I.F. Mirabel 1, O. Laurent 1, M. Sauvage 1, V. Charmandaris 2, L. Vigroux 1, C. Cesarsky 1, M. Sauvage 1, & T. X. Thuan 3
1 DSM/DAPNIA/SAp, CEA-Saclay, 91191 Gif sur Yvette Cedex FRANCE
2 Observatoire de Paris, DEMIRM, 61 Av. de l'Observatoire, F-75014 Paris, France
3 Astronomy Dept., University of Virginia, PO Box 3818, University station, Charlottesville, VA 22903-8181, USA
1) THE DARK SIDE OF STAR FORMATION: Mid-infrared observations show that the most intense starburst in the prototype colliding system of galaxies (The Antennae) takes place in an off-nucleus region that is inconspicuous at optical wavelengths. The analyses of the mid-infrared spectra indicate that the most massive stars are being formed in an optically obscured knot, which produces about 15% of the total luminosity from the Antennae galaxies between 12.5 microns and 18microns. The multiwavelength view of this nearby prototype merging system suggests caution in deriving scenarios of early evolution of high redshift galaxies using only observations in the narrow rest-frame ultraviolet wavelength range.
2) A BARRED SPIRAL AT THE CENTER OF THE GIANT ELLIPTICAL CENTAURUS A: ISOCAM provided a new view that penetrates into the central regions of CenA (NGC 5128), a giant elliptical galaxy that harbors the closest radio loud active galactic nucleus to Earth. Observations at mid-infrared and sub-millimeter wavelengths reveal that the dust distribution within 5 kpc of the active galactic nucleus (AGN) is similar to that of a barred spiral galaxy. The barred spiral is a bisymmetric stable structure formed at the center of the giant elliptical from the tidal debris of a gas-rich object(s) accreted in the past 109 years. The total size and mass of interstellar gas in the barred spiral of CenA is comparable to the small Local Group spiral galaxy Messier 33. The observations suggest that CenA could be the nearest example of a class of ``symbiotic'' galaxies composed by spiral and elliptical structures that share the same massive black hole at their center. These ISOCAM observations provide clues to understand how bars serve to funnel gas towards radio loud AGNs. It is proposed that the elliptical in CenA could be regarded as a smaller scale analogue of dark matter haloes where disk galaxies form.