THE NATURE OF THE OPTICAL CONTINUUM IN NGC-5128 (CENTAURUS-A)

We present long-slit observations of NGC 5128 in the range lambda lamb da 3200-9800 Angstrom, covering the nucleus and dust lane across the e ast-west direction, up to approximate to 2.2 kpc from the nucleus. The reddening-corrected continua and absorption features, when compared w ith those typical of a bulge stellar population, show that all locatio ns within 1.8 kpc from the nucleus present a diluting continuum in the blue. We investigate the nature of the continuum by constructing mode ls which consider the possible contribution of a metal-rich old bulge, a blue stellar population and a power-law component from scattered nu clear light. By matching the equivalent widths of the absorption lines , continuum slopes and 4000-Angstrom break in our spectra with those o f the models, we find that: (i) at all locations the dominant contribu tion (in flux at 5870 Angstrom) comes from a metal-rich old bulge; (ii ) the diluting continuum is well reproduced by a power-law component a lone in the two locations at the borders of the dusty disc; (iii) in t he locations closer to the nucleus along the dust lane, the diluting c ontinuum can be represented by the spectra of blue stellar populations subject to different amounts of reddening, denoting the presence of r ecent star formation at different depths along the dust lane; neverthe less, some contribution of a power law from scattered nuclear Light to the blue component cannot be ruled out by our data. From the optical continuum distributions, we conclude that the regions within a project ed distance d < 0.3 kpc from the nucleus are affected by a large redde ning, with A(V) approximate to 3 mag, whereas at d approximate to 1.8 kpc, A(V) decreases to approximate to 0.6 mag. Gas properties were obt ained from the stellar population-subtracted emission Lines, confirmin g previous findings that several emitting regions have H II region cha racteristics, consistent with the derived blue stellar population cont ribution. However, near-infrared lines also indicate some contribution from shocks. The nucleus and locations between 1.0 and 1.5 kpc to the west have LINER-like spectra. All the above properties can be underst ood within a scenario in which a dusty star-forming disc is interactin g with the giant elliptical radio galaxy. For the nucleus we are able to match with the same aperture, our visible-near-infrared spectrum wi th that obtained by the International Ultraviolet Explorer (IUE). A re ddening analysis of this spectrum shows that at different wavelength r anges we are probing different depths: while in the near-infrared A(V) approximate to 3.0 mag and the stellar population is dominated by the bulge, in the ultraviolet it appears to be dominated by that of a you ng disc with A(V) approximate to 0.9 mag.