Dissecting the planetary nebula NGC 4361 with MUSE

Context. Optical integral field spectroscopy of planetary nebulae (PNe) offers a unique tool to explore the spatial relationships between the complex mixture of the many components (neutral, low- and high-ionisation gas, dust, and the central star) and their underlying physical conditions. Aims. The optical line and continuum emission in the very-high-ionisation Galactic PN, NGC 4361, were mapped to study the distribution of ionisation, extinction, electron temperature, and density. Methods. Based on commissioning data, MUSE Wide Field (60×60″) normal-mode (4750–9300 Å) observations of NGC 4361 were reduced. The PN is larger than a single MUSE field and only the central 1 arcmin2 of the PN was observed in good conditions. Emission images in recombination and collisionally excited lines were extracted and the line ratios provided the dust extinction, electron density and temperature, and ionic abundances using standard techniques. A family of compact low-ionisation knots (dubbed ‘freckles’) was discovered and techniques developed to measure their spectra, independently of the extended high-ionisation medium. Results. The nebula is confirmed as optically thin in the H-ionising continuum, based on its very low He <ns0:sc>I</ns0:sc> emission, even to the edges of the field. The electron temperature, Te, is shown to have a large-scale spatially coherent structure, as indicated by a previous long-slit spectrum. Prior to this study, no low-ionisation emission had been positively detected, although MUSE revealed both weak extended [N <ns0:sc>II</ns0:sc>] and [O <ns0:sc>II</ns0:sc>] and >100 spatially unresolved knots. There are several linear associations of these knots, but none of them point convincingly back to the central star. They have low-to-moderate ionisation with Te ~ 11 000 K, Ne ~ 1500 cm−3 and generally exhibit a higher extinction than the extended high-ionisation nebula. Within the MUSE field, a low-redshift emission-line galaxy was serendipitously found to be hiding behind NGC 4361. The spectrum of this dwarf galaxy was carefully extracted from the bright foreground nebular emission and the galaxy’s line and continuum properties were then determined. Conclusions. NGC 4361 is not completely optically thin, as indicated by several extended regions and many compact features of lower ionisation emission. The low-ionisation ’freckles’ identified here do not clearly appear to differ in (He, N, O, S) abundance with respect to the extended high-ionisation gas. The spatial distribution and radial velocities of these features suggest that they belong to a thick disk oriented perpendicular to the large-scale nebular gas, which may perhaps be remnants of an earlier structure. The low-luminosity disk galaxy at ~87 Mpc has bright H <ns0:sc>II</ns0:sc> regions with metallicity 12+log(O/H) ≅ 8.4 and is suggested to be a Magellanic irregular or low-mass spiral.