN such as in the Milky Way. The temperature sensitive [O III]4363 Å line was weakly detected in 10 PNe, both [O II] and [O III] lines were detected in 30 PNe, and only the bright [O III]5007 Å line was detected in 7 PN. For 40 PNe with Cloudy models, from the upper 2 mag of the [O III] luminosity function, the most reliably estimated element, oxygen, has a mean 12 + log(O/H) of 8.52 with a narrow distribution. No obvious radial gradient is apparent in O/H over a range 2−20 kpc. Comparison of the PN abundances with the stellar population, from the spectra of the integrated stellar light on the multi-slits and existing photometric studies, suggests an average metallicity of [Fe/H] =  −0.4 and [O/Fe] = 0.25.

Conclusions. The masses of the PN central stars in NGC 5128 deduced from model tracks imply an epoch of formation even more recent than found for the minority young population from colour magnitude studies. The PN may belong to the young tail of a recent, minor, star formation episode or derive from other evolutionary channels, perhaps involving binary stars.