A flow diagram of the procedure used to discriminate extended objects
(EOs) against objects with a stellar point spread function (psf) in
the CCD frames is shown in Fig. 1 (click here). The R images, which are
deeper, were used to identify the EOs around the interacting
galaxies. In each field, a stellar reference psf was defined using
elliptical Gaussian fitting with the IRAF task "fitpsf'' on 10-15
apparent star-like objects. This psf is characterized by the major and
minor halfaxis widths (
, 
), see
Table 2 (click here). Then, an automatic search for flux peaks
was done with the IRAF routine "daofind'', from the "daophot'' package
(Stetson 1987; Stetson & Davies 1992). Daophot made typically 5000
detections on each frame, including the faintest objects visible by
eye on the CCD, but also many pixels with random high counts from
background noise. Elliptical Gaussians were then fitted to all these
detections, and those with a psf significantly narrower then the
stellar psf and below a minimum amplitude were discarded. The
remaining objects were fitted again with elliptical Gaussians to a
greater precision. The results of these fits were then used to create
a final list of objects (stellar and extended) which fulfilled the
following requirements:
and
For a secure final determination of the size of the stellar psf
(
), histograms of the distributions of
(and 
) were plotted
(Fig. 2 (click here), also Fig. 6a), where the stellar psf can be clearly
seen as a peak. Objects identified as EO were only those which were
not oversaturated and have a psf at least 1.4 times larger than
or . Both, the initial light source
search (daofind) and the Gaussian fitting (fitpsf) work reliably only
within a range of Gaussian widths, and some very extended galaxies
were missed. These galaxies can easily be seen on the frames and were
added manually. Visual scanning was also performed to remove those
objects which were obviously close double or multiple stellar
systems.
As an example of our findings, the R band image of NGC 3226/7 is shown
in Fig. 3 (click here). Marked in rings are the objects identified as
extended.
Figure 1: Flow-diagram of the selection procedure used to
identify objects as EOs or stellar like
Figure 2: Distribution of the major halfaxis, , of all identified
objects in the field around NGC 3226/7. The dashed line is a fit (a
combination of a Gaussian with a linear quadratic function) whose peak
was used to determine the final size () of the stellar psf.
The units of are in pixels; 1 pixel = 0.55 arcsec
Figure 3: R band CCD image of NGC 3226/7.
Marked in rings are the objects identified as EOs. The big cross
is the position of the reference coordinate of
this field