An unambiguous observation of the radial expansion of the faint giant halo of the planetary nebula NGC 6826

Spatially resolved observations at high spectral resolution of the O III 5007-Å line profiles from the faint, giant halo of the planetary nebula NGC 6826 have been obtained with the Manchester echelle spectrometer combined with the Isaac Newton Telescope. The observed profiles were fitted with single Gaussian line profiles. When the widths of the Gaussian profiles are plotted as a function of radius from the centre of the halo of NGC 6826, a trend to smaller widths at higher radii is found, as expected for a radially expanding halo. The radial expansion velocity of the halo is found to be 9.8 km s−1 when modelled with a thin shell but ≈ 13 km s−1 for a thick shell model. Comparison of the observed data to numerically simulated data suggests that the halo of NGC 6826 is more likely to be a thick shell than a thin shell.