Lines of high excitation in NGC 4151 : new measurements of [Fex] and [Fe Xiv]

Abstract

We provide in this paper new measurements from high resolution data of the [Fe X] 6374.5 Å and [Fe XIV] 5303 Å emission lines in NGC4151. Line blends have been analyzed properly through a quantitative separation technique: compared to previous results the accuracy is substantially improved. The [O I] 6300.3 Å and 6363.8 Å line profiles match that of the [O III] 5006.8 Å line. Taking into account this property, we find that the [Fe X] 6374.5 Å line emission can be represented by a single component corresponding to the [O I], [O III] core emission, as long as kinematical parameters are concerned. In other words, the prominent [Fe X] line emission in NGC4151 occurs at the emission systemic velocity - is not blue-shifted - with a value FWHM ~ 200 km s⁻¹. In the framework of AGNs photoionized models, this observation implies that the ionizing radiation may travel out to large distances and reach the NLR without being much attenuated. This result points towards a small covering factor in the NLR. Considering the set of previous [Fe X] line intensity estimates, together with their large uncertainties, we find no convincing evidence for time variability of this feature over the past 15 years. We definitely detect the [Fe XIV] 5303 Å and [Ca V] 5309.2 Å emission lines in NGC4151. The [Fe XIV] emission is marginally found to decrease when both the continuum and the broad Balmer components drop in this object. Comparisons of the [Fe X]/[Fe XI] and [Fe XIV]/[Fe X] line ratios with theoretical predictions do not allow readily to discriminate between collisional and photoionized models.