How to read the filtered images?
The image is the result of stacking all the available frames in one band
(the I-band in most cases) after their pre-processing.
It is an false-colour image. The colour table was individually choosen to get
the best contrast.
- This is a contour map of the adaptively smoothed original image.
- The contour levels were set by hand to get more or less equidistant
isophotes. No kind of calibration was done.
- The only aim is an easier imagination of the shape of the galaxy and
the visiualization of possible isophote twisting, internal structure etc.
The image represents the first derivative of the surface brightness
distribution. Since the gradient is a vector, only its absolute value is
coded here in false colour (red => steep gradients).
The gradient is particulare suited to visualize sharp edges (even
if they have low contrast), like shells or ripples.
Because these types of structure are relatively rare in galaxies, for the
majority of the objects the Laplacian image is more instructive to show
the faint structures.
The image represents the second derivative (i.e. the curvature)
of the surface brightness distribution. For convenience, the curvature is
inverted: negative values (i.e. the blue part of the colour table)
corresponding to concavity and positive values (i.e. the red part of the
colour table) to convexity.
The images do not retain any photometric information and must be used
only to disentangle structures and to study their morphology.
The Laplacian image, however, provides an independent way to estimate
the size and trace the geometry of the hidden features, thus helping in masking
out the structures in order to model a ``clean'' galaxy to be subtracted
from the original image (cf. the case of NGC 3384,
Busarello et al. 1996).
Every convex spike - such as a star - is surrounded by a concavity. Due
to the fact that the filter has a square shaped response, spikes usually
have square-shaped negative artifacts. This squared shape, far from
being a nuisance, helps in disentangling artifacts and real structures.
Along the minor axis, and outside the seeing blurred galaxy core,
there is almost always a negative signature produced
by the concavity of the luminosity profile (cf. the r1/4
law which is usually a good approximation
of the surface brightness profile). However, since the two-dimensional
curvature is the average of the one-dimensional curvatures measured
in the two main curvature directions, in elongated systems the very
central convex part is more extended along the major axis
(in the case of high axis ratios
there is no negative curvature along the major axis at all). This
effect has nothing to do with what we call inner disks,
which are clearly differentiated from the aforementioned fake-features
by their profiles and their well-defined edges along the main axes.
Since it preserves the spatial resolution, the Laplacian morphology allows
us to detect edge-on or close to edge-on disks and rings as patches of enhanced
emission which are more or less symmetric with respect to the nuclei of the
galaxies. It should always be kept in mind, however, that the same
Laplacian morphologies may be produced by completely different structures
such as weak polar rings or bars.
The notes on structures:
- We describe the structures found in the filtered images for every
object and give our interpretation here.
compare Capaccioli et al. 1997.
Created by Petra Böhm, updated: 1998-04-22