BIDIMENSIONAL SPECTROSCOPY OF THE CENTRAL KILOPARSEC OF NGC-4151

This article presents 61 bidimensionally distributed spectra covering the circumnuclear region of NGC 4151 within a radius of about 6".5. The spectra were obtained using a fiber optic system designed to perform bidimensional spectroscopy of extended objects. From the analysis of the H-alpha, [N II] lambda-lambda-6548, 6584, and [S II] lambda-lambda-6716, 6731 lines, the local physical conditions and the mean velocity field are studied. The alignment of the line intensity maps in the direction of higher ionization is confirmed. The electron density is fairly isotropically distributed, although at the inner parts it seems to be more aligned with the major radio axis. Its radial dependence is consistent with a r-1.5 law. A relatively low-ionization zone extending about 350 pc is found around the nucleus. No traces of bars or similar features have been detected near the nucleus, which has a relatively high ionization. Over the entire area studied, line ratios are incompatible with an ionization spectrum typical of H II regions. Solar abundances and a range for the spectral index are required to fit the observed line ratio with power-law photoionization models. The velocity field shows a strong S-shaped distortion, although it allows sinusoidal fitting. The whole region observed seems to be globally blueshifted with respect to the galactic systemic velocity determined from the 21 cm line. In addition, there are zones showing greater blueshifts where outflows are likely to exist. This seems to be the case of a zone surrounding the nucleus at 1".3 and of a region at a distance of about 4" in the southwest direction. The bidimensional distribution of line broadening is anisotropic, roughly transverse to the direction of higher ionization. The relationship between broadenings and blueshifts is complex, and it may be the result of the different degree of collimation with respect to the line of sight of the gas outflow in different zones. The radial dependence of line broadening is a slowly decreasing function, the FWHM at the nucleus and in the outer parts showing numerical coincidence with the stellar velocity dispersions. A systematic ordering in broadenings and blueshifts (H-alpha > [N II] > [S II]) at all distances is also noticeable, and it could indicate that the ionization parameter varies with distance to the galactic plane. Although the morphological picture which may be inferred for the circumnuclear region of NGC 4151 is complex, showing anisotropic features, a gradual connection between the galaxy and the nucleus is also found. This paper gives a test of the actual performance of fiber optics systems applied to bidimensional spectroscopy, which show clear advantages over other systems.