Using the present set of observations we were able to confirm the binary
association of 17 out of 26 pairs. Approximately 30% of the original
sample are found to be false pairs. Adding these new observations with the
ones already existing in the literature we have 50 pairs of the list of
Soares et al. (1995) with known radial velocities. In
Fig. 1 (click here) we present an histogram of these objects as a function of
the projected separation. The dashed line represents the distribution found
on Karachentsev's sample of 487 systems . We remark that due to the
isolation criteria the sample of Karachentsev (1987) is
strongly populated by tightly bound systems. In fact, he estimates that
about 90% of the pairs with separation larger than 100 kpc are omitted by
his isolation criteria (Karachentsev 1990). On the other hand
the isolation criteria adopted in Soares et al. (1995) takes
into account the local density enhancement. Therefore, this sample includes
a larger fraction of loose pairs, at the expense however of a larger
contamination of false pairs. Actually the pairs in our sample are well
described by an exponential distribution with a characteristic mean
separation of 
(solid line in Fig. 1 (click here)). This value can be compared
with the mean separation of 
found on Karachentsev's
sample, after corrections to evaluate only the isolated pairs.
Figure 2: The velocity difference distribution is well described by a linear
relation (continous line) similar to the one found on Karachentsev's sample
(dashed line)
In Fig. 2 (click here) we present the velocity difference distribution for our
sample. The solid line represents the relation found for our sample, while
the dashed line stands for the relation valid for Karachentsev's sample
normalized for the number of objects in our sample. We can see that the
velocity distribution of both sample are quite similar and it also agrees
with Reduzzi & Rampazzo (1995). The differences in the
angular coefficient is probably related to the fact that our sample is
richer in wide separtion pairs. Nevertheless we can infer that pairs with
high velocity difference (
) are quite
probably not bound and should be considered as false pairs as also suggested
in previous work by Chengalur et al. (1993).
Since our spectra were flux calibrated we could also compare the shape of
the spectra of binaries and field galaxies. For comparison we observed the
elliptical galaxies NGC 3923 and NGC 3557 which will be used as templates of old
population (Bica & Alloin 1987). In order to compare the
spectra we first normalized them relative to the continuum at 5500 Å. A
residual spectra was then defined as the difference between the normalized
spectra of each galaxy (
), and the normalized spectra of
the elliptical galaxy NGC 3923 used as a template of old population
(
). In Fig. 3 (click here) we present the spectra of NGC 3923
and the residual spectra of NGC 3557. Since both objects are representative
of old population we can see that the residuals deviated less than 10%
showing no systematic trends with wavelength.
Figure 3: Spectra of the field elliptical NGC 3923 used as a template for old
stellar population. The residual spectra of the elliptical NGC 3557 show
that both objects have a similar population
Figure 4: Upper panel- residual spectra of g351a, an early type
galaxy with blue excess. Bottom panel- early type
galaxy with a residual spectra typical of an old population object
However, when we examine the residuals of ellipticals in pairs we verify
very clearly that the blue continuum of these objects, in the region around
, tend to be more intense than the template object.
In Fig. 4 (click here) we present two extreme examples of early-type galaxies
having large (g351a) and small (g362a) deviations in the blue region. In
Fig. 5 (click here) we present the same plot for late-type galaxies (g222b and
g266b). The steeper blue continuum can be used as a clear signature of young
population (Bica & Alloin 1986), which suggests that galaxies
showing this effect have had recent star formation episodes. Using the
residual of the normalized spectra (
) we can measure the extent of this effect by defining the
residual (R40) evaluated at 4000 Å. As a first approximation we
assumed that the spectral energy distribution of a galaxy is composed by the
contribution of an old stellar population, similar to the adopted template
object, (
) and a younger counterpart (
). Using this
approximation we obtain,
where 
and 
are equal to
the mean normalized flux of the young and old population, respectively. We
can see from this result that the mean residual contains an information
relative to the difference between the normalized spectra of the old and
young population respectively. In Col. 13 of Table 1 (click here) we present
the residual parameter measured for each galaxy in our sample. Actually, we
can use the observations of Bica & Alloin (1986) to verify
that the young globular clusters of LMC with ages between 
and 
, and metallicities 
have 
. On the other hand, a
typical old globular cluster have 
,
close to the ratio of 0.58 observed in NGC 3923. Therefore, the
difference between the mean normalized flux of young and old population
is bounded to an
interval limited by 
. Obviously the exact value
of this parameter for a given object will depend on the mean metallicity and
age of the assumed young and old population. However, this quantity have a
typical value 
. Therefore, within the
uncertainties due to the mettalicity and age history of the underlying
population, the fiducial residual result to be approximately equal to the
fractional contribution of the young stellar population flux at the
reference wavelength (
).
On these grounds we can conclude from Fig. 4 (click here) that the young
population contribution of g362a is probably less than 10%, but it is
substantially higher (
) on g351a. For spirals the young
stellar contribution is obviously higher as can be seen in Fig. 5 (click here).
Figure 5: Two examples of spirals showing a strong young population (g222b),
and a moderate blue excess
Our spectra are representative of the central region of the objects,
therefore we do not sample the same spatial dimensions in nearby and in the
more distant objects. The slit aperture used in this work was 2.35 arcsec
projected in the sky corresponding to an spatial region in the range
, with a mean representative value of 
. Since there exist a gradient in surface brightness, and
also in the star formation rate, in the central regions of interacting
systems, the R40 indicator might be affected by this aperture effect.
Actually if we consider the 
profiles of the list of strong
interacting systems analysed by Bushouse (1987), in the same
spatial region that we have sampled our objects, the flux drops by a factor
of 10. Since the profiles are roughly exponential we can predict that the
mean flux in the slit, for the nearest objects, should be
approximately 86% of the central flux, while for the more distant it will
drop to 59% of the central flux. Therefore we expect that one object, if
observed in the same radial velocity range as our sample, would present a
variation of the mean detected star forming rate of 45%. To test for the
presence of this effect we present in Fig. 6 (click here) a plot of R40 as
a function of the mean radial velocity of our pairs. We can observe that
the high star forming region of this diagram is populated basically by
nearby objects. However we caution that this is actually due to the
Malmquist bias due to the limiting magnitude of our sample. In fact all the
objects with R40 higher than 0.35 are faint late type irregular spirals
with mean absolute blue magnitude of 
, and therefore cannot
be seen beyond 
with apparent magnitude brighter than
15.5. For the brighter spirals we could not detect any systematic variation
with distance. The absence of a strong aperture effect among spirals is
probably related to the fact that our sample is not populated by highly
interacting systems. As pointed out by Kennicutt et al.
(1987) the contribution of the nuclear emission to the
luminosity, in a sample of paired galaxies similar to ours, amounts to
13% and is smaller than the one found on strongly interacting systems.
For early type objects there is a correlation with radial velocity pointing
the presence of an small systematic effect. For the low velocity objects
(
) the mean value of R40 is 
, while for the ones
with higher velocity R40 is 
.
The residual parameter R40 shows no clear correlation with the
projected separation, normalized to the galaxy diameter, as can be seen from
Fig. 7 (click here). For the early type objects there is a very marginal
correlation (r=0.20) in the sense that loose pairs tend to present a
lower star forming rate. However, this is quite probably an spurious
correlation resulting from a side effect of the slit aperture. As we
mention before the residual parameter in more distant objects tend to be
slightly smaller due to the larger spatial region sampled by the slit and
the gradient on the star formation rate. On the other hand nearby wide
separated pairs are far more difficult to detect due to chance projection of
field galaxies along the line of sight. Therefore pairs of larger
separation are more common at a higher distance and since in these objects
the slit effect is larger an artificial correlation is introduced by this
effect. In the case of spirals there is not even a marginal trend of the
star formation with pair separation. The dominant effect is a larger
dispersion of the residual parameter among objects of distinct morphological
types. The average residuals of early-type objects in pairs is consistent
with a young stellar contribution of the order of 
, while for
spirals this contribution can be as large as 40%, and even larger for
late spirals. It is worthwhile mention that the star formation rate
inferred from CO observations of a sample of binary galaxies (Combes et al.
1994) points to a weak but detectable correlation with component
separation in the range 
. We remark that these two
results are not contradictory since the sample of Combes et al.
(1994) was richer in close interacting pairs having a higher level of
tidally induced star formation rate.
Figure 6: Residual parameter, R40, for early type objects (
),
spirals (
) and irregulars (
) showing no clear trend with the
mean radial velocity
Far-infrared fluxes of the galaxies at 60 and 
were
obtained by consulting the IPAC/IRAS Faint Source Catalog. A total of 7
objects out from the list of 17 confirmed binaries were detected. In 3
pairs (g280, g331, g363) the spatial resolution of IRAS was not enough to
separate the emission of each member, and we assume that most of the
emission comes from the spiral component in those cases. The mean infrared
luminosity for these objects is 
.
Due to the presence of a larger number of late type spirals, the proportion
of IRAS sources detected in the complete sample, described by Soares et al.
(1995), is higher, 172 objects out from 189 pairs, when compared
with the sample observed in the present work. After correcting for those
cases where we could not separate the emission of each member we are left
with 162 objects, and in this case we obtain 
. These values can be compared with Xu & Sulentic
(1991) that have obtained 
for a list of SS binaries where both components are spirals, extracted from
the sample of Karachentsev (1987) and 
, for a list of isolated field galaxies. Therefore we
conclude that the FIR luminosities is also enhanced in our binary pairs when
compared with field galaxies.
The absence of a clear correlation of R40 with separation in a larger scale and the enhancement of FIR luminosities is probably related to the fact that even the small tidal interaction between wide separated pairs is enough to stimulate the mean star forming activity. Actually the detailed mechanism of the interaction depends on variables like the mass ratio and the coupling between spin and orbital angular moment, which are quantities that are almost impossible to be inferred for wide pairs. However, we notice that even galaxies with small residuals, like g362a, show an increase in the blue continuum not seen in the field objects like NGC 3557. Furthermore, the absorption lines in the Mg region (5175 Å) are much more intense, resulting in large negative values of the residuals. Therefore, galaxies in pairs are probably affected by either an increase in the star formation rate or a higher metallicity, when compared with objects in the field.
Figure 7: Residual parameter, R40, for early type objects (),
spirals () and irregulars () showing no clear trend with the
projected pair separation