A catalog of UGC isolated galaxy pairs with accurate radial velocities
aa r X i v : . [ a s t r o - ph . GA ] N ov A catalog of UGC isolated galaxy pairs with accurateradial velocities
Pierre Chamaraux and Laurent Nottale GEPI, CNRS, Paris Observatory, 92195 Meudon CEDEX, [email protected] LUTH, UMR CNRS 8102, Paris Observatory, 92195 Meudon CEDEX, [email protected]
July 17, 2018
Abstract
The present paper is devoted to the construction of a catalog of isolated galaxypairs from the Uppsala Galaxy Catalog (UGC), using accurate radial velocities. TheUGC lists 12921 galaxies to δ > − ◦ ′ and is complete to an apparent diameterof 1 arcmin. The criteria used to define the isolated galaxy pairs are the following:1) Velocity criterion: radial velocity difference between the members lower than500 km s − ; 2) Interdistance criterion: projected distance between the memberssmaller than 1 Mpc; 3) Reciprocity criterion: each member is the closest galaxyto the other one, which excludes multiplets; 4) Isolation information: the cataloglists the ratio ρ between the projected distance to the closest UGC galaxy (havinga velocity difference smaller than 500 km.s − ) and the pair members interdistance,thus allowing one to choose any isolation criterion (beyond the chosen limit ρ > . − ). Our final catalog contains 1005 galaxy pairs with ρ > .
5, of which 509 have ρ > ρ >
10 (27% of the pairs, i.e. 4% of the UGCgalaxies). Then we give some global properties of the pair catalog. We display thehistograms of the radial velocity differences between the pair members and of theirprojected interdistances (median 0.29 Mpc). Finally, we provide an estimate of thecontamination by cosmological false “pairs”, which is about 10% up to a velocitydifference of 380 km s − , beyond which all pairs are probably false. Keywords : catalogues, galaxies, galaxy groups1
Introduction
The isolated galaxy pairs represent the simplest gravitational systems of galaxies. Theirdynamical study is especially fruitful to estimate the masses and mass-luminosity ratiosof their members, and possibly to check the presence of massive haloes and of darkmatter (see for instance the basic works by Peterson (1979) [15] and by Chengalur et al.(1996) [4]). Such studies need a large catalog of those galaxy pairs, with accurate radialvelocities of their members. A pioneering catalog of isolated galaxy pairs has been devisedby Karachentsev (1972) [9]; it lists 603 pairs north of the declination δ = − ◦ and it hasbeen used in several studies. Since then, other catalogs of galaxy pairs have been madeavailable, for instance by Soares et al. (1995) [16] which completes Karachentsev’one inthe Southern hemisphere, and especially by Karachentsev and Makarov (2008) [10], whichgathers 509 bound pairs in the Local Supercluster with radial velocities V r < − .Recently, several works have been devoted to the study of the enhancement of starformation activity in pairs of galaxies due to gravitational interaction, resulting in newpairs catalogs. The most important number of pairs (nearly 13 000) have been used forthis purpose after extraction from SDSS-DR2 and 2dFGRS by Alonso et al. (2006) [2].In the present paper, we construct a catalog of isolated galaxy pairs from the UGC(Uppsala General Catalog of Galaxies, 1973) [13]. The UGC covers the Northern Sky at δ > − ◦ ′ , and it is complete in apparent diameters to 1 arcmin. Since the linear diameterfunction of galaxies is known (Hudson & Lynden-Bell (1991) [7]), one can consequentlymake appropriate corrections for the loss of galaxies with the distance in our catalog whennecessary.Our main purpose is a dynamical statistical study of the isolated pairs of galaxies,which will be developped in next papers. A specific study will be devoted to the statisticaldetermination of the actual velocity differences and the actual distances between themembers of the pairs, which are needed in order to understand their dynamics.The paper is organized as follows: section 2 is devoted to a short description of theUGC and to its diameter function; in section 3, we present and apply the criteria used inorder to define the isolated galaxy pairs; the gathering of accurate radial velocities of themembers is detailed in section 4; the final catalog of 1009 galaxy pairs is given in section5, some of its general statistical properties and corrections for cosmological false pairs arediscussed in section 6 and we conclude in section 7. The Uppsala General Catalogue of Galaxies (UGC) is an essentially complete catalog ofgalaxies to a limiting diameter of 1.0 arcminute and/or to a limiting apparent magnitude2f 14.5 on the blue prints of the Palomar Observatory Sky Survey (POSS), constructedby P. Nilson [13]. Coverage is limited to the sky north of declination − . Using our UGC galaxy velocity distribution, we have the opportunity to check the Hudsonand Lynden-Bell diameter function [7] and possibly improve it. Indeed, this diameterfunction is defined by: Φ e ( D ) dD = Φ ∗ e exp( − D/D ∗ ) d ( D/D ∗ ) (1)where Φ e ( D ) dD is the number of galaxies by volume unit with diameter in the range[ D, D + dD ], Φ ∗ e is the density of UGC galaxies by volume unit (for all diameters) and D ∗ is a characteristical diameter.A galaxy at a distance r is included in the UGC if its apparent diameter is largerthan 1 arcmin, i.e. if its linear diameter D r > πr/ . D r in kpc and r in Mpc).Therefore, from Eq. (1), the number dN ( r ) of UGC galaxies between the spheres of radii r and r + dr is given for an homogeneous distribution by dN ( r ) = 4 πf Φ ∗ e exp( − D r /D ∗ ) r dr, (2)where f is the fraction of the sky surface covered by the catalog ( f = 0 .
42 for the UGC,accounting for the obscuration zone from our Galaxy).If we transform the distance r into radial velocity v through the Hubble law, we obtain dN ( v ) = 4 πf Φ ∗ e H exp( − v/v ∗ ) v dv (3)for the number of UGC galaxies in the interval v to v + dv , where v is the Hubble velocityat distance r and v ∗ is the velocity of a galaxy of diameter D ∗ seen with an apparentdiameter of 1 arcmin.We have least-square fitted with this law the tail of the new velocity data ( v > − , in order to remove nearby fluctuations due to clustering) and we have obtained(on ≈ v ∗ = 1997 ±
85 km s − (see Fig. 1). By integration, we obtainΦ ∗ e = N H / (8 πf v ∗ ), with N = 12141 (the number of galaxies having available radialvelocities). This leads to a value of the catalog density Φ ∗ e = (0 . ± . h Mpc − .These results agree within error bars with the Hudson and Lynden-Bell [7] values v = 2200 ±
150 km s − and Φ ∗ e = (0 . ± . h Mpc − and improve them. Notethat the Hudson-Lynden-Bell value of v was a fit made with a smaller number of radialvelocities N = 1510 available at that time. The error they quote, 150 km s − , takes intoaccount systematic errors specific to the UGC apparent diameters.3 s - N u m be r Figure 1:
Histogram of UGC galaxies radial velocities (blue line), plotted with a bin of 100 kms − ; the ordinate is the number of galaxies per bin. The red curve is a fit of the distant part ofthis distribution ( v > − ) through the diameter function (assuming an homogeneousdistribution). The UGC also includes, in the description given of each galaxy and its surrounding area, amention (pw : “pair with”) of its possible membership to a pair with another galaxy of thecatalog. From this information, a Nilson list of 596 pairs has been extracted [14]. However,Nilson used qualitative criteria (by-eye estimation on the POSS) for the definition ofisolated pairs, and few radial velocities were available at the time of completion of theUGC.In the framework of our construction of the new pair catalog based on quantitativecriteria and accurate radial velocities for almost all galaxies of the UGC, we have put tothe test the validity of Nilson’s pairs. We find that 395 among the 596 Nilson pairs havea radial velocity difference between their members smaller than 500 km s − , i.e. 66 % ofthe pairs, and 362 smaller than 350 km s − (61 % of the Nilson pair list).Now, if we compare with our new catalog (see the following), we find 348 pairs amongthe 362 which satisfies our other quantitative criteria (of projected separation of themembers and of relative isolation), i.e. 96 % of the pairs. However, the total numberof pairs in the present new UGC pair catalog is 1246, i.e. 3.5 times the number of real(non-cosmological) pairs in Nilson’s list.We conclude that the main problem in the constitution of Nilson’s pair list was thelack of radial velocities; moreover, his qualitative isolation and separation criteria arevalidated by our quantitative ones, but they appear as more stringent as our’s, since only40 % of our pairs are in the Nilson list. A galaxy pair is characterized by 6 variables, the three coordinates of the galaxies interdis-tance ( x, y, z ), and the three coordinates of the velocity difference between its members,( v x , v y , v z ). However, only three among these 6 parameters are observationally available.Namely, (assuming the z -axis is oriented from the observer and therefore ( x, y ) is in theplane of the sky), only x, y and the radial velocity difference v z are observable. Fromthe x and y coordinates, we can compute the interdistance between the pair membersprojected on the sky plane, r p = p x + y . In practice, this projected interdistance isderived from the observed angle on the sky θ between the galaxies and the pair distance D , i.e., r p = D θ (since θ ≪ v z and r p . We have made the choice to take not too constrain-ing limits, in order not to miss possible real pairs having remote members. Doing that, weexpect to introduce false (cosmological) pairs, which will be accounted for and excludedin the subsequent analysis.Our criteria of selection are: • Small interdistance: projected separation r p < r lim , where we have taken r lim = 1Mpc (recall that the interdistance between the Milky Way and M31 is 0.7 Mpc). • Small velocity difference: we require a radial velocity difference v z < V lim , where wehave chosen the large value V lim = 500 km/s (knowing that we shall subsequentlycorrect for the false pairs, the “members” of which have large velocity differencesmainly due to Hubble expansion). • Reciprocity criterion: we require that, if the closest galaxy to a galaxy A is B, thenA is also the closest to B. This is a first isolation criterion, which also allows toexclude multiplets. • Relative isolation information: d being the projected distance of the closest galaxyfrom the UGC to the pair (also such that | ∆ V r | <
500 km.s − with respect tothe pair center), we have computed the ratio ρ = d /r p for all pairs satisfying thethree first criteria. The final pair catalog lists all pairs such that ρ > ρ l = 2 . ρ for each pair. Therefore, sub-catalogs of weakly isolatedpairs ( ρ > . ρ >
5) and highly isolated pairs (e.g. ρ > ≈ /ρ that exerted by the other member, allowing the isolatedpairs to be dynamically independant and physical.5 Additional isolation criterion: the previous isolation criterion is biased at the lowdiameter limit (1 arcmin) of the catalog, since possible smaller galaxies present inthe pair environment will not be members of the UGC and will be missed. Thereforewe have decided to use a deeper catalog, Zwicky et al.’s Catalog of Galaxies andClusters of Galaxies (CGCG, [19]) for searching galaxies in the pair environment.The magnitude limit of the CGCG ( m < .
7) corresponds to a mean limitingUGC diameter (blue major axis) of ≈ .
8. Finally, we have kept all pairs suchthat the diameters of both members are d ≥ . ρ Z = d Z /r p (as previously done on UGCgalaxies) on CGCG galaxies having a radial velocity difference from the pair center | ∆ V r | <
380 km.s − . Then all pairs such that ρ Z < . d ≥ . ρ Z < . d (cid:144) r p N u m be r Figure 2:
Observed distribution of ρ = d /r p for all 2280 reciprocal pairs; r p is the projectedinterdistance between members of a pair; d is the projected distance between the center of thepair and the nearest UGC galaxy. The limit of our pair catalog ( ρ > ρ l = 2 .
5) is shown (verticaldashed line). The theoretically expected inner depletion due to the partial isolation constraintinvolved by the reciprocity criterion (see Fig. 3 and Eq. 4) is displayed as a red dashed line. A ρ − fit of the outer part is displayed as a dashed red curve. The method used to apply these selection criteria is the following.We start from a numerical version of the UGC catalog [13], which lists 12939 galaxies.We have completed it by radial velocities from the HyperLeda extragalactic database[8, 11]. We take the velocity V lg corrected to the centroid of the Local Group. Therewere 12141 available radial velocities at the time of the completion of this version of the6 BO Figure 3:
Illustration of the isolation constraint included in the reciprocity criterion. Thiscriterion excludes the presence of a third galaxy in the two circles of radius r p centred oneach member (A and B) of the pair of center O. This exclusion is complete from ρ = 0 to ρ = √ / ≈ .
866 (dashed circle), then partial at larger values (red circle) and it finally ends at ρ = 3 / UGC. The distances D of the galaxies were taken from the HyperLeda “mod0” (redshift-independent modulus), which is an estimate of the luminosity-distance independently ofthe redshift. Otherwise, they were derived from the corrected redshift V lg through theHubble relation D = V lg /H , using H = 70 km s − Mpc − .We have written a Mathematica program searching around each galaxy of the UGCcatalog all close-by galaxies such that their projected distance on the sky is d < d = θD and d = θD , D being the cosmological distance of the referencegalaxy and D the cosmological distance of each of the selected galaxies. Then we havekept only those for which the radial velocity difference with the reference galaxy is | ∆ V r | <
500 km.s − .This results in 6489 non-isolated galaxies (for which we have found one or more com-panions with these criteria), which means that about 50 % of the UGC galaxies areisolated (see their list in [ ? ]).Then we have applied the reciprocity criterion to the galaxies with companions. Thisresults in 2280 reciprocal pairs (where each galaxy of the pair is the closest to the otherone, allowing to exclude multiplets like triplets, chains, etc.).Finally, we start from this reciprocal pair list and we search the closest galaxy tothe pair center in the whole UGC catalog with relative radial velocity difference | ∆ V r | <
500 km.s − . Then we keep only the pairs for which this closest galaxy shows a relativeprojected distance ratio d /r p > .
5. We are left with 1321 pairs satisfying the threecriteria.Concerning the pair isolation, we note that the reciprocity criterion has excludedmultiplets, chains, etc.. Now the question of the influence of the galaxy closest to the7air depends on the kind of study considered by the catalog user. Therefore, instead ofchoosing an arbitrary isolation criterion, we have determined for all 2280 reciprocal pairsthe distance to the nearest galaxy of the UGC (up to a limit ρ = 10). The correspondingdistribution is shown in Fig. 2. The value of ρ is given in the catalog, allowing the userto extract any sub-catalog of isolated pairs according to his (her) own choice.The criterion ρ > . . < ρ < ρ > ρ > ρ − of the ρ probability distribution (see Fig. 2).The additional isolation criterion using CGCG galaxies (with the isolation parameter ρ Z ) has decreased the sample of isolated pairs to a final number of 1005 (16% of UGCgalaxies). The statistics remain essentially the same: 509 isolated pairs have ρ > ρ >
10 (27%of the pairs, i.e. 4% of the UGC galaxies), where ρ is still the isolation parameter referingto UGC galaxies.An inner depletion is seen in this distribution for ρ < / ρr p centered on O (the red circle in Fig. 3, infact a narrow ring) which is outside the two circles of radius r p centered on A and B(the two members of the pair). This results in two symetrical arcs of (C) seen from Ounder an angle α . If we admit that the density of UGC galaxies is constant along (C),the proportion p of those reciprocal pairs is p = α/π . This also implies that there is noreciprocal pairs for ρ < √ / ≈ .
866 (dashed circle in Fig. 3), and that all the pairs arereciprocal if ρ > / α by analytical geometry, namely one finds α = 2 arctan ρ − / p − ( ρ − / ! . (4)This results in a proportion p increasing quasi linearly between 0 and 1 when ρ variesbetween √ / /
2, in excellent agreement with the observed distribution of reciprocalpairs (see Fig. 2).
One of the main purposes of our study of pairs of galaxies is to analyse the distribution ofthe actual (unprojected) velocity differences between their respective members. Such ananalysis requires to use radial velocities V r of the pair members of our catalog as accurate8s possible. We are not satisfied with the V r values given by Hyperleda, because they are aweighted average of all the measurements, some accurate and some other ones inaccurate.Even though the Hyperleda automatic method takes the uncertainties into account in theweighting, the presence of a very uncertain value is often enough to strongly bias the finalvelocity. Therefore, we prefer to select the most accurate ones and use only them.In order to collect our accurate V r values, we have used the bibliography up-to-date inHyperleda (by March 2014) and the NASA extragalactic database (NED, [12]) which givesfor each galaxy listed the various V r measurements with their uncertainties (unfortunatelynot totally up-to-date).There are mainly two sources of V r measurements, namely those from the radio 21cmHI line, and those from optical lines. The HI measurements are generally more accuratethan the optical ones, except for the optical data coming from the Sloan Digital SkySurvey (SDSS-III), which have an accuracy better than 5 km s − , equivalent to HI ones.For the members of a pair of galaxies, the HI measurements present a risk of con-fusion when the angular distance between the members is smaller than the Half PowerBeamwidth (HPBW) of the radiotelescope. Such a confusion is not necessarily visible inthe HI spectra if the velocity difference between the members is small. In order not tointroduce such an error, we have excluded the HI measurements relative to such pairs,taking only optical values if available; if not available for the two members, the pair hasbeen rejected from our catalog. Note that the smallest HPBW of a single-dish radiotele-scope is Arecibo’s one (3.3 arcmin); thus only optical measurements have been taken intoaccount for pairs with angular distances lower than 3.3 arcmin.Our main sources of accurate V r are the following: • Optical measurements:(i) Aihara et al. [1]: SDSSIII DR8 ; ε V r < − .(ii) Huchra et al. [6]: 2MASS redshift survey ; ε V r <
30 km s − . • HI line measurements:(i) Springob et al. [17]: Homogeneous compilation of HI spectral parameters for9000 galaxies; median of ε V r = 4 km s − .(ii) Haynes et al. [5]: Arecibo ALFALFA survey; very accurate V r , ε V r < − .After exclusion of pairs with possibly confused members, pairs with inaccurate V r ( ε V r >
100 km s − ) and two wrong pairs ( { UGC6539, UGC6545 } and { UGC11913,UGC11919 } , identified by their very large velocity difference between the members), ourfinal pair catalog lists 1246 isolated pairs of galaxies with ∆ V r <
500 km s − . The uncer-tainty on the radial velocity difference between the pair members is ε ∆ V r = p ε + ε , ε and ε being the respective uncertainties on the radial velocities of the members.9able 1: The UGC isolated galaxy pair catalog: example (first and last pairs). The fullcatalog is available at CDS in VizieR [3].UGC UGC V ε V V ε V | ∆ V r | ε ∆ V r p ρ
18 23 7864 51 7975 4 111 51.2 0.222 10.20 12905 4153 22 4104 20 49 29.7 0.399 10.34 36 6137 2 6299 24 162 24.1 0.243 3.9... ... ... ... ... ... ... ... ... ...12906 12919 5366 6 5486 4 120 7.2 0.391 3.112908 12911 4903 30 4768 7 135 30.8 0.018 10.12914 12915 4371 8 4336 7 35 10.6 0.019 10.
The resulting pair catalog (Table 2) contains, for each pair (classified according to increas-ing UGC number of the first member): column (1), the UGC number of the first member;column (2), the UGC number of the second member; column (3), the radial velocity V relative to the Sun of the first member, in km s − ; column (4), the uncertainty ε V on V ;column (5), the radial velocity V relative to the Sun of the second member, in km s − ;column (6), the uncertainty ε V on V ; column (7), the absolute value | ∆ V r | of the radialvelocity difference between the pair members, in km s − ; column(8), the uncertainty ε ∆ V on | ∆ V r | ; column (9), the projected distance on the sky r p between the two pair members,in Mpc; column(10), the ratio ρ = d /r p between the projected distance d of the nearestUGC galaxy to the center of the pair and the pair interdistance r p (a value 10 . in theTable means that ρ ≥ A detailed analysis of the catalog, in particular regarding statistical deprojections of thevelocity difference and distance between members of the pairs, will be done in followingpapers. Presently we will present some general properties of the catalog.
The distribution of cosmological radial velocities (and therefore of the distances) of thepairs is shown in Fig. 4 and compared to the distance distribution of the UGC cataloggalaxies.The agreement between the two distributions is globally satisfactory, suggesting thatthe pairs have been correctly drawn at random from the parent UGC distribution andthat the rate of pairs up to ≈ − ≈
200 Mpc is almost constant.However, one can notice that the decrease of the number of UGC galaxies at largedistances ( V r > − ) due to the apparent diameter limit, is slower than that of10 s - N u m be r Figure 4:
Distribution of the pairs according to cosmological distance, measured by radialvelocity (thick blue line). It is compared to the distribution of all galaxies in the UGC (thinred line) after normalization in the range [0 − − . The brown dashed curve is a fit bythe expected exponential diameter function for the decrease of the number density of galaxiesobserved beyond ≈ − . The black dashed curve is a fit of the expected loss of pairs,whose relative value quickly increases for increasing distances beyond ≈ − (see text). the galaxy pairs. As shown in Sec. 2.2, the decrease of the number of UGC galaxies is inexcellent agreement with the Hudson and Lynden-Bell exponential diameter function for V r > − , at distances which are not affected by galaxy clustering. One can alsoaccount for the faster decrease of the pair distribution at those distances thanks to thisdiameter function. Indeed, the density ρ ( v ) of UGC galaxies at a distance measured bythe cosmological radial velocity v is: ρ ( v ) = ρ (0) exp (cid:16) − vv ∗ (cid:17) (5)from Eq. (3).Let α be the proportion of visible UGC galaxies which belong to pairs at this distance.Only those pairs with a second galaxy visible at this distance can be identified as such,i.e., a proportion p of real pairs given by p = exp (cid:16) − vv ∗ (cid:17) , (6)assuming that the galaxy pairing does not depend on the linear diameters (which alsomeans that α is constant with the distance).11hus the density of visible pairs at distance given by v is: ρ p = α ρ (0) exp (cid:16) − vv ∗ (cid:17) , (7)resulting in dN ( v ) = α × πf ρ (0) H exp (cid:16) − vv ∗ (cid:17) v dv, (8)where f = 0 .
42 for the UGC (see Eq. 3).Figure 4 shows the corresponding curve dN ( v ) for v > − , in good agreementwith the observed pair distribution. Figure 5 shows the histogram of ε ∆ V r ; note the large number of small ε ∆ V r -values. Themedian of ε ∆ V r is ε med = 10 km s − , which is much lower than the median ≈
200 km s − of the difference of radial velocities between the members of the pairs. Ε D Vr , km s - N u m be r Figure 5:
Histogram of the measurement uncertainties ε ∆ V r = p ε + ε on the differencebetween radial velocities of pair members. The bin width is 2 km s − . It shows a sharp peak ofaccurate velocity measurements at 4 km s − (radio HI and SDSS measurements) and a tail ofless accurate optical values. .3 Distribution of radial velocity differences between pair mem-bers We give in Fig. 6 the histogram of the radial velocity differences between the membersof the galaxy pairs, which is decreasing for increasing radial velocity as theoreticallyexpected [18]. The small number of values beyond the catalog limit 500 km s − comesfrom improved velocity values obtained once the catalog was completed. s - N u m be r Figure 6:
Histogram of the radial velocity differences between pair members, for a bin widthof 40 km s − . We theoretically expect a monotonously decreasing distribution as a consequenceof the projection effect (for randomly distributed orientations of velocity vectors): the obtainedhistogram supports this expectation. The dashed horizontal line is an estimate of the maximalcontribution of cosmological false pairs, which becomes 100% beyond ≈
380 km s − . We shall study in more detail this velocity distribution and its deprojection in aforthcoming paper.
We give in Fig. 7 the distribution of projected distances between pair members in ourcatalog. Two well defined populations clearly appear, with two different distributions ofnumber density, one with d < . . < d < .
75 Mpc. Thedensity almost vanishes at this value 0 .
75 Mpc. Some pairs remain between d = 0 .
75 Mpcand the cut-off of our catalog 1 Mpc, which are compatible with being false (cosmological)“pairs”. The mean projected distance of the whole sample is < d > = 0 .
29 Mpc.13 .0 0.2 0.4 0.6 0.8 1.0050100150 Projected distance, Mpc N u m be r Figure 7:
Histogram of the projected distances between pair members, in bins of 0.04 Mpc.The dashed line is an estimate of the maximal contribution of cosmological false pairs.
Some of the pairs are expected to be the result of mere projection effects, their memberslying actually at a large relative distance. Indeed, the radial velocity criterion | ∆ V r | < − was chosen in order to include very tight pairs, which have large intervelocities dueto Kepler’s third law. However such a limit for the velocity difference corresponds also toa large cosmological distance, d c = DV /H = 7 Mpc (taking a Hubble constant H = 70km s − Mpc − ). One exceeds our interdistance criterion r p < | ∆ V r | = 70 kms − .We have attempted to estimate this contamination by cosmological false “pairs” byusing its expected dependence on radial velocity and projected interdistance. The volumedefined by our criteria is almost a radial cylinder along the line-of-sight, so that thenumber of false pairs depends linearly on | ∆ V r | . Therefore the corresponding rate inthe interval [ | ∆ V r | , | ∆ V r | + d | ∆ V r | ] is constant (see Fig. 6). Concerning the projectedinterdistance, the volume depends on it quadratically, so that the expected dependenceon r p in the histogram is linear, N ∝ r p . We have performed numerical simulations whichhave confirmed these expected dependences.The observed distribution of | ∆ V r | supports this expectation, since it becomes almostflat beyond ≈
380 km s − (then falls down around the limit of the catalog 500 km s − ).The value 380 km s − is a reasonable limit for the maximal velocity difference betweenmembers of real pairs. Identifying this flat tail (containing ≈
60 pairs) to the expected flatcosmological pair distribution yields an upper limit to the false pair contribution. Sincethe observed constant rate in the range ≈ −
500 km s − is ≈
20 pairs by bins of 40 km14 − (see Fig. 6), we estimate by this way the maximal total number of false pairs to be ≈ ≈
25% of the pair catalog. This rather large number is a consequence of ourvoluntary choice to (i) take a large limit for our radial velocity criterion | ∆ V r | <
500 kms − , then (ii) correct for cosmological contamination. We have preferred such a strategyrather than to risk having missing pairs.Concerning the projected interdistances, the observed distribution is also compatiblewith an expected linear false pair contribution (see Fig. 7). Indeed, the histogram showsa clear change of behavior, decreasing up to a minimum at ≈ .
75 Mpc, then increasingagain up to the catalog limit at 1 Mpc. By considering all pairs beyond this minimum asfalse cosmological pairs, we find another estimate of the maximal number of these pairsequal to 210, in reasonable agreement with our previous determination from velocities(250). Finally, the number of false pairs in the reduced catalog | DV | <
380 km s − and r p < .
75 (containing 860 pairs) is at most 90 ±
20, so that its rate has fallen down to lessthan only 10%.
In this paper, we have carried out the construction of a sample of isolated galaxy pairs,using the Uppsala General Catalog of Galaxies (UGC). For this purpose, we have selectedaccurate quantitative criteria to define isolated galaxy pairs, namely: 1) Low radial veloc-ity difference between the pair members: | ∆ V r | <
500 km s − ; 2) Small projected distancebetween the pair members: r p < ρ between the distance of the closest UGCgalaxy (having a velocity difference with the pair smaller than 500 km s − ) and the pairmembers interdistance for ρ > .
5, thus allowing any choice of isolation criterion beyondthis value; 5) Additional isolation condition for excluding pairs having close-by CGCGgalaxies.Our final catalog contains 1005 galaxy pairs with ρ > .
5, of which 509 are isolatedwith ρ >
5; this sub-sample contains ≈
50% of the pairs, i.e. ≈
10% of the UGC galaxies.This rate is the same as obtained by Karachentsev and Makarov [10] for isolated pairs(using different criteria). Finally, 273 pairs are highly isolated with ρ >
10 ( ≈
25% of thepairs, i.e. ≈
5% of the UGC galaxies), which is the same rate as in the pioneering 1972Karachentsev’s catalog [9].Applying those criteria leads to a sample of 1005 galaxy pairs, for the members ofwhich we have collected today’s most accurate radial velocities. The distribution of thepairs versus their cosmological distance is nearly identical to that of the UGC galaxies,excluding a possible distance bias. The proportion of “false” cosmological pairs, i.e.galaxies which seem close to each other due to projection effects, is estimated to be lessthan 25% in the full catalog, and less than 10% in the reduced catalog | ∆ V r | <
380 kms − and r p < . Acknowledgements.
We acknowledge the use of the HyperLeda database(http://leda.univ-lyon1.fr). This research has made use of the NASA/IPAC ExtragalacticDatabase (NED) which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics and Space Administration.We acknowledge very helpful remarks from an anonymous referee concerning the isolationcriterion, which have allowed to improve the catalog.
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