4.3. Spectral characteristics of active galaxies

In addition to an active nucleus, Circinus is known to also have a circumnuclear starburst ([Moorwood et al. 1996]); salient features of its spectrum (fig. 2), apart from the large number of fine structure lines, are the presence of infrared bands, the continuum bump due to hot dust and the broad silicate absorption around 9.7 µm. In a subsequent study, using the ISOCAM-CVF, Moorwood (1999) showed that the infrared band emission does not originate in the starburst ring, but is in fact caused by the remnants of an earlier starburst episode.

Another interesting case is that of Centaurus A, the closest active galaxy, at only 3.1 Mpc, which is suspected of hosting a misaligned BL LAC nucleus (e.g. [Bailey et al. 1986]) at the center of an elliptical galaxy. An ISOCAM map by Mirabel et al. (1999) reveals the presence of a bisymmetric structure, similar to that of a barred spiral galaxy, in the central region of the giant elliptical galaxy. This strange "galaxy" is composed of the tidal debris of small galaxies accreted by the elliptical galaxy in the last 10⁹ years. Marconi et al. (2000) recently observed this object with NICMOS, on board HST, and found Pa emission indicative of enhanced star formation at the edges of the bar seen with ISO, probably due to shocks associated with the bar. The ISOSWS spectrum of Centaurus A ([Alexander et al. 1999]) is that of an AGN with a circumnuclear starburst, and has been modeled by these authors with a 3.6pc torus inclined at 45°. ISOCAM-CVF spectra from 5 to 16 µm permit to distinguish very clearly between the nucleus dominated by a fast rising continuum, and the star formation regions emitting essentially in the infrared bands.