Surface photometry of binary galaxies. I. A multicolour study of morphologies due to the interaction

Astronomy and Astrophysics Supplement Series, Jul 2018

We present the multicolour (B, V, R) surface photometry of 53 double galaxies. All the pairs belong to the catalogue of Reduzzi & Rampazzo (1995) containing objects selected according to the Karachentsev (1972) criteria. We comment on the morphological, structural and photometric characteristics of pairs and their members. Different classes of interaction induced phenomena, both among early and late-type galaxies, are considered. We found that few early-type galaxies show fine structures. Grand design structure is more frequently detected in binary than in field spirals both for barred and non barred, confirming Elmegreen & Elmegreen's (1982) study. The colour of the tails is consistent with the stripping hypothesis since it is similar to the progenitor galaxy outskirts. Among our objects we have no evident sign of induced star formation in tails. Rings appear on average bluer than the disc as a whole.

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Surface photometry of binary galaxies. I. A multicolour study of morphologies due to the interaction

Ntot Surface photometry of binary galaxies. I. A multicolour study of morphologies due to the interaction? L. Reduzzi 1 2 R. Rampazzo 0 1 0 Osservatorio Astronomico di Brera , Via Brera 28, I-20121, Milano , Italy 1 Send o print requests to: L. Reduzzi 2 Istituto di Fisica Applicata, Universita di Milano , Via Brera 28, I-20121 Milano , Italy | We present the multicolour (B, V , R) surface photometry of 53 double galaxies. All the pairs belong to the catalogue of Reduzzi & Rampazzo (1995) containing objects selected according to the Karachentsev (1972) criteria. We comment on the morphological, structural and photometric characteristics of pairs and their members. Di erent classes of interaction induced phenomena, both among early and late-type galaxies, are considered. We found that few early-type galaxies show ne structures. Grand design structure is more frequently detected in binary than in eld spirals both for barred and non barred, con rming Elmegreen & Elmegreen's (1982) study. The colour of the tails is consistent with the stripping hypothesis since it is similar to the progenitor galaxy outskirts. Among our objects we have no evident sign of induced star formation in tails. Rings appear on average bluer than the disc as a whole. galaxies; photometry | galaxies; elliptical and lenticular | galaxies; spiral | galaxies; interactions | galaxies; evolution 1. Introduction Beginning in the '70s numerical simulations of encounters (Toomre & Toomre 1972; Barnes & Hernquist 1992 and reference therein) and the development of photometric models (Larson & Tinsley 1978; Kennicutt 1990 and references therein) both indicated that galaxy evolution is modi ed by interactions. This suggested a connection between the morphological disturbance and the di erent levels of activity (from nuclear activity to star formation, SF hereafter, enhancement) in galaxies. This connection is not yet completely understood and a clear correlation between morphological distortions and sign of activity does not yet exist (Dahari & De Robertis 1988). The presence of strong SF processes in galaxies with nuclear activity indicates that nuclear activity, enhanced SF and gravitational interaction are connected properties. Recently, Moles et al. (1995) studied the morphology of the galaxies showing Seyfert or LINER activity in the Veron-Cetty & Veron (1991) catalogue concluding that what \seems necessary for the nuclear activity to appear is the ability of a given galaxy to produce a global, well organized response to the nonaxisymmetric component of the potential. This could explain why active galaxies are not so frequent among strongly interacting systems, where the galaxy is torn apart." Nuclei and rings seem privileged places of SF, but the dispersion in the colour of the rings is due also to the gas richness of the galaxy. Pogge & Eskridge (1993) found that HI rich S0s either show H emissions with a ring-like structure or are concentrated in the centre. The bar, in fact, tends to remove the gas kinetic energy and angular momentum driving the gas towards the centre. Rampazzo et al. (1995) , studying a sample of interacting pairs, showed that LINER phenomena and a shock induced inversion of the ratio [NII]/H can happen in the very centre of the nuclei, typically inside 1 Kpc, in conjunction with the presence of a bar. Marston & Appleton (1995) , studying asymmetric rings, probably due to a head-on encounter, found that \ring galaxies have similar H luminosities as starburst galaxies, with star formation being almost exclusively concentrated in the ring". Since the detailed local mechanisms of star-formation are not known, simulations of interacting galaxy encounters formulate di erent expectations concerning SF development and SF loci during the collision. Several groups have tried to quantify the SF enhancement due to tidal perturbations adopting di erent SF laws. Noguchi & Ishibashi (1986) adopted a star formation linked to cloudcloud collisions. They found that the SF rate could be enhanced by a factor 6 after pericentre, during a close passage. The gas collects in the centre and in a transient ring. The order of magnitude enhancement they found was the same as observed by Bushouse (1987) in H . Olson & Kwan (1990 a, b) assumed that the SF follows the disruption of giant molecular clouds by collisions. Adopting a SF e ciency, they obtain reasonable agreement with the observed luminosity-to-gas ratios in interacting and merger systems. However, they can account for the highest ratios only by considering the gas depletion into star formation. The increase in SF e ciency is then not due to an increase in burst luminosity, but to a gas content decrease. This does not take into account possible gas infall to fuel the star-burst, as suggested by the Combes et al. (1994) result of enhanced molecular content in interacting galaxies. Mihos et al. (1991) in their simulations adopted a Schmidt law for the SF rate. They found that most of the time the SF rate is enhanced in interacting galaxies, since the gas is gathered in spiral arms, and driven towards the centre. In some cases, the gas is so depleted in tidal tails that the SF rate is decreased with respect to an isolated galaxy. However, they have not included self-gravity, which considerably underestimates radial gas flows. Thus, we still need observations in order to answer some basic questions: 1) Is it possible to de ne the necessary and su cient conditions which connect the activity to the morphological features produced by the interaction? It is clear that a more quantitative morphological classi cation of galaxies and features is needed. 2) What dynamical conditions may induce/enhance the activity ? When is SF enhanced during an interaction episode (just after the perigalactic passage or later)? Do we also observe a dimming in the SF? 3) Where does SF occur (in the circumnuclear region as in some prototypical galaxy with nuclear activity (Heckman et al. 1986) , in the ring, in the disc, etc.)? How is this connected with the amount of gas in the interacting members? 4) What are the mechanisms through which the di erent phenomena are activated (bar and gas driven mechanisms (see Combes et al. 1994) , cloud fragmentation, etc.)? Most of the above questions require a statistical approach on a carefully selected sample. Due to their environmental simplicity, compared to that of groups or clusters, isolated binary galaxies provide a unique opportunity to study galaxies and their sub-components (stellar bulge and disc, gas and dust), in a non-equilibrium con guration. The compilations of isolated double galaxies tend to select nearby, typically interacting, galaxies. The aim of such samples is to isolate the population of binaries for which the potential energy of interaction is by far larger than the interaction energy with any other nearby galaxy. Pairs may then o er several pieces of information about the questions sketched above. In this paper we present the multicolour photometric study of a sample of 53 pairs selected from Reduzzi & Rampazzo (1995: RR95 hereafter). Our attention is centred on the detailed galaxy morphology and on the colour of those features that simulations credit to the interaction. The sample contains a wide range of pairs, from gas rich (spiral/spiral) objects to those (early-type/earlytype) containing members where SF can be at threshold levels. The sample is among those more homogeneously selected for the twodimensional study of the colours of features in interacting objects. Previous samples (see e.g. Schombert et al. 1990; Schmidt & Hintzen 1991) contain a mixture of pairs and multiple objects. The RR95 catalogue has been extracted from The Surface Photometry Catalogue of the ESO-Uppsala Galaxies (Lauberts & Valentijn 1989: ESO-LV) . The photometric parameters in the ESO-LV catalogue are derived from analysis of plate material. Global parameters are given for more than 15000 galaxies, but the material in the B and R colours, suitable for a morphological classi cation, cannot give information about detailed structure of the objects, for which deep and high resolution material is needed. Colours give information neither about local variations possibly due to interaction penomena nor on their shape (rings etc.) since only apertures at standard distances are given. Isophotal shape is not given in ESO-LV. Our detailed photometric data then complement ESO-LV information. In Sect. 2 CCD observations and the reduction technique are presented. In Sect. 3 the general sample properties and the results on single objects are discussed. In Sect. 4 objects are grouped and discussed according to the di erent topics previously sketched. In particular, the systematics of morphological distortion induced by the interaction and connected signs of the SF activity deduced from colours, are discussed in the light of current models. In a forthcoming paper (Paper II) a detailed pro le decomposition analysis and the determination of photometric parameters will be performed with di erent aims among which the location of early-type members of pairs in the fundamental plane. 2. Data acquisition and analysis A journal of the photometric observations is given in Table 1. The observations were done in di erent runs between 1992 and 1994. All the objects were imaged using the 0.92 m ESO-Dutch telescope placed at La Silla, Chile, and with similar instrumental set up. In this sense, our data can be considered as obtained under quite homogeneous observational conditions. The original frames were in the Bessel b, v and r bands. The image cleaning, dark and bias subtraction, flatelding, cosmic ray removal and the nal calibration and image manipulation were performed using the IRAF package. In Table 2 for each separate frame are given the values of seeing and the sky surface brightness measurements. The latter have been estimated avoiding lowsurface-brightness features, halos of bright stars or any other objects that might degrade the estimate. The table reports also the errors in the sky determination. The zero points are based on measurements of photometric standards. When the night was not fully photometric (see in Table 2 the quality parameter) we used the aperture photometry available in ESO-LV. The standard stars used for calibration purposes were obtained in the elds of Rubin 149, PG09-18, PG13-23, PG16-33, Markarian A, T Phe, SA 98. These elds contain few calibration stars each and crowding was not a problem as in the typical elds surrounding globular cluster areas. We used the standard photometric transformation equations tailored to the ESO (La Silla) systems and extinction coe cients: (B − V ) = b1 + b2 (bs − vs); V = v1 + vs + v2 (vs − rs); (V − R) = r1 + r2 (vs − rs); where bs, vs and rs are the instrumental magnitudes in the Bessel bands and r1, r2, b1, b2 , v1, v2 are the transformation coe cients. We used a total of 25 standard stars. Their colour range is −0:296 (B − V ) 1:553 and −0:149 (V − R) 0:849. This large range is crucial for any project dealing with star-forming galaxies. Determination of the luminosity pro les of early-type members were accomplished through interpolation of the isophotes with an ellipse. A di erent methodology has to be used for spiral galaxies where the presence of arms perturbs any ellipse interpolation (except for the bulge dominated region). We tested the equivalence of two reduction 3.1. The sample strategies that have been discussed in the literature (e.g. 3. Discussion Boroson 1981) . The rst technique derives the luminosity pro le as the average of a sample of radial traces at di erent position angles adequately corrected for inclination. The second one uses an azimuthal pro le obtained by tting ellipses with a xed position angle and ellipticity previously determined on the external isophotes. While in the external regions these procedures tend to give similar results, both techniques do not produce an adequate pro le of the bulge. For this reason we decided that our nal pro le would be the composite of a completely free azimuthal pro le (free selection of the position angle and ellipticity of the isophote) to represent the bulge component and a xed pro le (where the position angle and the ellipticity are de ned by the outer isophotes) to describe the galaxy outskirts. Table 3 reports the salient properties of the observed pairs and their members. In Figs. 1.1 and 1.2 are shown some of the properties of the sample. The ESO-LV catalogue reports the parameter Ntot which corresponds to the total number of galaxies in a square degree around a given galaxy. This is a measure of the surface density, although the authors state that it correlates also with the volume density. Since according to the authors a galaxy is in a rich cluster when Ntot > 9 − 10 most of our pairs, see Fig. 1.1, populate very low density environments. Following the classi cation given in the ESO-LV the morphological type of the members of galaxy pairs cover nearly all the Hubble sequence. There is no indication that the observed pair members have similar morphological types. The nal results consist of isophotal maps for each pair Actually, as shown in Fig. 1.1, also the regions in which (Fig. 2) and luminosity plus colour pro les for each mem- mixed morphological types are present, are rather popuber (Fig. 3) . For the early-type members the geometrical lated. We know the velocity di erence for 60% of the pro les (ellipticity, twist, a4=a) are given as well. The de- sample. Two pairs, namely 292 and 143, have to be considviation from the elliptical shape, described by the a4=a ered optical. Because the fraction of optical pairs ( V > parameter, is discussed in the text for each early-type ob- 1000 km s−1) in the RR95 catalogue is 11%, we expect few ject. The geometrical pro les do not change as a function other cases among the sample here discussed. The separaof the colour. Foreground stars have been removed before tion between the members in the observed sample is very any tting procedure. small, 1 galaxy diameter (h a25;a/a25;bi = 1:2 0:7). We are then mainly dealing with tight pairs, although in projection. 3.2. Comments on individual objects Figure 4 presents 2D (B − V ) or (B − R) colour maps for the pairs showing the most evident signs of interaction. In order to control the noise in the low signal regions a Gaussian smoothing 200 200 has been applied to the colour frame. A continuous scale, rather than colour slices, has been selected in order to represent colour variations in the galaxies and in their features. Colour pro les given in Fig. 3 report the formal errors in the measured colour. Due to the smoothing procedure adopted, in the 2D colour maps the latter can be considered as an upper estimate of the error in the di erent regions of the galaxy under examination. The foreground stars in the eld have not been subtracted from the frame, only spikes, where present, have been masked. The smoothing can then create ctitious patchy regions but they can be easily identi ed comparing the 2D colour maps with the original frames in Fig. 2. In this section we describe the observed features of pairs and their members. This will be the basis for the general morphological distinction and evolution considerations developed in the next Section. For the classi cation we follow ESO-LV complemented by RC3 (de Vaucouleurs et al. 1991) . Usually there is agreement between the two classi cation systems; when we notice a di erence we report it in the text. In the case of early-type galaxies (from Es to late S0s) we try to parameterize the shape of the luminosity, ellipticity, twist and a4=a pro les for a more quantitative classi cation (see Capaccioli et al. 1990) . It can be easily foreseen that some di erences will emerge when comparing classi cation obtained from plates and from high resolution CCD images. A particular attention will be dedicated to the presence Redshifts are available for 60% of the objects and of ne structures in the early-type component. A `classisince z 0:025, no K-correction has been applied to our cation' of ne structures has been given by Schweizer magnitudes and colours. (1992). Among the more important for the origin of the early-type galaxies hosting them are ripples/shells since We also describe in the text a search for faint struc- their origin is generally attributed to merging or acquisitures such as ripples, shells, X-structure and dust-lanes tion phenomena. The origin of other features like isopho (see Schweizer 1992 and references therein) in the early tal shape (boxiness or disciness: see Bender et al. 1989) type members of the pairs. The method adopted for en- are more uncertain (see Stiavelli et al. 1991; Governato hancing ne structures is discussed in Reduzzi et al. et al. 1993) . Goudfrooij (1994) suggests that the dust in (1994). early-type galaxies, as opposed to spirals, has probably an external origin since the mass of the cool dust is independent of the optical luminosity of the galaxy. X-structure is among the most rare features and its origin is also attributed to the merging of two galaxies (see Binney & Petrou 1985; Hernquist & Quinn 1989) . We are aware that Forbes & Thomson (1992) found that the detection of this latter feature is connected in a tangled way to boxy isophotes. For this reason, attention has been paid in order to disentangle real from spurious e ects. Median integrated total (B − V ) colours of galaxies according to their morphological class are given in Roberts & Haynes (1994 : RH94 hereafter). In their Table 1 the median (B−V ) has a monotonic decrease from E-S0 (0.90), to Sm-Im (0.42) through S0a-Sa (0.78), Sab-Sb (0.64), SbcSc (0.55) and Scd-Sd (0.48). RH94 report that the \range of colour among Sa galaxies overlaps that of Sc: some Scs are as red as some Sas and some Sas as blue as Scs". The samples used by RH94 are rather inhomogeneous in terms of environment but the interacting objects were excluded from their analysis. So they interpreted the spread in (B − V ) colour as presumably due to current SF rates in individual objects. Balcells & Peletier (1994) studied colours and colour pro les of the bulges for a set of earlytype spirals. The average central colour (B − R) for 18 objects is 1:54 0:12. (B − R) colour pro les show different trends. Some objects have central values that reach (B − R) 2 even if the measure is obtained on the less contaminated side (e.g. along the minor axis). Also in the Balcells & Peletier work interacting galaxies have been avoided. Thus we may consider the above ones as comparison and reference samples. Notes: an estimate of the photometric quality of the night is given in the last column with the following legenda: 1=good; 2=intermediate; 3=low. NP indicates that the luminosity pro le has not been extracted (see individual notes for an explanation). Pair 24 (AM0115-444). Due to the small separa- 500and 1000, a small deviation from the de Vaucouleurs law tion of the galaxies we did not compute the luminosity is visible in the luminosity pro le in correspondence to a pro les. In fact, each member is strongly contaminated by variation of the ellipticity trend. In the same region 62a the light of the companion. The complexity in the isopho- is up to 1% boxy, while outside the a4=a pro le is quite tal structure prevented us from modelling each member irregular. The twist is less than 10 . 62b is a spiral (Sb in and then to try an iterative subtraction. One of the ob- the ESO-LV classi cation) seen nearly edge-on and the exject is classi ed as Sb while the second is a lenticular. The ponential disc is clearly visible. The pair is then a typical 2D (B − V ) map shows that the colour of the two objects E+S and the colour pro les are typical of the two classes. is quite similar and consistent with the late type morphological classi cation. The distorted structure of the arms suggests that they are probably created/modi ed by the interaction, but their (B − V ) colour is indistinguishable from that of a normal spiral. Pair 94 (AM0449-612/AM0450-612). This is a hierarchical pair with no obvious sign of interaction. The ESO-LV classi es the members as late spirals (Sbc/SIrr). 94a shows a prominent bar, visible also in the luminosity pro le, from which arms start. The spiral struc Pair 62. 62a is classi ed as a lenticular but the lu- ture is grand design. In 94b the disc is predominant as seen minosity pro le can be well represented by an r1=4 law. in the pro le. Colour pro les are typical of their morphoOutside the region contaminated by the seeing, between logical classes according to RH94. Pair 98 (AM0459-753). Sbc+Sa pair following the ESO-LV classi cation. Arp & Madore (1987 : AM87 hereafter) suggest that the pair is probably part of a quartet. 98a is a spiral with grand design structure. Colours are typical of spirals in all galaxy regions. The (B − V ) colour map shows that arms are bluer although foreground stars contaminate the eld. 98b shows an exponential disc and, outside the seeing disc, a twist less than 10 . No arms are visible in our frame since the galaxy is seen at high inclination. The colour is quite red also in the outskirts; we suspect strong absorption features caused by dust whose presence can be deduced by the asymmetries of the isophotes (more compressed in the South-Eastern side). This fact influenced also the measure of the a4=a pro le which appears strongly discy (up to 8%). Pair 99 (NGC 1738/39). Interacting Sb+Sbc pair. In both pro les an exponential disc is visible. 99a is very disturbed with multiple arms. 99b shows a bar in the centre; its colour pro le is flat and blue also in the centre unlike the companion which has a nearly similar morphological type. Pair 100 (AM0459-340). In the RR95 catalogue this object is classi ed as a triplet (see also AM87) since a small galaxy North of 100a is also interacting with it. This latter is not listed in the ESO-LV catalogue because its diameter is less than 10. A bridge of matter links it to 100a. RC3 classi es 100a as a uncertain spiral. The luminosity pro le of 100a has structures (a bar?) superimposed. The colour pro le is blue along all the galaxy with the exclusion of the very centre (the bar?) as is clearly visible in the 2D colour map. This latter shows that the stretched arm/tail is blue ((B − V ) 0.5) with clumpy structure. 100b is less luminous than the companion. The bulge is well developed. The luminosity pro le can be basically tted with an r1=4 law although there is a residual structure and, together with the arms, it justi es the ESO-LV classi cation as S0a. The arms are quite open and di used; a signi cant twist is visible from the isophotal contour. Pair 101 (AM0501-225). Following the ESO-LV classi cation, 101 is composed of an E/S0+E. Luminosity pro les con rm such classi cation. The pair does not show evidence of interaction. In both objects the isophotes are strongly discy (larger than 10% in 101a and up to 5% in 101b) as visible from the luminosity, the a4=a and the ellipticity pro les. The isophotes are not signi cantly twisted. The colour pro le of 101a tends to be bluer in the outer parts. Pair 105 (AM0505-230). In the ESO-LV the pair members are classi ed E/S0 and Sd. The colour pro le of 105a is typical of an early-type galaxy and can be tted with an r1=4 law. No ne structures are visible since a model made with the parameters shown in Fig. 3 perfectly subtracts the galaxy. 105b is a spiral where the disc predominates. The disc shows asymmetries in the light distribution. The case is similar to that in 204b. Pair 106 (NGC 1811/12) (AM0506-292). Sa+Sa pair in the ESO-LV. 106a has a dust-lane along the galactic plane. Its colour pro le is stable and redder than the companion 106b which has the same morphological classi cation but no visible sign of dust. In fact, the colour pro le of 106b becomes very blue in the outskirts for an average Sa indicating active star formation. 106b has a bar in the centre and a ring. Pair 120 (NGC {/2144) (AM0545-820). Isolated, hierarchical and very separated (8:05) Sd+Sa pair. The fainter member, 120a , is very disturbed. The arms are like tails, but an underlying disc is visible. The luminosity pro le of 120b can be tted with an r1=4 law but a luminosity structure is visible in the residuals. The colour prole is quite noisy and red suggesting that di use dust can be present. The isophotes of 120b, the dominant member, are asymmetric in the East-West direction (faint arms?). The colour pro le is more similar to that of an early-type galaxy and the luminosity pro le can easily be tted by an r1=4 law, although signi cant residuals are present. Pair 129 (AM0608-333). The pair, in the ESO-LV, is composed of Sbc+S0a galaxies (in RC3 both are classi ed as S?). The members are strongly interacting. The RR95 indicates that 129 is probably part of a group. Only the pro le of 129a has been obtained; 129b is very distorted. The colour pro le behaves like that of `normal' spirals and is very blue in the outskirts (see also the colour map). 129b is later in type than 0.0 given by the ESO-LV, since arms, probably created/modi ed by interaction but bluer than the disc, are visible. Pair 131. The pair consists of two spirals of type later than Sbc. The luminosity pro les in fact are dominated by the exponential disc. The colours are typical of late spirals. Pair 133 (NGC 2211/12). The ESO-LV classi ed the pair as an S0+S0. 133a is a classical lenticular as can be seen by the exponential disc. The ellipticity grows monotonically up to 0.6 in the inner 1000and then stabilizes at this value up to the last observed point. An X-structure is present in the central part (see Fig. 4 in Reduzzi et al. 1994 for the image) . The X-structure is concentric with the outer isophotes since the twist, outside the seeing disc, is negligible. The colour pro le is constant with (B − V ) 1.1, so the colour of the galaxy is not modi ed by the presence of this structure. 133b is unusually blue, starting from the very centre, for the morphological classi cation assigned by the ESO-LV. In the isophotal map a strong twist is visible probably due to a bar-like Pair 155. The pair is an Sb-c+Sc in the ESO-LV catastructure. logue. 155b is seen nearly edge-on. 155a has a grand design structure, a prominent bar and an inner ring. The (B −R) Pair 136 (AM0631-341). The pair is classi ed as and the (B − V ) colour pro les of 155a have a di erent S+E/S0 in the ESO-LV. The luminosity pro les suggest shape. A nearby bright star (just outside the frame) can that both galaxies have an exponential disc. 136b has be responsible for the anomalous behaviour in the R-band. colour pro les redder than a typical early-type galaxy, The (B − V ) colour pro le is quite red ((B − V ) 0.9) for while those of 136a are typical of spirals of Sc class. The this nearly face-on Sb-c galaxy. a4=a pro le of 136b is up to 1.5% boxy inside 1000, while outside the galaxy shows signi cantly discy isophotes (up Pair 161 (AM0830-593). Sbc+Sa pair in the ESOto 2%). LV while 161b is classi ed as uncertain spiral (S?) in RC3. 161a has a bar and the arms are very open and grand de Pair 143 (NGC 2305/07) (AM0648-641). The ve- sign. The colour pro les have the typical trend of objects locity di erence of this E+S pair, as deduced from the with the given classi cation. RR95 catalogue, is quite high (1067 km s−1). Even if the Pair 162. S/Irr+Sa pair in the ESO-LV. The objects pair is really isolated and the magnitude di erence be- are visibly interacting as shown by a tail South of 162b. In tween members is only 0.56 B-mag this could be an optical both luminosity pro les appear the exponential disc. The pair. 143a is an elliptical with some sub-structures visible galaxies have similar colour pro les although the morphoboth in the ellipticity and in the a4=a pro les. It appears logical types are quite di erent. This could be a possible in the Knapp et al. (1989) compilation among early-type example of `Holmberg e ect' in action, since the morphowith IRAS (upper limit) flux (S12 =0 27, S25 =0 22, logical type jumps from 1 to 7. S60 =0 40, S100 =0 91). 143b shows a well developed Pair 175 (AM1003-435). Sa-b+S/Irr in the ESO-LV bar and a ring which are frequently found in interacting classi cation. Long tails are indicative of the interaction objects; the arms' structure is grand design. Since both of the two members. It is presumable that the internal galaxies are very bright, the background could be probably rotation of the two galaxies is signi cant. The luminosity overestimated with the typical cut-o sign in the fainter pro le is not given because of the strong contamination part of the pro le (see 143a). The colour pro les are nor- of one object on the other. Further, the numerous stars mally red for 143a and typical of its spiral class for 143b. in the eld and the two particularly bright ones North of In particular the 2D colour map of 143b evidences that 175b hinder the interpretation of the 2D (B − V ) colour the bar has a nearly homogeneous colour ((B − V ) 1). map. We observe that the (B − V ) colour of the Eastern Pair 153 ({/IC 2200) (AM0727-621). S0+S pair tail is the less contaminated and it is 0:8 − 0:9. The in the ESO-LV. 153a has a signi cant isophotal twist Western tail seems bluer but stars are present. We cannot (larger than 20 ) and o -centring in the outer part of then infer much from the tails. 175b have then the typical the galaxy, in a direction which is not that connecting colour distribution of a late spiral. We may only suggest the galaxy nuclei. The colour is typical of an early-type that (B − V ) colour distribution in the 2D map of 175a is galaxy (see also the colour map). A model subtracted from reminiscent of a disturbed early-type object rather than a the galaxy enhanced a spiral-like feature visible also in the spiral. original image (see Fig. 5 in Reduzzi et al. 1994) and the Pair 177 (AM1006-380). Sa+E pair in the ESO-LV asymmetries are created by this peculiar kind of `arms'. catalogue while in the RC3 177a is an S0a with outer and This kind of features are also visible in a simulation done inner rings and peculiarities. The presence of a bright star by Weil & Hernquist (1993) in a merger remnant. In our near the pair makes the photometry di cult. 177a shows case we notice that the asymmetry creates an area, visible a strong bar and a distorted ring probably created by the in the colour map, redder than the overall disc suggest- interaction itself. In the 2D colour map it is possible to ing that it could be a spatial re-distribution of bulge stars distinguish the two rings reported by RC3 which appear driven by the interaction. This galaxy deserves a spectro- slightly bluer than the rest of the galaxy. In the colour proscopic study in order to verify the presence of other signs le the internal ring is clearly visible as a sudden change disentangling the possible merging origin. 153b is a late- in the trend. 177b is an elliptical galaxy. The luminosity type spiral (Sb-c). The arms are grand design. As visible pro le follows an r1=4 law, although the contamination of also in the 2D colour map the galaxy is quite red also in the companion is visible in the outer parts of the prothe outskirts with a (B − V ) 0.7. RH94 report that the le. The colour is typical of such morphological type. In variation among the colour of spirals is quite large. Al- the outer part the a4=a pro le becomes signi cantly boxy though the presence of a di erent amount of dust (and up to 1.5−2%, but the contamination of the light by the inclination) may contribute to the large dispersion among nearby companion (which is in any case masked during the the spiral classes, this is a good example of the so called modelling) could account, in part, for this e ect. Outside `Holmberg e ect' under which paired galaxies with di er- the region contaminated by the seeing the position angle ent morphological type tend to have the same colour. of the isophotes is nearly constant. diameter (at B=25) ratios of the pair members obtained from the ESO-LV. Pair 181. S0+S0a in the ESO-LV. Both galaxies show arms may have been created by the interaction. They are tails (181b) or asymmetries in the isophotal structure reminiscent of the `arms' seen in 210b. (181a). These characteristics are probably due to the ongo- Pair 197 (AM1116-290). Sb+Irr in the ESO-LV cating interaction. 181a has a twist of the isophotes of 20 . alogue, while the RC3 classi es 197a as an uncertain spiA structure, probably a lens, is visible in the ellipticity, ral. 197a, the brighter member, is strongly disturbed and a4=a (larger than 2%) and luminosity pro les. 181b is with multiple arms. Open arms (or tails, we do not see, in later in type than the companion but not yet a spiral. The fact, an underlying disc) connect 197b to the companion. colours of both galaxies are consistent with their classi - The colour pro le of 197b becomes bluer going toward the cation. The tail of 181b is similar in colour ((B − V ) 0.8) outskirts of the galaxy (see also 2D colour map). Isophotes to that of 175a. No evident signs of star formation are of the galaxies show also a signi cant twist (larger than present. The pair is possibly one of the objects described 30 ), while a4=a indicates that the inner isophotes are in Davoust & Prugniel (1988 : DP88) discussed in the next slightly boxy (a small bar?). section. This case is particularly interesting since the pair Pair 204 (AM1158-345). Sa+Sb pair, strongly inis quite large in comparison with those previously studied, teracting. Open arms starting from a well visible central suggesting that the phenomenon is long-lasting as indi- body (nucleus + bar or lens) in 204a with characteristics cated by the Combes et al. (1995) simulations. similar to the case of 197b. Also the trend of the colour Pair 182. Irr+Sc pair. Only the brighter object, 182b, pro le is similar. The `arms' of 204a remain red. This is of this pair has been studied. The pair is among the most better visible in the 2D colour map where the colour of hierarchical in the RR95 catalogue. While 182b does not the `open arms' is approximately the same as that of the show evident sign of distortion, the companion appears main galaxy body. In Noguchi's (1990) simulations open quite asymmetric as described by its classi cation. arms develop at the beginning of a disc/disc encounter. Pair 184 (AM1023-291/2). S0+S pair. 184a is clas- Stars and possible gas are in an expansion phase caused si ed in the ESO-LV as a lenticular. The luminosity pro le by the interaction. The disc of the companion, 204b, which and colours con rm such classi cation. It is disturbed by is seen edge-on, is strongly distorted. The 2D colour map an interacting, small satellite which shows asymmetries in indicates that the latter has quite blue and clumpy areas the isophotal structure similar to tails. 184b is a face-on along the disc. spiral whose colour pro le has a normal behaviour: red in Pair 210 (NGC 4105/06) (AM1204-292). E+S0 the bulge part (where also in the 2D map the bar and a in the ESO-LV. The galaxies cover the frame and the sky lens structure are visible) and becomes bluer in the out- determination is quite critical in these cases. We probably skirts. slightly overestimated the sky background. The elliptic Pair 187 (NGC {/3289). Sa+S0a pair. 187a is a spi- ity and twist pro les of 210a are quite stable, while the ral with very blue colour also in the central part where a a4=a pro le is boxy in the centre and discy in the outbar is present. The galaxy is seen nearly face-on so the in- skirts (where the light contamination by the companion ternal absorption does not influence signi cantly the mea- is stronger ); furthermore, it has dust in the centre. 210a sured colour. This behaviour is di erent from that of 184b appears in the Knapp et al. (1989) compilation among although the morphology of the two objects is remarkably early-type with IRAS (upper limit) flux (S12 =0 21, similar. The arms' structure is typically grand design as in S25 =0 36, S60 =270 40, S100 =740 135). The comthe 184b case. We are probably looking at similar galaxies panion 210b is classi ed as lenticular in the ESO-LV. A at di erent stages of an encounter. This information can di use, rounded arm (or tail, we do not clearly see the come from the structure of the arms, which in the case of symmetric arm), possibly produced by the interaction, is 187a are much more open than in 184b. Noguchi's (1990) clearly visible. The structure of the latter is similar to the simulations show that during an encounter a bar forms arms in 187b, while the central structure is reminiscent of and arms, induced by interaction, tend progressively to that visible in 204a. 210b also appears in the Knapp et close around it. If this interpretation is correct, 184b is al. (1989) compilation among early-type with IRAS (upseen at a later stage of its encounter with respect to 187a, per limit) flux (S12 =0 21, S25 =0 27, S60 =230 22, and the star formation has had the time to fully develop S100 =630 151). The colours of both galaxies are consisalso in the centre. The gas, in fact, as shown by the same tent with their morphological classi cation. simulations, tends to fall towards the centre. A case simi- Pair 211 (NGC 4112/{) (AM1204-395). This is lar to 184b is seen in 143b. 187b is classi ed as lenticular an Sbc+Sa pair. The proximity to bright stars makes the but di use arms are visible in the frame and the bulge is photometry of 211a di cult. In both galaxies the lumiquite boxy (a bar?). The galaxy is seen at high inclina- nosity pro le shows the presence of the exponential disc. tion and the absorption can play a signi cant role in the 211b has an isophotal structure very similar to 100b. The measured colour. The overall colour, in fact, is as red as colour pro le tends to be bluer in the very centre. that of an early-type. Obvious morphological signs of in- Pair 215 (IC 3152/{). E/S0+Sc pair. 215a is an elteraction are not visible, if we exclude the possibility that liptical with normal colour pro le. Outside the seeing disc, the isophotes show a signi cant twist, while their shape in the isophotes. The twist is signi cant ( 20 ). The a4=a does not deviate from the elliptical form. 215b is a nearly pro le indicates that the galaxy has discy isophotes (up edge-on spiral galaxy with a prominent dust-lane which to 3%). The companion, 248b, shows a strong dust lane de nes the galactic plane. along the galactic plane which we see slightly inclined. Pair 216 (IC 3290/NGC 4373) (AM1222-392). Both colour pro les tend to become bluer towards the S0a+E/S0 pair. The inner structure in 216a is similar to outskirts that in 204a but the arms' con guration suggests that it Pair 248 (NGC 5152/53) (AM 1325-292). Sb+E is probably seen in a later stage of the encounter. 216a pair in the ESO-LV classi cation. 248a is classi ed as Sb has a central bar and the colour pro le is typical of an but the arms show unusual shape and debris of matter early-type. The luminosity pro le of 216b is tted by an are present. On the other side, the companion does not r1=4 law so it is rather an elliptical with discy isophotes as show obvious signs of interaction. The pro les were not evidenced by the a4=a parameter. The ellipticity and twist computed because of the reciprocal light contamination. pro les are quite flat outside the region contaminated by We preferred to plot a 2D (B−R) colour map which shows the seeing. After a model subtraction, a di use lamen- that in the outer part 248a has a colour typical of a spiral. tary structure centred on the nucleus and extended on the The colour of the plume of matter visible North-West of opposite side of the companion is enhanced. Irregularly 248a's outskirts is contaminated by the presence of two distributed dark areas are present in the residual, prob- bright stars in the eld. ably indication of a dust presence. The galaxy appears Pair 250 (NGC {/5193). S0+S0 in the ESO-LV in the Knapp et al. (1989) compilation among early-type classi cation, while 250b is classi ed as E in RC3. The luwith IRAS (upper limit) flux (S12 =0 25, S25 =0 28, minosity pro les indicate that 250b is an elliptical, while S60 =0 45, S100 =0 169). the CCD frame shows that 250a is probably an S0a since it Pair 217 (AM1226-402). Sc/Sd+Sd pair in the seems to show incipient arms as in the case of 187b. Both ESO-LV classi cation (217b is classi ed Sdm with a bar galaxies have discy isophotes, 250a up to 4% and 250b in RC3). The exponential disc dominates in both lumi- up to 2%. The colour pro le of both galaxies is consistent nosity pro les. 217a has a quite red central region. On the with the classi cation we sketched above. 250b appears opposite, 217b is very blue starting from the very centre. in the Knapp et al. (1989) compilation among early-type No obvious signs of interaction are visible. with IRAS (upper limit) flux (S12 =0 26, S25 =0 40, S60 =150 27, S100 =470 64). We expect then the pres Pair 222 (AM1248-255/4). S0+Sa pair in the clas- ence of di use dust. si cation of ESO-LV, while 222b in RC3 is classi ed as Pair 267 (AM1401-243). Both members are clasS0a. Between the two galaxies (Fig. 2) `clouds' of matter si ed as Sc in the ESO-LV. 267b shows a grand design are visible. Their con guration makes the origin unclear, structure of the arms, which are open and starting from although it may be connected to the ongoing interaction. a bar. The colour 2D pro le shows normal behaviour 222b shows a strong bar and an inner ring, visible also in for an Sc with a slightly redder centre (the bar) and the luminosity pro le. The colour pro le is stable and typ- blue, patchy outskirts. 267a has quite asymmetric and ical of galaxies earlier than spiral, although arms are evi- o -centred isophotes. The overall appearance of this endent. 222a is classi ed as a lenticular galaxy and a strong counter has some analogy with that described for Pair 129. disc is, in fact, visible both in the luminosity and in the In particular we note the strong asymmetry of 129b and a4=a pro les. The twist is negligible. 267a. Pair 225. S0+E/S0 pair in the ESO-LV classi cation. Pair 273 (NGC 5516/{) (AM1412-475). No obThe luminosity pro les suggest that both galaxies are bet- vious signs of interaction are visible in both objects which ter classi ed as Es. Series of shells and/ripples are present are early-type galaxies (E/S0+E/S0 in the ESO-LV classiin 225a (we detected 4 di erent layers). The boxiness of cation). 273a is quite red in colour. The twist is negligible the a4=a pro les is due to the presence of the ripples. The (lower than 10 ). The isophotal shape parameter a4=a intwist is not signi cant. 225b is an elliptical galaxy, up to dicates that the isophotes do not signi cantly deviate from 2% boxy in the outskirts. A faint ring (a shell?) in the elliptical. The isophotes of the companion, 273b, are discy inner region is enhanced by unsharp masking. The colour up to 4.5% (see also the increase of the ellipticity at the of the galaxy tends to be bluer in the outskirts. same distance from the centre). Pair 244 (NGC 5124/26) (AM1322-300). E+S0a Pair 280 (AM1440-241). We did not attempt to obpair in the ESO-LV classi cation. The RR95 catalogue tain luminosity pro les and geometrical information since indicates this object as a possible quartet (in the ESO- the two galaxies overlap and a bright star's corona inLV the other two faint galaxies in the eld are not listed). fluences the light distribution. Only the overall isophotal 248a is classi ed as elliptical but the luminosity pro le can map is given. 280a is classi ed as a lenticular galaxy. The be better decomposed adding a disc. The galaxy does not unsharp masking technique evidences that 280b (Sa in the show obvious signs of distortion if we exclude a small warp ESO-LV ) has a strong dust-lane system. Pair 284 (AM1529-272). The pair is formed by late- Pair 293 ({/ IC 4782) (AM 1846-553). Sc+Sc type galaxies (Sc+Sab in the ESO-LV classi cation). Both pair. Asymmetries in the isophotes and signs of distortion objects show asymmetries in the isophotal structures: in in both members of the pair. The colour pro les are quite both cases they are elongated towards the companion. The blue in the outskirts. The exponential disc dominates in colour pro les of 284a show a dip at about 2000. the luminosity pro le of 293b. Pair 285. Both galaxies are classi ed as lenticulars. Pair 296 (IC 4817/{) (AM1902-561). Both galaxThe pair is quite hierarchical and there are no obvious ies are late type. (Sbc/Sc). Both members show distorsigns of interaction. In the North-West direction a dust- tions. In particular 296b arms appear open, di use and lane system is visible. From the luminosity pro le we may asymmetric. The overall morphology is reminiscent of deduce that 285a is better classi ed as E since it follows 267b. Exponential discs dominate in both luminosity prostrictly an r1=4 law. Its isophotes are discy up to 2%. The les. The colour pro les are consistent with their morphotwist is negligible. 285b is embedded into the companion logical type. and the photometry is quite di cult owing to the light contamination. Both colour pro les are quite red (the low galactic latitude may play a signi cant role) and constant along all the galaxy. Pair 297 (NGC 6734/36) (AM1902-653). This is a E/S0+E pair in the classi cation reported in the ESOLV. Luminosity and colour pro les are consistent with this classi cation. 297a is discy in the inner part. No ne struc Pair 287 (AM1724-622). The `Sacred Mushroom'. tures are visible. 297b is only slightly discy and, in fact, A bridge of matter links the two galaxies. Around the after the subtraction of the galaxy model the residual is a brighter one, 287a, the interaction has developed an ex- di use, elongated structure which probably is not a reguternal (asymmetric) ring. The 2D colour map shows that lar disc. Other two small galaxies are in the eld. For this this ring is bluer than the 287a disc but the large number reason in the AM87 catalogue AM1902-653 is classi ed as of foreground star superimposed makes di cult to draw a quartet. ttsccagadbwihhlozeiomiaiseendtdsmtheiessyslritrutaaetrmhnrtmnohaehaiftetssitetagoe2lftrhtptlrhet8hyhtcaohci7eoepmsbeaaannertpvcccirtoltiedrloasheurimgmfssacasesutpgitegiohtreaainhsevdonsc0o.aeufaio:rncto8rtTauoeinlooatlmih(fonnsnsaeksueok2.eseinrre8srr(Imt7itBtatpsesFahnasriioesoo−ligofsSfta.lfweal2WValesn3s.8noio)t)a7maTiglSbctpcliuahu0i.noonllaellaasatlosgtxleyr2uiioti.pborDreoaelnasTeddoslhghc.iftwioaaeao(lsltgs1hoahEesr9aiuxeeeecS9rymceeh4O.omm)mstslT-.ohueaLnahunIagiVnpnoetrt- fttcaanuTowhocnllehreatengoeirSt2lPio.icasn90gonoaTmu8aciblibioao+chrjisllennueeeEavc2sarintAifi/t9ssosdpoeSit8Mrdbrrea0aobnlr8(alebietoo{7nlyints/ewnvhctg.tIetlaihtarCthFmarhyehseoleslen4ritccmsetehll8ehboaaeebe2erssseoiiese3rcntgrit2th)gcsaha9.ldcnel(8amtaaaeAdbhsxtns4eseicyMuiam=aorcsantifcdbblw1oeaEpieegr9suitrr+dsiti0s2sovtio,jn97beneoo8ncoy-lt.feft6b+uOsiDb2,4ttvcy9hasut0ePoi8hretd)n8tesase.ehi8diigqdsTce.petonuoenTehhpElisreotoethhatShefueuotpOcerriiitreaoenreb-anpirnLtsriaretrenV.errgoriigaeess---. Pair 290 (NGC 6438/{) (AM1806-852). The pair is made by a lenticular and an irregular galaxy, following Pair 307. S0+E pair in the ESO-LV catalogue. 307a the ESO-LV classi cation. The encounter has been dis- shows sign of distortion and a presence of a single arm, ruptive for the late type member. The geometry of the probably created by the interaction. The arm has a blue early-type member, 290a, has already been investigated colour (B −R) 1.0−1.1. The early-type companion, 307b, by Rampazzo & Sulentic (1992: pair #54 in their compi- seems completely unperturbed. Twist is negligible, while lation). de Mello et al. (1995) found that the dominant the shape of the isophotes jumps from slightly boxy (1%) population in the nucleus of this galaxy is old and metal to strongly discy (up to 3%). de cient ([Z=Z ]max = −0:5). However, an important flux Pair 317 (IC 5013/{) (AM1947-445). S0+Sb pair fraction at 5870 A ( 37%) arises from a young compo- in the ESO-LV classi cation but we do not see arms in nent (age 5 108 years). 290b has arms with HII regions. 317b. The colour pro les are in fact consistent with an Their (B − V ) colour is about 0.4−0.5. Although fore- early-type classi cation for both objects. 317a has discy ground stars in the eld enhance the noise, the 2D colour isophotes (up to 4.5%) but the luminosity pro le is tmap evidences the distribution of star formation areas. ted by a simple r1=4 law. The twist is negligible, while Since the morphological type of 290b is very late, it is im- the isophotes of 317b show a twist (30 ) which is partly possible to draw conclusions about possible SF enhance- the consequence of the light contamination caused by the ment due to the interaction. proximity of the brighter companion. 317a is an IRAS Pair 292 (AM1840-622). Although we obtained the emitter (Knapp et al. 1989) whose flux is S12 =0 31, photometry of this pair we consider it an optical align- S25 =0 33, S60 =200 25, S100 =600 80. ment, since its V = 3191 km s−1. It appears in the Pair 322 (NGC 6935/37) (AM2034Journal of Observations but has not been considered in 521). Sa+Sbc pair. The luminosity pro les indicate that the following analysis. both galaxies have an exponential disc. The colour pro le ESO-LV ident. Notes: morphological types, total B magnitudes and the galaxy density parameter Ntot are obtained from ESO-LV. The morphological type given in the notes, when present, is deduced from the inspection of the CCD frame. Columns labelled with numbers are: (1) Ripples, Shells, (2) Tail-Plumes, Jets, (3) X-Structure, (4) Dust, (5) Asymmetries of 322a becomes extremely blue in the outskirts, while it is consistent in values and in the general trend with that of a galaxy morphologically earlier that an Sa, possibly an S0a. 322b shows multiple, open arms starting from a prominent bar, while 322a has small, central open arms. Pair 353 (AM2143-464). The pair is in a rich envi ronment. The two galaxies are both late type (S0a+Sbc). Colour and luminosity pro les may be contaminated by the light coming from the companion. Both galaxies are very di cult to model and consequently the contribution of each cannot be realistically subtracted. The 2D colour map con rms the classi cation of the ESO-LV since the colours are typical of their classes. This pair, although less disturbed, is reminiscent of pair 290, but probably represents an earlier phase of the encounter. The colour of 353b is similar to that of 290b. 4. Quantitative morphology and photometry RR95 found that there is no correlation between the morphological types of the pair members contrary to Karachentsev (1972) . We have reclassi ed the galaxies in our sample on the basis of the CCD material. We found that 25% of the entire sample of pairs have at least one of the members misclassi ed. This happens especially among early-type objects, with an average error hT i= 1. In our sample 13.5% are pairs in which a `bona de' elliptical is associated with a spiral (E+S), a fraction which is in agreement with the ndings by Rampazzo & Sulentic (1992) . E+S0 are also represented in the sample with a similar fraction, but the bulk of the present sample is composed by disc/disc pairs (S0+S or S+S) with a limited presence of irregular galaxies. Some of them are classi ed as irregular but they may easily be completely distorted spirals (see e.g. pairs members 290b, 296b). 4.1. Morphological distortions in early-type pairs Among very close and isolated E+E pairs, DP88 identify a class of objects showing morphological distortions. In this class the outer isophotes of each galaxy are displaced with respect to the inner ones in opposite sense along a direction nearly perpendicular to the line joining the centres of the two galaxies. The deformation appears symmetric with respect to the centre of the pair establishing with a high con dence that they represent physical binaries. Combes et al. (1995: and references therein) simulations show that this kind of feature is produced during an interpenetrating encounter. The displacement is visible also out of the plane of the encounter and it is long-lasting (108 years). We looked for this e ect in our early-type pairs. The pair 298 and possibly 181 show this e ect. 14% of the RR95 southern catalogue of pairs is made by E+E. Any de nite conclusion about the frequency and duration of the DP88 phenomenon needs a study of a statistically signi cant set of objects extracted from the catalogue. 4.2. Fine structure in early-type objects Early-type galaxies in pair (54 objects in total) have been analysed with the purpose of evidencing the presence of ne structures like shells/ripples, X-structure, tails/jets etc. following the scheme outlined by Schweizer (1992) . The structures detected are reported in Table 4. Both shells/ripples and X-structure are rarely detected (3.7% and 1.8% respectively). A large uncertainty about the frequency of ne structures exists since all samples available in the literature are incomplete. Schweizer (1992) extracted a representative sample of galaxies brighter than BT =13.5 but the analysis and the frequency refers to part of them (74 objects out of 145 in total). The observations were conducted using the KPNO 0.9 m telescope. The higher frequency of shells detected (56% for E and 32% for S0s) refers to this sample, which is composed, according to the author, mainly of eld galaxies. On the opposite, Malin & Carter (1983) in their southern search for galaxies with shells, found that the nal sample represent 16.5% of isolated ellipticals in RC2. Furthermore, they suggest that these structures are much rarer in denser environments, like groups and clusters. Our results then suggest that the occurrence of shells/ripples in early-type members of pairs is similar to that found by Malin & Carter (1983) for dense environments. In Reduzzi et al. (1995) the comparison with a sample composed of isolated (i.e. in low density environment) early-type galaxies is discussed. Eight objects show clear, well organized dust-lane structures. Six early-type galaxies, namely NGC 2305, NGC 4105, NGC 4106, NGC 4373, IC 5013 and NGC 5193 are detected in the IRAS survey mainly at 60 m and at 100 m (see individual notes). The isophotal shape of these objects is either nearly elliptical or discy. The fraction of IRAS-detected early-types is then very small. Also Rampazzo & Sulentic (1992) found a similar result studying 22 E+S pairs. The current interpretation is that FIR luminous objects contain cool interstellar matter (108 M , Jura 1986) in the form of dust that is reprocessing starlight. Dust is actually detected in the centre of NGC 4105 and seems more di use in NGC 4373. No special signs of interaction are evident in the other objects if we exclude NGC 4106 in which bar and di use arms seem created by the interaction. 4.3. Spiral structure in binaries Elmegreen & Elmegreen (1982) developed a 12-division morphological system to classify spiral galaxies according to the regularity of their spiral arm structure. This spiral arm classi cation system correlates with the presence of density waves, e.g. grand design galaxies tend to contain prominent density wave modes. Using these arm classes they classi ed some samples of spirals belonging to di erent environments, among them a binary sample derived from Turner (1976) . The sample of Elmegreen & Elmegreen (1982) is incomplete and moreover the Turner (1976) binary galaxy sample has been strongly criticized since it included many pairs belonging to groups and even clusters (White et al. 1983) . Elmegreen & Elmegreen found that 1) the binary membership probably influences the spiral structure in non barred galaxies, 2) the binary galaxies with bars are almost always grand design (93%) and this fraction is larger than the fraction of grand design galaxies among eld spirals. Following the Elmegreen & Elmegreen (1987) revision we succeeded in classifying 36 spirals (Table 5). Some of the spirals in the global sample are, in fact, seen nearly edge-on, strongly interacting or simply do not t into the Elmegreen & Elmegreen classes. Four galaxies, namely 175a/b, 197b, 204a, 248a, 287b are generally classi ed late-type in the ESO-LV although their `open arms', interpreted in the framework of Noguchi's (1990) simulations, developed during the interaction. In the case of 280b the presence of di use dust makes the arm classi cation di cult. Among barred spirals 21 out of 22 (corresponding to 95%) objects have a grand design structure while among non barred galaxies only 7/14 (50%) galaxies have a grand design structure. The fractions are consistent with the results of the analysis of Elmegreen & Elmegreen (1982) . In particular the grand design structure is more frequently detected in binary than in eld spirals both for barred and not barred. Grand design structure is then probably connected with the binary membership. 4.4. Colours of interaction features and induced star formation A fraction of our objects may be interpreted in the framework of the simulations performed by Noguchi (1990 and reference therein). The simulated galaxies, in fact, have similar sizes not di erent from those in our sample. The bar selection criteria applied (see RR95), tend, in fact, to bar? avoid strongly hierarchical pairs. Besides, a large fraction of our pairs possesses a disc, with a di erent gas richness. Noguchi's models follow both the stellar and gaseous components evolution in a disc galaxy during the encounter. an asymmetric way with the largest concentration in the In the simulations di erent `phases' may be evidenced. centre. 1/4 of the total initial gas mass is gathered within Just after the perigalactic passage (0.4 disc rotations at 2 Kpc from the nucleus. In this scenario of `moderate' inthe outer edge) completely open arms (integral sign) are teraction the bar develops quite soon and it is long lasting, created by the passage of the companion. A second phase while the ring develops later and the gas follows the con(between 1 and 1.4 disc rotations) may be individuated guration of the ring. We then expect that the ring could when a bar develops and arms start to close. The gas is be a place where SF might occur. The gas nds a stable still dispersed all over the disc but it starts to follow the con guration for about 2-3 disc rotations and may have star con guration. After about 2 disc rotations the arms the time to form stars. are completely closed around the bar and form a ring. The As noticed before a large fraction of the spirals in our gas is now mainly concentrated in the centre and in the sample shows a bar (60% of those classi ed in Table 5), ring. The last phase described by the models is when the which could be a transient product of the interaction in ring starts to be disrupted by the internal dynamics and Noguchi's models. The bars present in our sample are in the overall appearance of the galaxy is nearly asymmet- the average quite red h(B − V )i 0:91 0:14 compared ric (after 4 disc rotations). Also the gas is distributed in with the galaxy outskirts h(B − V )i 0:66 0:17. We detected a total of six rings, namely in 106b, 143b, frequency of the shell phenomenon compared to isolated 155a, 177a, 222b, 287a, while the presence of blue struc- samples. ture in the centre of our objects is rare. The average Six early-type galaxies in our sample are detected by h(B − V )i colour of rings is 0.77 0.15. In 177a the dis- IRAS. Half of them show clear dust structures. torted ring appears bluer than the galaxy body as seen Two pairs seem to show the e ect described by DP88. in Fig. 4. Asymmetric rings may have a more `traumatic' The study of the kinematics of these objects would give origin. In the case of 287a, the `Sacred Mushroom', the us information about the presence of a U-shape rotation ring origin could be a nearly head-on encounter (Wallin et curve foreseen by simulations (Borne 1990; Combes et al. al. 1994) . Our (B − V ) and (B − R) colours (Fig. 3) of the 1995) and expected from the ablation of the galaxy outmain body of the galaxy agree very well with those given skirts. by the previous authors, (B − V )=0.87 and (B − R)=1.52 101a and 133b show in the central part an anomalous respectively. The ring present in 287a appears bluer than blue colour possibly indicating star formation areas. the body of the galaxy ((B − V ) 0.6). From the observational point of view there is no clear Arms of spiral galaxies in pairs are of grand design indication that SF in tails is enhanced: most of the tails type in 95% of cases when a bar is present, while 50% maintain the colour of the outskirts of the progenitor of non barred galaxies are grand design. These results are galaxy. Only in few cases there seem to be knotty blue consistent with ndings of Elmegreen & Elmegreen (1982) . areas which Schombert et al. (1990) interpreted as \short In both cases (barred and non barred) spirals in pairs have (107 yr), weak bursts". On the basis of Noguchi's (1990) grand design arms in a larger fraction with respect to their models, integral sign arm con guration are made by the eld counterparts, indicating a correlation between pair distorted disc material (reminiscent of tails), in the rst membership and grand design structure. phase of the encounter. Thus they are expected to pre- We analysed the loci of SF in a selected subsample serve the colour of the progenitor. This could be the case made of galaxies in the framework of the simulations perof 175a, 197b and 204a. Also the tail of the members of formed by Noguchi (1990) . In this context we observe that the pair 181 shows the colour of the progenitor. The long among the studied objects the SF in the bar is not modelongated `tails' visible in 24a and 24b may be interpreted i ed during the encounter. At the same time the ongoing as long asymmetric arms matching the colour of the arms SF is not depleted in the outskirts during the encounter. A of a typical spiral. Blue knots are visible in 100a, 175b (al- possible variation in SF rate is detected in the ring which though stars are superimposed) and 287b. In any case the can develop during de nite phases of an encounter. Simmain body of the latter galaxies are gas rich spirals and ulations show that the gas tends to collect in a ring while SF is already expected to happen in the outskirts. Sum- the larger fraction collapses towards the centre. In our marizing, in our sample we have indication neither that sample, the rings tend to be bluer than the nearby parts SF is going on in tails connected to an early-type galaxy of the galaxy, i.e. the stellar disc as a whole. This is not progenitor nor that there is a depletion of the SF due to only true for late-type galaxies but also for some galaxies the interaction. which are presumed to be early-type like 177a and 287a, the `Sacred Mushroom'. It is clear that the SF e ciency is correlated to the quantity of gas present which may be 5. Conclusions parameterized as a function of the morphological type. At The study presents the B, V , R surface photometry of the same time, the gas may also be compressed during 53 pairs of galaxies extracted from a catalogue of south- an encounter. Combes et al. (1994) , in fact, studying CO ern pairs (RR95). We use the luminosity pro le and the in pairs show that there is a correlation between the ingeometry of the isophotes in order to give a quantitative teraction class and the CO emission. Tidal tails tend to morphological classi cation of each pair member. We are show the colour of the galaxy outskirts, consistenly with looking for secular evolutionary e ects in the photomet- the stripping hypothesis. We have no clear indication that ric and structural properties of galaxies in pairs. The 2D SF can be induced/stimulated in them by the encounter, colour maps have then been produced in order to study at the same time SF is not suppressed when it is active in the connection between interaction morphology and sign the progenitor. of induced SF activity. In general, our data suggest that a moderate interac One of the pairs, namely Pair 292, has V = tion has the function to constrain the gas into certain areas 3191 km s−1 and is then considered an optical alignment. basically the nucleus and the ring. As noticed by Moles et It has not been used in the general discussion. al. (1995) the galaxies produce a \global, well organized Very few objects among early-type galaxies (−5 T response" to the nonaxisymmetric potential created by the 0) show peculiarities. Approximately 30% show ne struc- interactions. This is especially evident for gas poor galaxtures. Shells are rarely observed in our sample (3.7%). ies which may develop SF in those areas. The formation of As previously suggested by Malin & Carter (1983) early- an (asymmetric) ring can be also the response to a more type galaxies in high density environments show a lower disruptive encounter (Marston & Appleton 1995) as in the case of the `Sacred Mushroom' in this work (see also simulations of Wallin & Struck-Marcell 1994) . The frequent presence of bars, a transient phenomenon following simulations, indicates that this is the most used mechanism to transfer the gas to the centre and possibly activate both SF and nuclear phenomena. Acknowledgements. This paper has bene ted from corrections and useful suggestions of the referee Dr. E. Davoust. LR acknowledges the kind hospitality and the use of the facilities of the Osservatorio Astronomico di Brera. The tables of the photometric and geometrical pro les may be requested to LR. We thank Dr. Francesca Rampazzi for the patient debugging of our English. Fig. 2. Images of pairs in R band. The bar, 30 arcsec wide, indicates the scale of the frame (North is at the top, East at the left). The step of the isophotes is 0.5 mag/arcsec2; the external isophote is at 23 mag/arcsec2 while the last grey level reaches 24 mag/arcsec2. (To be seen in landscape) Fig. 4. (B − V ) or (B − R) colour maps. The pixel size is 0.4 00/px. The grey scale an the correspondent colour value are given at the top of the gure. For (B − V ) the isophotal contours are: 0.4, 0.6, 0.8, 1.0; for (B − R) are: 1.1, 1.3, 1.5, 1.7. 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L. Reduzzi, R. Rampazzo. Surface photometry of binary galaxies. I. A multicolour study of morphologies due to the interaction, Astronomy and Astrophysics Supplement Series, 515-571, DOI: 10.1051/aas:1996133