Double-barred starburst galaxies viewed by ISOCAM

H. Wozniak ¹, D. Friedlik ², L. Martinetk ², & D. Pfennigerk ²

¹ IRAP/Observatoire de Marseille, 2, Place Le Verrier, F-13248 Marseille cedex 4
² Geneva Observatory, Switzerland

Double-barred galaxies has gained strong new interests from the point of view of observations (e.g. Wozniak et al. 1995; Friedli et al. 1996), numerical simulations (e.g Friedli & Martinet 1993; Friedli 1996) and theory (e.g. Shlosman et al. 1989). Indeed, double-barred galaxies or galaxies with central non-axisymmetric deformations are very common, especially in early-type disk galaxies. Moreover, bars within bars systems might play a crucial role in fueling central activity of galaxies and shaping circumnuclear starbursts.

The present study is part of a global project intended to clarify the interplay between central starburst activity and the non-axisymmetric dynamics of barred galaxies. Our general approach makes use of multiwavelength observations coupled with self-consistent numerical simulations. ISOCAM observations in LW2 and LW3 bands offer us exploring the characteristics of the radiation coming from ``unidentified infrared emission bands'' and hot dust. An extended comparison with the radiation at optical, near-infrared and millimeter wavelengths allows to describe the typical characteristics (SFR, age, stellar populations, etc.) of the circumnuclear star forming regions.

In this talk, a special emphasis is given on the fresh 6.75 and 15m data that have been obtained with ISOCAM on board of ISO for two galaxies, namely:

• NGC1097 is a SB(s)b galaxy which hosts a Seyfert 1 nucleus. This object is also a starburst galaxy whose rate of transformation of gas into stars is significant ( ). Near-infrared observations have clearly shown the existence of a nuclear bar misaligned from the large-scale one. Both bars are separated by a prominent circumnuclear ring of star formation. ISOCAM images reveal six distinct peaks regularly spaced along the ring, distributed two by two with respect to the nucleus. This symmetry cleary suggests that star formation is organised at large-scales along the ring. No peaks are present close to secondary bar ends. The strongest peak in LW2 and LW3 bands coincides with the position of the type II supernova SN1992bd. We suspect that a non-negligible part of the flux is due to the supernova event.

• NGC4321 (M100) is a late-type giant, moderately barred spiral galaxy in Virgo. Its inner region has been extensively studied at several wavelengths. Optical, H, near-infrared, and CO images have revealed many intricate features associated with its mild star formation activity in the central kpc. A nuclear bar almost perfectly aligns with the large-scale bar occupies the region inside the ring. Several H II regions lie on a 1kpc circumnuclear ring crossing the long stellar bar. The LW2 and LW3 images show an inhomogeneous emission resulting in an incomplete ring-like distribution. Two bright spots near the nuclear bar ends dominate the emission in both bands. We have recently suggested that the high LW2/LW3 ratio in these two star forming region could be due to Wolf-Rayet stars (Wozniak et al. 1998, A&A 330, L5).

We compare NGC 1097 to NGC 4321. Recent results from numerical simulations bring new clues to understand the major and striking differences between the MIR emission from NGC 1097 and NGC 4321.