For the galaxies in the radio sample we have observations in
B, R and K' bands for several objects and in 
for the
remaining. For the former we have available two colors to compare
with the synthetic spectra while in the latter case we have only
one color. With the degeneracy mentioned in the previous section, it is
not very meaningful to carry out a best fit for burst age and
strength since such a fit will provide a
large range for both the parameters. So, instead, we have plotted
the curves showing the age-strength relation. We have also
plotted our objects
on a similar diagram to see what possible values the parameters are
likely to take. In our work we have used synthetic spectra kindly
provided by Dr. Bruno Guiderdoni. A detailed prescription
of how these are generated can be found in
Guiderdoni and Rocca-Volmerange (1987).
Details on models other than the burst model can be found in
Rocca-Volmerange and Guiderdoni (1988).
The initial mass function (IMF, 
) is one of the most important
inputs for the generation of synthetic spectra. The IMF gives the
distribution of the number of stars as a function of mass.
The IMF and the law of star formation together provide the
number of stars forming in a given mass range at a given time, t.
These stars are then placed on the zero age main sequence (ZAMS)
in the Hertzprung-Russel (HR) diagram.
Theoretical stellar tracks are used to evolve the
stars at each time step. The distribution of the stellar populations
in the HR diagram 
, is estimated.
Using a bolometric correction and the luminosity class,
is converted to 
. A synthetic stellar spectrum, 
is then constructed. A nebular component 
resulting from the Lyman continuum photons is added. A correction for
internal extinction 
is applied.
The synthetic spectra used here are based on the
Scalo
IMF. This has the form 
, with
The stellar tracks used are from the Geneva group. The stellar
spectra are from Kurucz (1992) for hot stars ( 
),
and from Bessel et al., Fluks et al. (1994) for cold stars.
The tracks include four evolutionary stages viz. Main Sequence (MS),
Giant Branch (GB), ``red'' Horizontal Branch (HB) and Asymptotic
Giant Branch (AGB). Along each evolutionary sage, the input stellar
tracks are interpolated in mass to get differences of successive masses
lower than 1 Gyr. Mass loss of massive stars is taken into
consideration.
The basic spectra that we have made use of to obtain colors and
magnitudes
cover a range of wavelength from 
(1221
distinct wavelengths).
We used spectra corresponding to the following 219 distinct ages:
,
, 
.
The spectra were generated for different metallicities
viz. 0.001, 0.004, 0.008, 0.020 and 0.040. However, we actually
used only the solar metallicity spectra with Y=0.28 to 0.30 and
Z=0.02. The whole Hubble sequence is represented with one
metallicity since the mean metallicity of stars rapidly approaches
the solar value. Also, with the scanty data that we have, adding
a further variable Z would not have been vary meaningful.
In this subsection we describe how colors were obtained
for a E + A model from the synthetic spectra.:
(1) The age of the old galaxy was assumed to be 
.
(2) A young burst was then superimposed onto the old galaxy.
The burst age was allowed to vary between 0 and 2 Gyr and
it was allowed to contain a mass fraction of upto 
.
(3) The resulting spectrum was convolved with individual
filter response curves, after appropriate corrections for
redshift and cosmological dimming were applied, to obtain
magnitudes for those filters.
Thus, for each burst age and each mass fraction, a number (magnitude) is obtained for each filter. Colors and isochrones are then obtained from the magnitudes. These are finally compared with the observed colors of the galaxies to estimate the age and strength of the starburst. We in fact, shifted the colors of the radio sample galaxies to a redshift of zero so that all the galaxies can be simultaneously compared with the zero-redshift isochrones.
