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Mon. Not. R. Astron. Soc. 000, 1­13; (2004)

Printed 14 July 2004

A (MN L TEX style file v2.2)

Star Formation in Close Pairs Selected from the Sloan Digital Sky Survey
B. Nikolic H. Cullen P. Alexander
Astrophysics Group, Cavendish Lab., Cambridge CB3 0HE 2004/03/14

ABSTRACT

The effect of galaxy interactions on star formation has been investigated using Data Release 1 of the Sloan Digital Sky Survey (SDSS). Both the imaging and spectroscopy data products have been used to construct a catalogue of nearest companions to a volume limited (0.03 < z < 0.1) sample of galaxies drawn from the main galaxy sample of SDSS. Of the 13973 galaxies in the volume limited sample, we have identified 12492 systems with companions at projected separations less than 300 kpc. Star-formation rates for the volume-limited sample have been calculated from extinction and aperture corrected H luminosities and, where available, IRAS data. Specific star formation rates were calculated by estimating galaxy masses from z-band luminosities, and r-band concentration indices were used as an indicator of morphological class. The mean specific star-formation rate is found to be strongly enhanced for projected separations of less than 30 kpc. For late-type galaxies the correlation extends out to projected separations of 300 kpc and is most pronounced in actively star-forming systems. The specific star-formation rate is observed to decrease with increasing recessional velocity difference, but the magnitude of this effect is small compared to that associated with the projected separation. We also observe a tight relationship between the concentration index and pair separation; the mean concentration index is largest for pairs with separations of approximately 75 kpc and declines rapidly for separations smaller than this. This is interpreted as being due to the presence of tidally-triggered nuclear starbursts in close pairs. Further, we find no dependence of star formation enhancement on the morphological type or mass of the companion galaxy. Key words: galaxies: evolution ­ galaxies: statistics ­ surveys

1

INTRODUCTION

It is well established, both observationally and theoretically, that galaxies do not evolve in isolation from one another but that their present day structure is the result of sequential mergers and encounters of galaxies and proto-galaxies. For example, Toomre & Toomre (1972) used numerical simulations to show that many of the features of peculiar galaxies can be explained by recent galaxy-galaxy interactions and mergers. Indeed, in the currently widely favoured hierarchical models of galaxy formation, all of the galaxies at the present epoch are the results of mergers and accretion (e.g., Cole et al. 2000). The problem of predicting the outcomes of galaxy encounters is difficult. Much progress has been made using numerical simulations (e.g., Mihos & Hernquist 1996), following the pioneering work of Toomre & Toomre (1972), but several outstanding problems remain. The fate of the inter-stellar medium (ISM) and even-

tual conversion to new stars is particularly difficult to follow due to the finite resolution of simulations and the complicated physics of the ISM. In this paper we investigate one aspect of this problem, the effect of tidal interaction on the star formation rate of galaxies, using data from the Sloan Digital Sky Survey (SDSS; York et al. 2000). The connection between star formation and galaxy interactions has been suspected ever since the first large surveys, biased towards star forming systems were conducted: first in the ultraviolet continuum, then in emission lines and the far infrared. The far infrared survey by the Infrared Astronomical Satellite (IRAS), in particular, found a population of star forming systems in the nearby universe with LFIR > 1 в 1011 L , corresponding to an inferred star formation rate greater than 22 M year-1 . These starforming galaxies exhibit, on average, more peculiar morphologies and are more likely to be mergers. Furthermore, these observational indications of interaction/merger become more common and pronounced in systems with higher star-formation rates. The most luminous of the IRAS sources, i.e., those with LFIR 3 в 1012 L , be-

Supported by a PPARC Studentship Supported by a PPARC Studentship c 2004 RAS


2

B. Nikolic et al.
panas Redshift Survey (Hashimoto et al. 1998). In all three studies a correlation is found between density of environment and star-formation rate in the sense that ongoing star formation is suppressed in the denser environments. Gomez et al. (2003) and ґ Hashimoto et al. (1998) have attempted to decouple this effect from the density-morphology (Dressler 1980) or radius morphology relations (Whitmore et al. 1993) finding that the density/star-formation rate relationship exists independent of the density-morphology relationship. The structure of this paper is as follows: In Section 2 we discuss the compilation of a volume- and luminosity-limited sample drawn from the SDSS, the method used to define galaxy pairs together with details of various physical parameters calculated for each system. In Section 3 we present our results and in Section 4 we discuss these results in the context of earlier work and consider the implications for our understanding of interaction-triggered star formation.

ing universally classified as strong interactions or mergers (Sanders & Mirabel 1996). A number of existing studies have used samples of interacting galaxies to examine statistically the causal relationship between galaxy interactions and star formation. For example, Larson & Tinsley (1978) found a much higher dispersion in the U - B versus B - V colour-colour plot for galaxies selected from Arp's atlas (1966) as compared to a control sample, indicating an enhanced star formation rate within the last 108 years. Kennicutt et al. (1987) used H line and far infrared observations to examine the influence of interactions on the global star formation of a complete sample of close pairs, and a sample of Arp systems. They found a higher than average star formation rate in interacting systems, but also found that a large fraction of galaxies in close pairs exhibit normal star formation rates. Similarly, Bushouse et al. (1988) examined the far infrared emission in a sample of strongly interacting galaxy pairs, finding enhanced emission in some, but not all systems. Bergvall et al. (2003) have looked at star formation in a sample of 59 interacting and merging systems, and 38 isolated galaxies, using spectroscopic and photometric observations in the optical/near-infrared. In contrast to other results, they find that the global U BV colours do not support significant enhancement of the star-formation rate in interacting/merging galaxies. Defining samples of interacting or merging galaxies to investigate the effects of interaction on star formation is not straight forward. One technique, based on the work of Toomre & Toomre (1972), is to select galaxies with peculiar morphologies, especially those showing tidal tails or galactic bridges. Such systems are often drawn from Arp's Atlas of Peculiar Galaxies (1966). This technique has been used by a number of authors, including, Larson & Tinsley (1978) and Kennicutt et al. (1987). The drawback, as illustrated by Toomre & Toomre (1972) themselves, is that the degree of morphological disturbance is sensitive to the orbital parameters of the interaction, and so can not be relied upon to trace all interactions. Additionally, the features characteristic of interaction are often of low surface brightness, requiring deep imaging for detection, and must be identified by visual inspection. When the interacting systems are separated by distances comparable to, or larger than, their optical extents, it is relatively easy to resolve the galaxies in imaging data and, based on their projected separation, identify them as a physical pair. If spectral data are available, the velocity separation can also be used. Using this technique, survey data can by analysed for close pairs yielding large samples of interacting galaxies, albeit usually missing the galaxies in the later stages of interaction and merger. Large area redshift surveys have enabled studies of large samples of interacting galaxies. For example, Barton et al. (2000) have analysed optical spectra from a sample of 502 galaxies in close pairs and N-tuples from the CfA2 redshift survey finding the equivalent widths of H anti-correlate strongly with pair spatial and velocity separation. Similarly, Lambas et al. (2003) examined 1853 pairs in the 100k public release of the 2dF galaxy survey and find star formation in galaxy pairs to be significantly enhanced over that of isolated galaxies for separations less than 36 kpc and velocity differences less than 100 km s-1 . In this paper we follow a similar approach and use the SDSS to define a large sample of 12861 galaxies with identified companions. Our aim is to establish the relative importance of galaxy interactions in determining star formation. A number of studies have established a link between density of environment and star-formation rate, for example using the SDSS (Gomez et al. 2003), 2dF (Lewis et al. 2002), and Las Camґ

2 2.1

THE DATA Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS) is an imaging survey in five broad photometric bands (u, g, r, i, and z, defined in Fukugita et al. 1996) with medium-resolution (R 1800­2100) spectroscopic follow up of approximately one million targets. When complete, it will cover most of the Northern Galactic Hemisphere. The sample used in this work is based on Data Release 1 (DR1) which covers 1360 deg2 of the sky; however, we use the improved spectroscopic data for each object from Data Release 2 (DR2). The Main Galaxy Sample (MGS, Strauss et al. 2002) was selected from the SDSS imaging data as a galaxy sample for spectroscopic observations. Objects were identified as galaxies in the SDSS imaging data if their r-band magnitude measured using the best-fitting galaxy light curve was at least 0.3 magnitudes brighter the magnitude measured using the point spread function model. All such galaxies with r-band Petrosian magnitudes brighter than 17.77 were selected for spectroscopic follow up and were observed, as far as possible, given the fibre-placement constraints and the finite number of fibres on any one plate. The resulting completeness is 93%. Of the 7% of galaxies that are missing, 6% are galaxies which could not be observed due to the minimum fibre separation constraint. The MGS was used to construct (Section 2.2) a volume limited sub-sample. Each member of this sub-sample was than paired with its nearest galaxy (Section 2.4) and its star formation rate estimated using two methods (Section 2.3).

2.2

Primary Catalogue

Our primary catalogue is a complete, volume- and luminositylimited, sub-sample of the MGS. It was derived from the MGS using a procedure analogous to that used by Gomez et al. (2003). ґ The volume of the sample was defined by the redshift constraint 0.03 < z < 0.1, where the lower limit was used to avoid subsequent large aperture corrections when estimating star-formation rates from H measurements (Section 2.3.1). The upper redshift limit allows us to construct a complete sample by retaining only galaxies with r-band absolute magnitudes Mr < -20.45. Two further criteria were needed to remove a small number of
c 2004 RAS, MNRAS 000, 1­13;


Star formation in SDSS close pairs
spurious entries. The first was the elimination of galaxies with redshift confidence less than 0.7. This relatively relaxed constraint removes most of the galaxies with poorly determined redshifts while introducing only a very small bias towards active emission lines systems. Secondly, entries in the SDSS with Petrosian z-band magnitudes fainter than 22.83, i.e., below the detection limit (Table 21 in Stoughton et al. 2002), were excluded. Since galaxies in the main sample have r < 17.77, entries with z > 22.83 have extreme colours and are almost certainly spurious detections. For the purposes of this study, we wish to remove systems in which an AGN provides the dominant contribution to the ionising radiation (in these cases an accurate estimate of the star-formation rate is not possible using the methods we employ). Following Veilleux & Osterbrock (1987), AGN were identified by their positions on the [N I I]/H vs [O I I I]/H and, in a few cases, [O I]/H vs [O I I I]/H planes. We required the signal to noise ratio for the line fluxes to exceeded 2 before taking them into consideration. Approximately 38% of galaxies from the volume limited sample were identified as having an AGN. Of these 64% are early- or mixed-type galaxies. A further 295 objects which were not classified spectroscopically as AGN but exhibited very broad H lines with > 570 km s-1 , were also excluded. 2.3 Star Formation Rate Estimators

3

Cardelli et al. (1989). If the H line was measured with a signal to noise ratio of less than 2, or if it was measured in absorption, no extinction correction was applied. If the measured H flux failed these criteria, the extinction was calculated assuming an upper limit for the H flux of 2.3 в 10-19 W m-2 which is approximately the detection limit. Finally, the H fluxes were corrected for the 3 -diameter circular aperture of the fibres, considerably smaller than most of the galaxies in our sample. The correction was done by scaling the H flux by the ratio of the total r-band flux to the r-band flux measured within the fibre itself. This procedure has been shown, by Hopkins et al. (2003), to provide excellent quantitative agreement with star-formation estimates from global galaxy properties such as radio, far-infrared and u-band continuum emission. This procedure assumes that the measured H equivalent width is representative of the whole galaxy. For the volume-limited sample the median physical size probed by the 3 aperture is 4.4 kpc. The fibre therefore probes a significant fraction of the galaxy and this accounts for the reported accuracy in Hopkins et al. (2003). 2.3.2 Far-infrared continuum

Kennicutt (1998) reviews a number of widely used estimates for galaxy star-formation rate (SFR), including H and [O I I] emission lines, together with ultraviolet, far-infrared (FIR) and radio continuum emissions. Two of these indicators with very different properties were used in this work: the H line and the FIR continuum emission. 2.3.1 The H line

Star formation rates, , for all the galaxies in the primary catalogue were estimated from H luminosities, LH , using the relation presented in Kennicutt (1998):

Far infrared traces recent star formation, where the natal dust clouds still surround the young massive stars and re-processes most of their bolometric output. The effects of obscuration at these wavelengths are far less prominent than at optical/UV wavelengths, removing a potential source of systematic errors. Flux densities at 60 µm and 100 µm were obtained for each galaxy in our volume limited catalogue by automated query of the SCANPI service1 , which co-adds individual IRAS scans at the position of each of the galaxies. The measurement in each band was rejected if the reported signal to noise was less than 2.5 or if the template correlation factor was less than 0.9. The FIR luminosity to SFR calibration we applied was the same as used by Hopkins et al. (2003), namely:



FIR

(M yr

-1

)=

(M yr

-1

)=

This relation is valid in the case of optically thin recombination, and when the star formation obeys the Salpeter (1955) initial mass function. H fluxes were calculated from line parameters measured by the SDSS spectroscopic data processing pipeline, outlined in Stoughton et al. (2002). As discussed above, while our sample is drawn from DR1, we used the DR2 spectroscopy data-products which are better calibrated and more robust. The pipeline uses a number of absorption and emission lines, and broader features, to determine redshifts from spectra. Therefore, the H line need not be prominent to be measured and, indeed, in the majority of objects in our volume limited sample it is measured below the noise level or in absorption. Corrections for dust obscuration, stellar line absorption and the 3 aperture were applied following the prescription presented in Hopkins et al. (2003). Both the H and H lines were corrected for stellar absorption. The equivalent width corrections applied ° ° were 1.3 A and 1.65 A respectively. As discussed in Hopkins et al. (2003), these values are lower than the true stellar absorption because SDSS spectra resolve the absorption profile. Dust absorption correction was calculated from the observed H /H ratio, assuming an intrinsic ratio of 2.86 and the extinction curve presented in
c 2004 RAS, MNRAS 000, 1­13;

LH . 1.27 в 1034 W

(1)

where 1+ f= 0.75 1 + 2.3.3

f LFIR , 1.39 в 1036 W
2.186в1035 W LFIR 2.186в1035 W LFIR

(2)

L L

FIR

> 2.186 в 10

37

FIR

2.186 в 1037 .

(3)

Specific Star Formation Rates

The star formation rates calculated above are not ideal for study of the triggering of star formation as they scale with galaxy mass. A more suitable measure is the specific star formation rate, defined as the SFR divided by the stellar mass. Approximate stellar masses ° were calculated from z-band (eff = 9097 A) luminosities, using Mz ( ) = 4.52, as used by Yasuda et al. (2001), and assuming one solar luminosity corresponds to one solar mass. The Hubble parameter H0 = 70 km s-1 Mpc-1 is used throughout, for consistency with Hopkins et al. (2003). To calculate the z-band luminosity, we used the reported Petrosian magnitudes as introduced by Petrosian (1976) and discussed
1 Available at http://irsa.ipac.caltech.edu/applications/Scanpi/. Provided by NASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.


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17 16 15 14 z

B. Nikolic et al.
150 125 100 0.6 75 0.4 50 N Fraction 1

0.8

13
25

0.2

12
0 0 0 1 2 z -band magnitude difference 3 4

11 9 10 11 12 K 13 14 15

Figure 1. A scatter plot of K - vs. z-band magnitudes for the galaxies in the volume-limited sample detected in the 2MASS XSC.

Figure 2. The distribution (solid line) and cumulative distribution (dashed line) of z-band magnitude differences in spectroscopically confirmed pairs.

extensively in the context of SDSS by Strauss et al. (2002). The fraction of galaxy light measured by Petrosian magnitudes depends only on the shape of the light profile of the galaxy and not on the surface brightness (Strauss et al. 2002). For this reason, if the effect of tidal interaction on near-infrared galaxy light distribution is to only change its characteristic length scale, there should be no bias in estimated masses of interacting and non-interacting systems. The z-band light will inevitably suffer some intrinsic extinction. To asses whether it is an un-biased estimator of galaxy mass we have correlated our volume-limited sample with the Two Micron All Sky Survey (2MASS) Extended Source Catalogue (XCS) which is described in Jarrett et al. (2000). In Figure 1, we compare the Petrosian z-band magnitudes with total K -band magnitudes for the subset of galaxies which were detected in the XSC ­ there is a good correlation between them with a dispersion of 0.25 magnitudes in z. Since K -band magnitudes are generally believed to give an extinction independent estimate of galaxy mass we conclude that for this sample the z-band magnitude also provides an un-biased estimate. As a further check, we have investigated this correlation for close pairs (r p < 50 kpc) and find it is unchanged from that shown in Figure 1. 2.4 Defining a Catalogue of Galaxy Pairs

ited sample is 9 ), the candidate was rejected. This is necessary to remove the small number of cases where the photometric pipeline incorrectly de-blends single galaxies into multiple components. · If the candidate has a measured redshift, the recessional velocity difference is evaluated: if it is less than 900 km s-1 the candidate is accepted, otherwise it is rejected. This is a relaxed constraint. which, according to Patton et al. (2000, 2002), includes all potentially interacting systems and avoids a bias toward dynamically bound systems. · If no redshift was measured, the z-band magnitude difference between the candidate and the primary galaxy is evaluated: if it less than 2 magnitudes the candidate was accepted, otherwise it was rejected. This procedure resulted in 12492 of the 13973 galaxies in the volume limited sample being identified with companions. Of these, 2038 have measured spectra, whilst the remaining systems have only imaging data.

2.4.1

Validity of Spectroscopic-Photometric Pairs

To investigate the effects of galaxy interactions, we have identified the nearest companion galaxy to each member of our primary volume limited catalogue. The method used for this was influenced by the selection constraints of the spectroscopic follow up sample of the SDSS. In particular, the 55 minimum separation between targets on any one plate introduces a significant bias against finding close pairs of galaxies if only galaxies with measured spectra are considered. Therefore, we base our search for pairs on all the galaxies identified in the imaging data, using measured redshifts where available. The procedure used was as follows: for each galaxy in the primary catalogue, a list of candidate companions out to a projected separation of 300 kpc was assembled from the SDSS imaging galaxy catalogues. Each of these candidates were then, in the order of increasing separation, evaluated according to following rules until a satisfactory match was found: · If the angular separation between the primary galaxy and the candidate was less than three times the r-band Petrosian half-light radius of the primary (the mean of this value for the volume lim-

As described above, candidate pairs were not restricted to the MGS, but were taken from all the galaxies identified in the imaging survey. However, in the absence of redshift information, photometric properties must be used to determine if the pairing is just a projection effect or if the galaxies are physically close. We adopted a relatively simple criterion, comparing the Petrosian z-band magnitudes of the two galaxies. This technique suffers both from incompleteness, when there is a large intrinsic luminosity difference between true companions, and contamination by background galaxies. As both of these effects depend sensitively on the cut-off brightness ratio, we now justify the choice of this quantity. Our choice of mz = 2 implies that companions which are fainter than the primary galaxy by more than about a factor of 6.3 in the z-band will be rejected, unless they have a measured redshift. The z-band was chosen for this comparison as it most closely traces the mass of the galaxy. To assess the significance of this incompleteness we have calculated the distribution of z-band magnitude differences in all pairs which have measured spectra, and therefore do not suffer from the above effect. The results, presented in Figure 2, show that mz = 2 is in the tail of the distribution and that the completeness for this choice of mz is approximately 80%. We have also investigated the probability that a detected pair is due to background contamination as a function of the maximum
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Star formation in SDSS close pairs
1 Probability of pro jection pair

5

0. 8 0. 6 0. 4 0. 2 0 0 0.05 0. 1 0.15 0. 2 0.25 0. 3 0.35 Pair Separation (Mpc)

(1994). The concentration index, C, is defined as the ratio of Petrosian 50%- to 90%-light radii as measured in the r-band. Low values of this quantity correspond to systems with high central concentrations of light which in turn are of an early morphological type. The correlation between classical morphological classification and the concentration index for a sample of galaxies taken from the SDSS is presented in Shimasaku et al. (2001). We use the concentration index both to exclude early-type systems from our study of triggered star-formation and to examine the efficiency of this triggering as a function of morphological type. For the purpose of removing early-type systems, we exclude all galaxies with a concentration index less than 0.375, larger than the 0.33 value proposed by Shimasaku et al. (2001). In doing so, we reduce contamination of our late-type sample to less than 5% (see Figure 11 of Shimasaku et al. 2001), at the expense of completeness. 2.6 Pair Morphology

Figure 3. Mean probability of a non-physical pair as a function of separation from the primary galaxy.

magnitude difference. To do this we have used the galaxy count from Yasuda et al. (2001). The parameterised integrated counts, where m denotes the z-band magnitude, are N (z-mag < m) = 11.47 в 10 deg2
0.6(m-16)

.

(4)

The probability of a detected pair being due to contamination is then calculated as a function of physical separation and averaged over the whole of the primary catalogue. The results are presented in Figure 3. They show that at projected separation of over 150 kpc, contamination of our pair sample by background or foreground galaxies becomes a significant problem. Therefore, our results at separations greater than 150 kpc are significantly diluted by nonphysical pairs; the reported separations in this regime can, however, be regarded as reliable lower limits of separation to the true nearest companion.

In the later stages of interaction it is often not possible to resolve the nuclei of interacting galaxies, especially in visible light, and even when they are resolved the automated algorithm in the SDSS photometric pipeline might not de-blend them. More importantly, the algorithm used to select nearest companions (Section 2.4) rejects all objects closer than three times the Petrosian half-light radius. As a result, our pair sample does not contain the closest pairs or merging objects. At the median redshift, and using the median halflight radius, the minimum galaxy separation corresponds to 14 kpc. To assess the importance of this effect, we have visually inspected three sub-samples, each containing approximately 30 galaxies. The first consisted of the most actively star-forming objects, the second of medium star-forming objects and the third was a random control sample. In each case we evaluated the possibility that the galaxy is in the process of gravitational interaction via the presence of tidal arms and double nuclei. The results are presented in Section 3.3.

2.5

Concentration Index 3 RESULTS Our primary sample consists of 13973 galaxies of which 12492 have an identified companion. The distribution of morphological types, as determined by the concentration index (Section 2.5), of the the galaxies in the primary sample, together with their companion, is shown in Table 1. The distribution of absolute star formation rates, , and specific star formation rate, m , are shown in Figure 4. Here we distinguish between three morphologicallydefined sub-samples: late-type (C > 0.375), early-type (C 0.33) and mixed (0.33 < C 0.375). As expected, given the good correlation between concentration index classification and Hubble type, the vast majority of star-forming systems are classified as late-type. The mean star formation rates are 0.7, 1.7, and 3.3 M yr-1 for the early, mixed and late type sub-samples respectively. In this and following sections, we consider each pair to consist of a primary galaxy and companion and examine the star-formation rate of the primary galaxy in terms of the properties of the interacting pair. 3.1 Star formation as function of projected separation

Galaxy morphology has long been known to correlate strongly with the star-formation rate (e.g., Kennicutt 1998). Additionally, numerical simulations by Mihos & Hernquist (1996) show that the time dependence of gas inflow during interactions, and the star formation which results, depend sensitively on the structure of the progenitor galaxies, and in particular on the bulge to disk mass ratio. In order to measure accurately the effect of interactions on star formation, it is important to consider these effects. The traditional classification of galaxy morphology is the Hubble sequence. There is a well established correlation between H equivalent width and Hubble type, equivalent width increasing from zero (within observational errors) in E/S0 galaxies to 20­ ° 150 A in late-type spiral and irregular galaxies, with the same trend shown between Sa through to Sc spirals (see Kennicutt & Kent 1983). The Hubble classification is based on three characteristics: the bulge-to-disk light ratio, tightness of spiral arms and degree of resolution of spiral arms. Besides being hard to automate, the Hubble classification is non-ideal for studies of star formation as the last of the three criteria in particular depends significantly on star formation. That is, the spiral arms are more likely to be resolved if there is significant star formation along them. In this work we adopt an alternative morphological classification scheme, the concentration index, which is described in detail by Morgan (1958), Okamura et al. (1984), and Abraham et al.
c 2004 RAS, MNRAS 000, 1­13;

To investigate the dependence of specific star formation rate, m , on true pair separation we consider firstly the dependence of m on projected separation, looking subsequently at the dependence


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B. Nikolic et al.
Table 1. Distribution across concentration index classes of galaxies in the volume limited sample and their companions.

Primary Galaxy C > 0.375 0.33 < C 0.375 C 0.33 Total
a

Total 7936 3344 2693 13973

Companion Galaxy C > 0.375 0.33 < C 0.375 5120 2064 1455 8639 1500 681 576 2757

No companion C 0.33 555 270 271 1096 761 329 391 1481

a

The number of galaxies with no companion identified within the 300 kpc maximum search radius.

Table 2. The dependence of the specific star formation rate on projected separation is characterised by fitting a two power-law function to the specific star formation rate vs pair separation data (Fig. 5) after its been divided according to morphological type.

Concentration Index bin 0 < C 0.33 0.33 < C 0.375 0.375 < C 1.0

(10

a1 -11 yr-1 2.04 6.30 13.6


)

1

(10 -0.53 -0.41 -0.04

a2 -11 yr-1 0.11 0.70 2.90


)

2

1.19 0.73 -1.04

10

4

10

3

0.375 < C < 1 0.33 < C 0.375 0 < C 0.33

N

10

2

10

1

10

0

(a) 0 10 20 30 40
-1

on velocity separation. In Figure 5, we plot the mean specific star-formation rate against projected pair separation. The primary galaxy in each pair has been assigned, using the concentration index, to one of three morphological classes representing late-, mixed- and early