The narrow line region of active galaxies - I. Nuclear [O III] profiles

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Abstract
High-dispersion (O III) profiles of 42 Southern hemisphere active and H II galaxies are presented as the first part of a study of the Narrow Line Region (NLR) of active galaxies. A parameter scheme based on area measurements is defined which characterizes the width, asymmetry and kurtosis of different parts of the profile. Techniques for estimating errors and resolution corrections are described. A comparison with parameters defined by a cut at constant height shows the area parameters to be less sensitive to noise, more stable to the effects of poor resolution and more sensitive to information contained in the base and wings of the profile. As found in previous studies, there is a significant excess of profiles with blueward slanting asymmetry and strong wings relative to a Gaussian. This strongly suggests the presence of radial gas flows together with a source of line opacity and/or scattering. To infer the direction of flow from profile asymmetry alone is. however, highly ambiguous and requires additional information. Current H I absorption observations show no preference for blueshifts or redshifts although there is good evidence for outflow in at least one case. When combined with other data, the distribution of log FWHM is well fitted by a Gaussian of mean 2.54 (350 km s−1) and sigma 0.20. indicating somewhat narrower lines than many of the ‘classical’ Scyferts first discovered. The absence of very narrow lines allows weak constraints to be placed on the degree of disc rotation. The line widths do, however, closely match the stellar velocity dispersion found in the bulges of early-type spirals. The NLR velocity field may therefore be dominated by simple gravitational motion in the bulge potential. Preliminary comparison with existing low-dispersion data shows the [O III] line widths of LINER galaxies to be similar to those of Scyferts. In contrast, H II and Starburst galaxies have somewhat narrower and more symmetric profiles than Seyferts. As found previously, the emission-line centres (centroid, median or C50 parameter) are on average blueshifted with respect to the galaxy systemic velocity. However, a similar analysis of the profile peak position shows no such preferred shift.

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