The starbursts in ultraluminous galaxies take place in the nuclear region. One of the new findings with ISO is a class of very luminous dust-enshrouded extranuclear starbursts in nearby spiral-spiral mergers. When the pre-encounter galaxies do not have prominent buldges, namely, when the mergers are -for instance- two Sc galaxies, the most luminous starbursts may take place in extranuclear regions that are inconspicuous at optical wavelengths. These extranuclear starbursts have sizes 100 pc in radius and can produce up to 50% of the overall mid-infrared output from these systems. Furthermore, the analyses of the mid-infrared spectra indicate that the most massive stars in these systems are formed inside these optically invisible knots.
In Figure 4 is shown in contours the mid-infrared (12-17 µm) image of the Antennae galaxies obtained with ISO , superimposed on the optical image from HST. Below are shown representative spectra of the two nuclei and the brightest mid-infrared knot. It shows that the most massive stars are formed in an obscured knot of 50 pc radius, which produces about 15% of the total luminosity from the Antennae galaxies between 12.5 and 17 µm. A more extreme case is found in NGC 3690 , where it is observed an extranuclear region 100 pc in radius that radates ~ 45% of the overall mid-infrared output from this system. If the fraction of far-infrared fluxes is the same as in the mid-infrared, such compact region produces a luminosity of 2 × 1011 L. Therefore, the luminosity of a few compact starburst knots of this type would be comparable to the total bolometric luminosity of a ULIG such as Arp 220 (Figure 3d).
Figure 4. The upper figure from  shows a superposition of the mid-infrared (12 -17 µ, countours) image of the Antennae galaxies obtained with the Infrared Space Observatory, on the composite optical image with V (5252 Å) and I (8269 Å) filters recovered from the Hubble Space Telescope archive . About half of the mid-infrared emission from the gas and dust that is being heated by recently formed massive stars comes from an off-nuclear region that is clearly displaced from the most prominent dark lanes seen in the optical. The brightest mid-infrared emission comes from a region that is relativelly inconspicuous at optical wavelengths. The ISOCAM image was made with a 1.5" pixel field of view. Contours are 0.4, 1, 3, 5, 10, and 15 mJy. The lower figure shows the spectrum of the brightest mid-infrared knot and of the nuclei of NGC 4038 and NGC 4039. The rise of the continuum above 10 µm and strong NeIII line emission observed in the brightest mid-infrared knot indicate that the most massive stars in this system of interacting galaxies are being formed in that optically obscured region, still enshrouded in large quantities of gas and dust.
The multiwavelength view of this nearby sample of prototype merging systems suggests caution in deriving scenarios of early evolution of galaxies at high redshift using only observations in the narrow rest-frame ultraviolet wavelength range . Although the actual numbers of this type of systems may not be large, we must keep in mind that the most intense starbursts are enshrouded in dust and no ultraviolet light leaks out from these regions.