Max Planck Institute for Astronomy - Annual Report 2005

48 II. Highlights
and we see a temporary decline in the star formation rate.
But the galaxies then turn around, and during the final
coalescence a powerful burst of star formation occurs.
After the final merger, the star formation gradually tails
off as the gas is consumed.
These simulations predict that the total period of enhanced
star formation lasts for about two billion years.
During this time the bolometric luminosity increases by
almost one order of magnitude. However, the surprising
result shown by this work is that the observed UV and
blue-band luminosity stays almost constant because the
dust density also increases, leading to increased attenuation
of the light emitted by the young stars (Fig. II.8.3).
This radiation escapes preferentially when the galaxy is
viewed perpendicular to the disk plane so that the galaxy
appears up to twice as bright when viewed face-on rather
Fig. II.8.4: Simulated images of two merging Sbc galaxies at a)
0.6, b) 1.6, c) 1.7, and d) 2 billion years into the simulation.
a
b
than at an oblique viewing angle. After the merger of two
disk galaxies a spheroidal or elliptical remnant is left,
whose luminosity in the UV and blue band no longer
depends strongly on the viewing angle.
As mentioned above, the luminosity was calculated
for 22 wavelengths. Therefore from the 25 simulation
runs with 7 different galaxy types and 50 different points
in time a total of more than 100 000 images and 10 000
spectra were obtained. These images and spectra will
prove valuable for comparing with multi-wavelength
observations of real galaxies. Fig. II.8.4 shows the appearance
of the spiral-spiral merger at several different times
during the simulation. Fig. II.8.5 illustrates the effect of
the dust attenuation on the galaxy spectrum.
Based on the results of this large set of simulations,
the team developed a simple analytic formula describing
the fraction of light absorbed by dust as a function of
the galaxy's luminosity, mass, and metallicity. Fig. II.8.6
shows the attenuation predicted by the analytic formula
compared to the results obtained in the simulations. This
c
d