The nature of the optical continuum in NGC 5128 (Centaurus a)

We present long-slit observations of NGC 5128 in the range λλ3200-9800 Å, covering the nucleus and dust lane across the east-west direction, up to ~~ 2.2 kpc from the nucleus. The reddening-corrected continua and absorption features, when compared with those typical of a bulge stellar population, show that all locations within 1.8 kpc from the nucleus present a diluting continuum in the blue. We investigate the nature of the continuum by constructing models which consider the possible contribution of a metal-rich old bulge, a blue stellar population and a power-law component from scattered nuclear light. By matching the equivalent widths of the absorption lines, continuum slopes and 4000-Å break in our spectra with those of the models, we find that: (i) at all locations the dominant contribution (in flux at 5870 Å) comes from a metal-rich old bulge; (ii) the diluting continuum is well reproduced by a power-law component alone in the two locations at the borders of the dusty disc; (iii) in the locations closer to the nucleus along the dust lane, the diluting continuum can be represented by the spectra of blue stellar popUlations subject to different amounts of reddening, denoting the presence of recent star formation at different depths along the dust lane; nevertheless, 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 projected distance d < 0.3 kpc from the nucleus are affected by a large reddening, with Av ~~ 3 mag, whereas at d ~~ 1.8kpc,Av decreases to ~~ 0.6 mag. Gas properties were obtained from the stellar population-subtracted emission lines, confirming previous findings that several emitting regions have H II region characteristics, consistent with the derived blue stellar population contribution. 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 understood within a scenario in which a dusty starforming disc is interacting with the giant elliptical radio galaxy. For the nucleus we are able to match with the same aperture, our visible-near-infrared spectrum with that obtained by the International Ultraviolet Explorer (IUE). A reddening analysis of this spectrum shows that at different wavelength ranges we are probing different depths: while in the near-infrared Av ~~ 3.0 mag and the stellar population is dominated by the bulge, in the ultraviolet it appears to be dominated by that of a young disc with Av~~ 0.9 mag