1) The most intense starburst in the prototype merger NGC 4038/39 takes place in an off-nuclear region that is optically obscured. This confirms the interpretation of the high far-infrared luminosity in this system in terms of star formation.
2) The [Ne III]/[Ne II] emission line ratios in the mid-infrared indicate that stars as massive as 60 are being formed in that optically obscured region. Therefore, to derive the high mass end of the Initial Mass Functions of the starbursts in luminous interacting galaxies, observations at wavelengths longer than the optical are needed.
3) The mid-infrared observations presented here suggest that about 15% of the bolometric luminosity from NGC 4038/39 arises in an off-nuclear region that is 100 pc in size.
4) The most prominent dust lanes in the optical image appear displaced from the peak distribution of the warm dust and gas traced by the mid-infrared emission. This could be due to different spatial distributions of the warm and cool dust, and/or projection effects in the optical appearance of dark lanes.
5) The infrared image also shows strong mid-infrared emission associated with the optically prominent star forming ring in NGC 4038.
6) The effects of absorption by dust are even more dramatic in the UV. Therefore, images of high redshift galaxies in their rest-frame UV could lead to strong biases in the morphological classification, and therefore in the scenarios of the history of galaxy formation.
© European Southern Observatory (ESO) 1998
Online publication: April 15, 1998