and I images were taken at the Cassegrain focus of the
2.2 m MPI/ESO telescope with EFOSC2, CCD19 (
with a scale of 
) during 3 photometric
nights in la Silla, Chile, in October 1994. Typical exposure
times were 900 s in B, 300 s in V and 600 s in I.
Figure 1: Grey-scale V images of 9 Karachentsev's
pairs of galaxies in our sample. North is to the top and
East to the left. For the pair CPG548 the size of the image
is 5.8
5.8'. For the other 8 images the size
is 2.9 2.9'
Figure 2: Grey-scale V images of 5 Karachentsev's
pairs of galaxies in our sample. North is to the top and,
East to the left. For the pair CPG587 the size of the image is
5.8 5.8'. For the other 4 images the size is
2.9 2.9'
Observational conditions were photometric with an average seeing of 1.2''.
Initial reductions followed standard procedures, and were performed with IRAF. The bias level was subtracted from each image which were then divided by the averaged flat field exposure for the night. In Figs. 1 (click here) and 2 (click here) we show the grey-scale V images of the 14 pairs in our sample.
Standard stars from Landolt's list (1992), MarkA, T Phe and SAO 98 were observed for extinction and calibration purposes. To calibrate the images we used the following equations:
where 
and I are the standard magnitudes, b, v and i
are the instrumental magnitudes, X is the airmass of the
observation and the coefficients A, C, D, E and F are the
transformation coefficients. B was obtained from the relation
B = b + m0 - mxX, where m0 is
m0 = -A1 - A3((b-v) - E1)/E3
and mx = (A2 - A3E2/E3). Similar relations
were derived for V and I.
In Table 1 we present the values of the extinction and color coefficients, mx and m0, and the rms of the fit obtained for each filter. Mean extinction coefficients provided by ESO were used for comparison.
Two-dimensional surface photometry was accomplished using the last version of GALPHOT image analysis package developed and kindly provided by W. Freudling. This package runs in the IRAF environment and uses the ellipse-fitting routine available in the ISOPHOTE package of STSDAS following the method described by Jedrzejewski (1987). Prior to ellipse fitting, the sky background is removed and field stars and other features are automatically masked outside the outer isophotes of the galaxy.
| Filter | m0 | mx | rms |
| B | -0.0027 | 0.2402 | 0.006 |
| V | -0.0026 | 0.1531 | 0.012 |
| I | -0.0027 | 0.0649 | 0.009 |
Stars overlapping galaxies were carefully removed by extracting elliptical models which were substituted using an interpolation across the region. For pairs with overlapping galaxies such as CPG018, CPG083, CPG091, CPG099, CPG547, CPG577, we used the following procedure: first we built a model of the pair component a, called model a using the ELLIPSE and BMODEL. Then we subtracted this model from the original image, creating an image 2. From image 2 we built a model for galaxy b, called model b. Model b was subtracted from the original image, creating image 3, which was then used to do photometry of galaxy a (see Fig. 3 (click here)a for an example of image 3). With the same procedure we have created images 4, 5 which was used to do photometry of galaxy b (see Fig. 3 (click here)b for an example of image 5). In order to test this method we have applied it to synthesized images. Good results were obtained when the overlapped area was less than 10% of the total area. The goal of this procedure was to recover the overlapping external regions of the galaxies. For the cases where there were remaining features in the center of the subtracted galaxies, we masked these features before doing photometry of the companions. Photometry of CPG018 and CPG083 should be used with caution since they present a common envelope.
Figure 3: a) Image of CPG547 after the
model subtraction of galaxy b; b) Image of
CPG547 after the model subtraction of galaxy a)
The radial surface luminosity profiles of the galaxies were fitted by an exponential law of the form
where 
is the surface luminosity density
(in 
pc-2) corresponding to the surface
brightness, 
(in mag arcsec-2), and r is the
major axis radius. The fit parameters are then the central
surface brightness, 
, and the exponential
scale length, 
. The value of 
will depend on
the morphological type of the galaxy, being 1 for spiral
disks, and 1/4 for ellipticals or spiral bulges (de
Vaucouleurs 1948). The following relations were used in
order to fit the surface brightness versus radius
and obtain the fit parameters:
where is the disk scale length and 
is the
disk central surface brightness, 
and 
are the
effective radius and the surface brightness at
radius and surface brightness at . We present the radial
surface brightness distributions and the exponential fits
in Figs. 4 (click here) and 5 (click here).
Figure 4: Brightness profiles
(
, 
and 
) and exponential
fits (solid lines), in units mag/
, as a function
of the semi-major axis (a or a1/4) in units of arcsec,
for both galaxies of 9 pairs of our sample in B, V, I
filters. a) and b) on the top indicate the members
of the pairs
Figure 5: The same as in Fig. 4 for five other pairs
The total magnitudes were calculated by extrapolating the
luminosity profiles to infinity. For several galaxies, bulge
and disk components were deconvolved from the surface
brightness profile following a standard interactive procedure
(Boroson 1981; Schombert & Bothun 1987;
Marquez & Moles
1996). First, we have fitted the outer regions of the surface
brightness profile using an exponential law. We have
subtracted the fitted disk profile from the total profile
obtaining the bulge profile. Then, the bulge profile was
fitted using an r1/4 law. Finally, the fitted profiles
(disk and bulge) were summed and the total fitted profile
was compared with the total observed profile. After some
iteractions the method converges and we obtain the
lowest 
fit. The total magnitudes correspond to
the lowest . The major uncertainty on the total
(asymptotic) magnitude values depends on the error on
the surface brightness () calculated at the outermost
point reached by the profile determination (due to the
uncertainty on the sky level) and on the error due to
the extrapolation procedure.
Taking into account all
contributions, the total uncertainties are 0.18, 0.13
and 0.12 for B, V and I, respectively. The total magnitudes
B, V and I, were corrected for inclination following
Burstein (1979);
Binggeli (1980) and
Franx et al. (1991)
for the ellipticals.
For the spirals we have assumed round
disks. We have also corrected for galactic extinction
using Burstein & Heiles (1984) sample. For the galaxies
that are not in their sample we have followed the method
in RC2 (de Vaucouleurs et al. 1976).
Figure 6: Comparison between our values of
B magnitudes before corrections (galactic extinction
and inclination corrections) and mB from RC3. Filled
circles correspond to six galaxies that we had in common
with RC3 and with magnitude differences 
.
The others symbols correspond to the pair CPG587 (triangles),
the galaxy CPG575a (square), and the galaxy CPG580a (star)
whose differences related to RC3 are discussed in the text.
The error bars are those given in the RC3
In order to check the quality of our photometry we have
compared our magnitudes with mB in the RC3 (de
Vaucouleurs et al. 1991) and with other papers in the
literature, specially with Marquez & Moles (1994 and 1996,
MM hereafter) values since our magnitude determination
followed similar procedure. In Fig. 6 (click here)
we plotted the values of our B magnitudes before corrections
(galactic extinction correction and inclination correction)
and mB from RC3. CPG575b was already found as
discrepant by MM and our result is closer to MM value
than to RC3 (RC3 mB= 15.08, MM mB= 14.67,
our value
mB = 14.14). CPG580a is discrepant in
relation to RC3 by 0.86 magnitudes, but that could be due
to presence of a warped disk which makes the photometric
decomposition not straightforward. The other two discrepant
galaxies are CPG587a and b. The distortion on the disks could
also be responsible for this large difference in magnitudes
(0.83 for CPG587a and 2.00 for CPG587b), however there is
no additional magnitude determination in the literature for
comparison. The other six galaxies that we have in common
with RC3 have differences in magnitude .
CPG575a, for instance, is only 0.01 fainter in our
determination than in RC3 and agrees with MM within
0.11 magnitudes, when no corrections are applied.
A few galaxies in our sample were studied by other authors. Reshetnikov et al. (1993) has done photometry of CPG099, CPG587 and CPG575 however, we can not compare directly the total magnitude values because they have used another photometric system (the R Cousins system). Rampazzo et al. (1995) studied CPG548 but they did not provide total magnitude values, only surface brightness profiles. They have remarked that the photometric profile of CPG548b (NGC 6964) follows an r1/4 law, indicating that this object is a bona fide elliptical which is in agreement with our results for this galaxy. Heraudeau & Simien (1996) have done the photometry of CPG548a galaxy extrapolating the brightness distribution by an exponential function. They obtained total magnitude values equal to 12.06 and 11.24 for V and I bands, respectively. Our total magnitude values for this object are different from these values maybe due to the differences in the decomposition method and the extrapolation procedure.