The 2-dimensional light distribution of each galaxy was fitted with
elliptical isophotes, using a procedure based on the task
,
(STSDAS
package; Jedrzejewski 1987;
Busko 1996), which allows the interactive masking of unwanted
superposed sources. Starting from an interactively centered ellipse,
the fit maintains as free parameters the ellipse center, ellipticity,
and position angle. The ellipse semi-major axis is incremented by a
fixed fraction of its value at each step of the fitting procedure. The
routine halts when the surface brightness found in a given corona equals
the sky rms, and then restarts decrementing the initial semi-major axis
toward the center. Isophotes whose rms is greater than their mean value
are discarded. The fit fails to converge for some galaxies with very
irregular light distributions. In these cases we kept fixed one or more
of the ellipse parameters.
The resulting radial light profiles were fitted using one of four models
of light distributions:
1) a de Vaucouleurs r1/4 law (de Vaucouleurs 1948);
2) an exponential law;
3) a "mixed'' profile consisting of the sum (in flux) of an exponential
law, dominating at large radii ("disk''), and an exponential or a de
Vaucouleurs r1/4 law, dominating at small radii ("bulge'');
4) a "truncated'' profile consisting of an exponential or a de
Vaucouleurs r1/4 law, truncated by a steeper exponential law beyond
a certain critical radius ,
according to either of the following:
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The fits are performed from a radius equal to twice the seeing disk, out to the outermost significant isophotes.
Total magnitudes
are then obtained by adding to the flux measured
within the outermost significant isophote the flux extrapolated to
infinity along the fitted profile. The
error attached
to the total magnitude
combines the statistical error on
the flux at the outermost isophote with that on the fit parameters.
The effective radius
(the radius containing half of the total
light) and the effective surface brightness
(the mean surface
brightness within
)
of each galaxy are "empirically'' computed
(see Paper V). The relative errors are obtained combining the uncertainty on
,
as described above, with the scatter
along the
integrated-light growth curve.
Finally we compute other useful parameters: the concentration index
(C31), defined in de Vaucouleurs (1977) as the model-independent
ratio between the radii that enclose 75% and 25% of the total light
,
and, for galaxies fitted with a two component model, the bulge to
total flux ratio (B/T).
The derived surface brightness profiles are shown in Fig. 3: each galaxy is labelled with a prefix denoting the telescope (N00 for ESO-NTT or G99 for TNG), followed by its catalogue name and by the type of decomposition (see Table 4).
Copyright ESO 2001