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
[16],
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
[6], 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 [16] 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 [16]. 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.