Astronomy & Astrophysics manuscript no. (will be inserted by hand later) Formation of polar ring galaxies Fr´d´ric Bournaud1,2 and Fran¸oise Combes1 e e carXiv:astro-ph/0301391v2 4 Feb 2003 1 Observatoire de Paris, LERMA, 61 Av. de l’Observatoire, F-75014, Paris, France 2 Ecole Normale Sup´rieure, 45 rue d’Ulm, F-75005, Paris, France e Received 2 october 2002; accepted 17 january 2003 Abstract. Polar ring galaxies are peculiar systems in which a gas-rich, nearly polar ring surrounds an early-type or elliptical host galaxy. Two formation scenarios for these objects have been proposed: they are thought to form either in major galaxy mergers or by tidal accretion of the polar material from a gas rich donor galaxy. Both scenarios are studied through N-body simulations including gas dynamics and star formation. Constraints on physical parameters are drawn out, in order to determine which scenario is the most likely to occur. Polar ring galaxies from each scenario are compared with observations and we discuss whether the accretion scenario and the merging scenario account for observational properties of polar ring galaxies. The conclusion of this study is that the accretion scenario is both the most likely and the most supported by observations. Even if the merging scenario is rather robust, most polar ring galaxies are shown to be the result of tidal gas accretion events. Key words. galaxies: formation – galaxies: kinematics and dynamics 1. Introduction From the shape of the dark matter distribution in PRGs, we would like to get information on dark halos A polar ring galaxy (PRG) is made up of an early-type, around spiral galaxies in general. At least for this rea- lenticular, or elliptical host galaxy, surrounded by a ring son, it seems fundamental to know how polar rings form. of gas and stars orbiting in a plane that is nearly polar Polar rings are usually thought to be formed during a sec- (Whitmore et al. 1990). The presence of two dynamical ondary event around a pre-existing galaxy. The collapse of planes is thought to give an insight into the shape of dark a protogalactic cloud could create two misaligned systems halos in such systems, which explains why PRGs are inter- (Curir & Diaferio 1994), thus the polar ring and the host esting peculiar systems. The prototype polar ring galaxy, galaxy might be formed at the same time. However, polar NGC 4650A, has been the target of several kinematical rings appear to be younger than host galaxies: they are studies: Whitmore et al. (1987) concluded that the dark gas-rich (see HI surveys from Richter et al. 1994 and van matter halo is nearly spherical. Sackett & Sparke (1990) Driel et al. 2000, 2002), while the host galaxy is gener- and Sackett et al. (1994) have proposed a dynamical model ally depleted of gas; moreover polar rings contain a young that indicates that the dark halo is ﬂattened, with its ma- stellar population while host galaxies contain old stars, jor axis along the equatorial plane of the host galaxy. More as indicated by their color indices (Iodice 2002b, 2002c). recently, Combes & Arnaboldi (1996) have built a diﬀer- Thus, it seems reasonable to admit that PRGs are made ent model in which the dark matter is only needed to up of a previously-formed host galaxy and a more recent explain velocities at large radii in the polar ring, while polar structure. the velocities in the host galaxy are accounted for by the Two kinds of scenarios have then been proposed to visible mass; thus they concluded that the dark halo of account for the formation of a polar ring around a preex- NGC 4650A may be ﬂattened towards the polar ring. In all isting galaxy (Iodice 2001): these studies the common conclusion was that PRG were embedded in a dark halo, for the visible mass is not enough – the merging scenario: proposed by Bekki (1997, 1998), to account for the observed velocities in rings. According this scenario assumes a head-on collision between two to a diﬀerent approach based on the Tully-Fisher relation, orthogonal spiral galaxies (see Fig. 1). The ﬁrst one applied to several PRGs, Iodice et al. (2002c) have argued is called the intruder, the second one the victim. that the dark halo may be ﬂattened along the polar ring When the relative velocity of colliding galaxies is large, plane in most PRGs. such collisions are known to form cartwheel-like rings Send oﬀprint requests to: F. Bournaud, e-mail: (Horellou & Combes, 2001) that are not polar rings Frederic.Bournaud@obspm.fr for they do not surround a host galaxy and are tran-
2 F. Bournaud & F. Combes: Formation of polar ring galaxies to compare the probability of ring formation according to each one. We will also compare the rings that each kind of event creates with observations of PRGs. PRGs were ﬁrst classiﬁed by Whitmore et al. (1990). From these observations, and more recent ones (van Driel et al. 2000, Iodice et al. 2002a, 2002b, Arnaboldi et al. 1993, van Driel et al. 1995), we can summarize the main properties of PRGs that a formation scenario has to ac- count for (Iodice 2001): – Host galaxies are often SO or early-type galaxies: they morphologically look like SOs, but their photometry is typical of early-type spirals, as revealed by Iodice et al. (2002b, 2002c). Some are ellipticals, as in AM 2020- 504, or late-type gas rich spirals, as in NGC 660. – About 0.5% of the possible host galaxies in the lo- cal Universe (SO and early-type) are observed to have a polar ring or a robust polar ring candidate. Yet, only a speciﬁc line-of-sight makes it possible to de- tect the ring, so that Whitmore et al. (1990) estimateFig. 1. Initial conﬁguration of a polar ring forming galaxy the actual percentage to about 4.5%. It could be evenmerger (Bekki, 1997). The intruder galaxy is on the left. greater if one also accounted for polar rings that mayThe victim galaxy is gas rich, and contains gas at radii have been destroyed by more recent galaxy interac-larger than the stellar disk radius. tions. This percentage does not consider the faintest polar rings, such as the ring of our own galaxy, so that 4.5% is a lower limit to the actual percentage. sient expanding features. When the relative velocity – Polar rings may be about as massive as the host galaxy, is lower, Bekki (1997, 1998) has shown that the two as well as much lighter. galaxies merge, owing to dynamical friction: the in- – Radii of polar rings can be much larger than the host truder becomes the host galaxy, while the gas content scale-length, as well as of the same order. of the victim galaxy forms the polar ring. – Most rings are inclined by less than 25 degrees from the – the accretion scenario: this scenario (e.g. Schweizer et polar axis of the host galaxy, but some highly inclined al. 1983, Reshetnikov & Sotnikova 1997) consists of rings are observed, for instance NGC 660, in which the the accretion of gas from another galaxy by the host. ring inclination is about 45 degrees (see van Driel et Our galaxy has a faint polar ring accreted from the al. 1995). large Magellanic cloud (Haud, 1988). Rings around – Some polar rings contain an old stellar population, other galaxies are more massive, and would rather be which indicates that they are stable structures with accreted from massive gas-rich galaxies, i.e. late-type a lifetime of at least a few Gyrs. spirals. This scenario assumes that two galaxies inter- – Polar rings show various inner morphologies, such as act, yet no merging is needed: tidal interactions can helicoidal rings, double rings, or rings with spiral arms. account for gas accretion without any merging event. The two interacting galaxies may also merge after the In Sect. 2 we describe the numerical schemes that are formation of the ring. The formation of a polar ring re- used in this work. In Sect. 3 and 4 we study the merging quires that the donor galaxy is on a nearly polar orbit scenario and the accretion one respectively. We compare with respect to the accreting host galaxy. them in Sect. 5, 6, and 7. We conclude in Sect. 8.It has been demonstrated that both scenarios can explain 2. Numerical techniquesthe formation of polar rings surrounding preexisting galax-ies (Bekki 1997, Reshetnikov & Sotnikova 1997). Yet, they In this section, we describe the numerical schemes andhave not been compared in the same work, and the sta- the mass models for spiral and elliptical galaxies that webility of such rings, probably related to their self gravity will use to study formation scenarios. The N-body parti-(Sparke 1986, Arnaboldi & Sparke 1994), still has to be cle code includes gas dynamics, star formation and stellaraddressed with regard to the formation scenario (Iodice et mass-loss.al., 2002a). Stars, gas and dark matter are described by 2 to 20·105 In this paper, we study both scenarios through the particles. The gravitational potential is deduced from thesame numerical model. We wish to conﬁrm that they both particle positions via a FFT method. The particles areform stable polar rings, and to determine which among ﬁrst meshed on a Cartesian grid (generally 128 × 128 × 128them is the most robust. We will then address the depen- cells), according to the multilinear “cloud-in-cell” (CIC)dence of both scenarios on physical parameters, in order interpolation. The convolution of the density and the
F. Bournaud & F. Combes: Formation of polar ring galaxies 3Green function is computed via FFT techniques (James,1977). Two-body relaxation is overestimated in particlecodes because of the small number of particles. To sup-press the two-body relaxation, the gravitational forces aresoftened with a characteristic size of 300 to 1000pc, de-pending on the grid size. The dissipative nature of the interstellar medium ismodeled with a sticky-particles algorithm that reproducesinelastic cloud-cloud collisions (Schwarz, 1981). The rel-ative kinetic energy of gas clouds is lost, and the gasre-radiates this energy away (Combes & Gerin 1985).Collisions are detected on a grid, and when two particlescollide, their radial (along the line joining them) and tan-gential relative velocities are reduced by factors βr and βt .We generally choose βr = 0.65 and βt = 0.65. A detailed Fig. 2. Collisional parameters for the merging scenario.description of our sticky-particles code and a discussion of The intruder is on the left, the victim is on the right.βr and βt is given in Bournaud & Combes (2002). Ideal values of these parameters are Θ = 90, Φ = 90, Rcoll = 0. Rcoll is always given in units of the disk radius The code also includes numerical schemes for star of the victim.formation and stellar mass-loss that are described inBournaud & Combes (2002). Run ring radius R The models for spiral and elliptical galaxies and dark A1 2.5 kpchalos are described in Appendix A. A2 2.5 kpc A3 3 kpc A4 6 kpc3. Merging scenario A5 4 kpc A6 3.5 kpc3.1. Numerical simulations and relevant parameters A7 3.5 kpc A8 3.5 kpcThere are many free parameters for the merging scenario: A9 15 kpcthe parameters for each disk, bulge, halo, and the orbital A10 35 kpcparameters. Some numerical tests have allowed us to de- A11 27 kpctermine which parameters may inﬂuence the formation of A12 13 kpcpolar rings and the properties of polar rings after their A13 14 kpcformation. For instance, the bulge-to-disk mass ratio is Table 2. Ring radius in runs A1 to A13.not a relevant parameter, and we will not vary it. Theparameters that have a signiﬁcant inﬂuence are: 3.2. General properties – the stellar and gas mass in both galaxies (intruder and victim). 3.2.1. Morphology of the polar ring – the radius of the stellar disk and the gas radial extent Polar rings are obtained in runs A1 to A13. A double ring in both galaxies. is obtained in run A2. Spiral arms are observed in runs – the dark halo shape. A5 and A11. In run A4 we obtain a polar structure that – the relative velocities of galaxies before the collision, may be regarded as a polar disk: the center of the polar Vcoll . structure is partly but not totally depleted. In Table 2 we – the angle between the two disk planes, Θ. give the radius of each ring, deﬁned by: – the angle between the orbital plane of the intruder and the victim disk, Φ. ∞ 2 0 r µ(r)dr – the radius at which the intruder crosses the victim R= ∞ (1) disk, Rcoll , in units of the victim disk radius. 0 rµ(r)dr where µ(r) is the mean surface density of the polar ring. Parameters Θ, Φ, and Rcoll are illustrated in Fig. 2. When we compute the radius of polar rings, we only ac- We give in Table 1 the values of the most relevant count for the matter of the polar ring, i.e. the matter thatparameters for each numerical run. Runs A1 to A13 aim at was initially the gas of the victim. The intruder’s compo-studying the inﬂuence of the structure of each galaxy and nents and the victim’s stars are ignored.the morphology of the PRG. Runs B1 to B21 study the Our simulations conﬁrm that the merging scenarioinﬂuence of collisional parameters in order to determine provides an explanation for the formation of polar ringsthe probability that a major galaxy merger results in a around pre-existing galaxies, and explains the various in-polar ring. ner morphologies of polar rings. An example of galaxy
4 F. Bournaud & F. Combes: Formation of polar ring galaxies Run MS intr MG intr RG intr MS vict MG vict RG vict Eintr Evict Vcoll Θ Φ Rcoll a b c d e f g h i j k l A1 10 0 - 10 3.3 1 E0 E0 30 90 90 0 A2 7 0 - 14 3.5 1 E3 E3 30 90 90 0 A3 7 0 - 14 3.5 1 E3 E3 45 90 90 0 A4 6 0 - 18 5 1 E5 E5 40 90 90 0 A5 12 0 - 14 3.5 1 E4 E4 40 90 90 0 A6 12 0 - 14 3.5 1 E4 E4 80 90 90 0 A7 12 0 - 14 5 1 E9 E9 80 90 90 0 A8 12 0 - 14 5 1 E0 E0 120 90 90 0 A9 20 0 - 18 6 3 E3 E3 120 90 90 0 A10 15 0 - 14 6 7 E3 E3 120 90 90 0 A11 15 0 - 10 6 5.5 E3 E3 120 90 90 0 A12 15 0 - 15 10 2.5 E3 E3 120 90 90 0 A13 15 0 - 15 10 2.5 E9 E9 120 90 90 0 B1 10 0 - 12 6 3 E3 E3 65 70 90 0 B2 10 0 - 12 6 3 E3 E3 65 55 90 0 B3 10 0 - 12 6 3 E3 E3 65 45 90 0 B4 10 0 - 12 6 3 E3 E3 65 90 60 0 B5 10 0 - 12 6 3 E3 E3 65 90 40 0 B6 10 0 - 12 6 3 E3 E3 65 90 90 .3 B7 10 0 - 12 6 3 E3 E3 65 90 90 .6 B8 20 0 - 15 7 3.5 E5 E5 120 90 90 0 B9 20 0 - 15 7 3.5 E5 E5 120 70 90 0 B10 20 0 - 15 7 3.5 E5 E5 120 60 90 0 B11 20 0 - 15 7 3.5 E5 E5 120 50 90 0 B12 15 0 - 15 6 3 E5 E5 100 90 90 0 B13 15 2 1 15 6 4 E5 E5 100 90 90 0 B14 15 2 2 15 6 4 E5 E5 100 90 90 0 B15 15 4 1 15 6 4 E5 E5 100 90 90 0 B16 15 4 2 15 6 4 E5 E5 100 90 90 0 B17 15 0 - 15 6 2 E5 E5 100 90 90 .3 B18 15 0 - 15 6 2 E5 E5 100 90 90 .5 B19 15 0 - 15 6 2 E5 E5 100 90 90 .7 B20 15 0 - 15 6 3 E5 E5 100 90 45 0 B21 15 0 - 15 6 3 E5 E5 100 90 35 0Table 1. Run parameters. Units: 109 M⊙ , kpc, km.s−1 a Stellar mass (intruder) b Gas mass (intruder) c External radius of the gaseous disk in units of stellar radius (intruder) d Stellar mass (victim) e Gas mass (victim) f External radius of the gaseous disk in units of stellar radius (victim) g Dark halo shape (intruder); E0: spherical; Ex: spheroidal with ellipticity 1-x/10. h Dark halo shape (victim) i Relative velocity of the colliding galaxies j Angle between the two disks k Angle of the intruder’s path on the victim’s plane l Radius at which the intruder crosses the victim diskmerger that results in a polar ring is shown in Fig. 3 (run victim galaxy contains gas outside its stellar disk. BekkiA9). (1998) explained the formation of large rings such as in The main property of a polar ring, its radius, depends NGC 4650A by low velocity collisions (about 30 km.s−1 ),largely on the gas radial extent, and is only slightly in- in which the victim disk is not dissolved, but such relativeﬂuenced by other parameters. The potential of the host velocities are not common. It seems much more realisticgalaxy is thus not fundamental for the ring radius. This to think that if polar rings are formed by the merging sce-result is justiﬁed by an explanation of the ring formation nario large rings are related to victim galaxies that containin such galaxy mergers, given in Appendix B. The forma- gas at large radii (Sancisi 1983), which is more likely thantion of very large rings (R > 10 kpc) requires that the very low relative velocities.
F. Bournaud & F. Combes: Formation of polar ring galaxies 5Fig. 3. Particle distribution in run A9. Blue: intruder (stars) – Red: victim (star) – Green: victim (gas and starsformed from this gas after the ring). When the mean radius of a polar ring R is determined The values of ∆R/R that we obtain about 1 Gyr after theby Eq. 1, we deﬁne its radial extent ∆R by: ring formation vary from 10% to 40%, and most of them ∞ are around 25%. 0 (r − R)2 rµ(r)dr∆R2 = ∞ (2) 0 rµ(r)dr
6 F. Bournaud & F. Combes: Formation of polar ring galaxies3.2.2. Other componentsIn every run, as in Fig. 3, we notice the presence of: – gas from the victim disk fallen on the center of the host galaxy – a stellar halo, formed by the stars of the victim that are dispersed by the collision The fact that the stellar content of the victim remainsdispersed was expected. Before the formation of the polarring, gas is dispersed, and settles in a planar structure be-cause of its dissipative nature. Stars are dispersed by thecollision, too, but cannot settle in the polar plane. Theythen form a faint halo that surrounds the host galaxy andthe polar ring. Runs in which the relative velocity exceeds60 km.s−1 show a nearly spherical stellar halo: Its axisratio is larger than 0.75. When the relative velocity issmaller (which is much uncommon in the Universe), the Fig. 4. Run B2: the polar ring mean inclination from thevictim galaxy stellar remnant is more ﬂattened, yet re- polar axis is 24 degrees, while the initial value of Θ wasmains much thicker than the gaseous ring: its axis ratio 65 degrees (i.e. 35 degrees from polar). Most polar ringsstill exceeds 0.4, and diﬀerential precession makes it be- formed in galaxy mergers are nearly polar, and no ring in-come nearly spherical in a few Gyrs. In 28 of our 34 runs clined by more than 25 degrees has been obtained. Stars(> 80 % of the runs), more than 50% of the stellar mass from the victim disk that from a stellar halo are not rep-of the victim is found within the radius of the polar ring. resented.The presence of such a stellar halo is tested in observa-tions of NGC 4650A (in Sect. 6). Let us keep in mind thatthe merging scenario results in a stellar structure that is 3.2.3. Ring inclinationdistinct from the polar ring, even nearly spherical in mostruns otherwise moderately ﬂattened, and that more than The inclination of the polar ring to the polar axis is inﬂu-half of the victim’s stars are found within the polar ring enced by the value of Θ: small values of Θ result in ringsmean radius R in 80 % of the simulations. that are inclined with respect to the polar axis. Yet, even The polar ring does not contain all the gas of the vic- if Θ is only 55 degrees, we obtain no ring inclined by moretim’s disk. A few percent remain dispersed at large radii, than 24 degrees from the polar axis (run B2, see Fig. 4)but the most important point is that some gas falls into The ﬁnal inclination of the polar ring is diﬀerent fromthe center of the host galaxy (see for instance Fig. 3). This Θ. We have mentioned that a signiﬁcant fraction of gasgas settles in an equatorial disk, but does not replenish the falls inside the center of the equatorial disk. It takes alwayswhole disk of the host galaxy, and never makes the host a polar component of the gas angular momentum (withgalaxy become gas-rich. Even if the polar ring is larger respect to the host), so that the orientation of the meanthan the host galaxy, the gas fallen in the host galaxy re- angular momentum of the polar gas is modiﬁed. In othermains located at small radii, generally not more than 2 words, some gas from the ring falls inside the host disk,or 3 kpc, and never more then 4 kpc in all our runs. This then the ring changes its orientation to conserve the totalgas form a small central concentration in which it quickly angular momentum. When Θ is 55 degrees, the resultingforms stars, so that no gas remains after about 3 Gyrs. ring is closer to the polar axis, which explains why all theWhen the values of parameters are ideal (Θ=Φ=90 de- obtained rings are close to the polar axisgrees, Rcoll =0), the fraction of gas that falls in the centerof the host galaxy is quite small, around 10-20 percent 3.2.4. Stability of polar ringsin most runs, at most 25 percent (run A8). This frac-tion becomes larger when we vary Θ, Φ, and Rcoll (see Self-gravity is susceptible to stabilize polar rings (SparkeSect. 3.3). When the polar ring is formed, its angular ex- 1986, Arnaboldi & Sparke 1994). Yet, the stability of polartension is small enough to prevent diﬀerential precession structures in numerical simulations of formation should beto dissolve the ring and to make gas fall in the equatorial addressed (Iodice et al. 2002a). Thus, we have let all theplane, but before the ring is formed, gas is dispersed with polar rings we have obtained evolve for at least 2 Gyrs.various inclinations to the polar axis, which enhances dif- All were then stable. For some runs (A9, A11 and B2), weferential precession, and makes part of the gas fall in the even let the system evolve for 8 Gyrs after the formationhost galaxy. of the polar ring. Polar rings still appear to be stable.
F. Bournaud & F. Combes: Formation of polar ring galaxies 7 3.3. Constraints on parameters In this part, we investigate the inﬂuence of deviations from the ideal scenario. We wish to show in which range each parameter can be varied without preventing the forma- tion of a polar ring. Thus, we examine the results of runs B1 to B21. As explained in Sect. 3.2.3 and 3.2.4, polar rings form in a merging event only if the eﬀects of dif- ferential precession are small enough, which requires that gas initially orbits on nearly-polar paths. When we devi- ate from this condition, the larger diﬀerential precessionFig. 5. Gas and stars formed after the polar ring in run makes more gas fall to the center of the host galaxy. IfA11. The central condensation is related to gas that falls the deviation is too large, all the gas falls inside the hostat the center of the host disk during ring formation and galaxy and no polar ring is obtained.quickly forms stars. The polar ring is seen edge-on (left) In runs A1 to A13, the fraction of the victim’s gasand 10 degrees from edge-on (right). that falls to the center of the host galaxy does not ex- ceed 25 percent. We consider that the eﬀects of varying a parameter become signiﬁcant when this fraction is be-For instance, for run A11, we show the 8 Gyr old polar tween one and two third, and that a ring was bound notstructure in Fig. 5. to form if this fraction exceeds two thirds. This is justiﬁed As displayed in Fig. 5, most polar rings show a warp, by the fact that a large fraction of gas fallen on the hostthat generally appears after 1 to 3 Gyrs of evolution. center attests to a nearly unstable ring (because of largeTheorical studies of the stability of self-graviting polar diﬀerential precession eﬀects), and a nearly unstable ringrings predict the presence of such warps (e.g. Arnaboldi is likely not to form at all (see Sect. 3.2.4: no unstable& Sparke 1994): self-gravity counteracts diﬀerential pre- rings are obtained in this scenario). We then describe thecession through the warp. results of runs B1 to B19 in Table 3 in terms of “ring” No unstable polar ring has been observed in the merg- (the variation of parameters from their ideal value is noting scenario. As explained in Sect. 3.2.3, rings are always signiﬁcant), “light ring” (the variation of parameters be-nearly polar, and their inclination from the host plane al- comes signiﬁcant), “very light ring” (much matter is lostways exceeds 75 degrees. It seems that the polar ring forms by the polar ring, which is thus nearly not formed), andonly if its inclination is large enough to allow self-gravity “no ring” (the variation in parameters is too large).to counteract diﬀerential precession. Before the formationof the polar ring, gas is dispersed around the host galaxy. The results of runs B1 to B21 then allow us to deter-Dissipation and self-gravity tend to make it gather in a mine the constraints on each parameter, i.e. the range inpolar plane, while diﬀerential precession is already acting. which each parameter may be varied without preventingTwo situations may then occur: either the diﬀerential pre- a polar ring from forming. These constraints are:cession is too large to be counteracted by gravity and noring forms, or the diﬀerential precession is small enoughto ensure that the ring will both form and be stable. This – Θ > 50 degreessimply explains why all the polar rings obtained in the – Φ > 45 degreesmerging scenario where stable. We will see a diﬀerent sit- – The victim galaxy should contain 3 times more gasuation for the accretion scenario (Sect. 4), in which rings than the intruder, or have twice as large a gaseousmay form and be unstable. disk, in order to prevent all the gas from merging in a In Fig. 5, we also notice that: single, equatorial disk – the polar ring is not totally depleted, and the central – Rcoll < 0.5, in units of the victim disk radius – The relative velocity should be small enough to allow hole becomes less obvious after several Gyrs. Star for- the two systems to merge, and should typically not mation is responsible for such a dispersion of the polar matter: we have not obtained this result in the same exceed 150-200 km.s−1 . The larger the velocity, the larger the constraints on other parameters (see runs run without star formation. – in some lines-of-sight, the ring shows two spiral arms, B1-2-3 vs. runs B9-10-11). while no real arms are seen in the face-on ring. These spiral arms are actually related to the warp, and are observed when the polar ring is seen nearly edge- Each parameter can be signiﬁcantly varied, thus the on, but not perfectly edge-on. For instance, the spiral merging scenario is actually robust, and may explain structures observed in NGC 4650A (Arnaboldi et al. the formation of a signiﬁcant number of polar rings. We 1997) may not be density waves but be caused by a will compare constraints on both scenarios and determine warp. which is the most likely to occur in Sect. 5.
8 F. Bournaud & F. Combes: Formation of polar ring galaxies Run result Run result B1 ring C1 polar ring B2 ring C2 polar ring (25 degrees from polar) B3 no ring C3 polar ring B4 ring C4 no ring (no accretion) B5 no ring C6 polar ring B6 ring C7 polar ring B7 very light ring C8 polar ring (20 degrees from polar) B8 ring C10 polar ring B9 ring C11 polar ring (see Fig. 8) B10 light ring C12 polar ring B11 no ring C16(e) polar ring B12 ring C17(e) polar ring B13 light ring C18(e) polar ring B14 very light ring C19(e) polar ring (25 degrees from polar) B15 very light ring C20(e) inclined ring (30 degrees from polar) B16 no ring C21(e) equatorial dust lane B17 ring C22 polar ring (25 degrees from polar) B18 light ring C23 polar ring B19 no ring C24 polar ring B20 very light ring C25 inclined ring (40 degrees from polar) B21 no ring C26 inclined ring (40-45 degrees from polar)Table 3. Results of runs B1 to B21. “light ring”: a large C27 polar ring, then polar diskpart (more than one third) of the victim’s gas falls in- Table 5. Run resultsside the center of the host galaxy and is lost by the polarring during its formation. “very light ring”: less than onethird of the victim’s gas is found in the polar ring after its – the relative velocity V before the encounter. To deduceformation, which indicates a large rate of diﬀerential pre- the velocity at the beginning of the simulation, we ne-cession: such rings are nearly unstable, thus likely not to glect eﬀects of dynamical friction, and treat galaxies asform, since no unstable ring forms in the merging scenario two rigid bodies for which the conservation of kinetic(Sect. 3.2.4) energy is computed. – the angle Θ between the donor disk plane and the host equatorial plane, given before the accretion event (tidal3.4. Another model for the merging scenario eﬀects may make it evolve). – the angle Φ between the donor orbital plane and theA semi-analytical model for the merging scenario is pre- host equatorial plane.sented in Appendix B to explain why a ring forms in this – the minimal radius Rm between the host center andscenario. The main results of this model are: the donor center, in units of the host optical radius. – Polar gas oscillates at large and small radii before gath- In run C27, this value is zero, for both galaxies merge ering in the ring. It is not directly placed in the ring, after the ring accretion (notice that the ring is not for the galaxy encounter stimulates large radial mo- issued from the merger as was the case in the previous tions. In particular, even if the ring is larger than the scenario, but really formed by tidal tails before the host disk, polar gas will ﬁrst encounter the host galaxy merging of the donor and the host). material For runs C16 to C21, the host galaxy is an E4 ellipti- – The ISM dissipative nature is responsible for the for- cal. In fact, even if the host stellar distribution is changed mation of a polar ring rather than a polar disk. from a spiral disk, the large-scale potential is not funda-Detailed explanations of these results are given in mentally diﬀerent, and we notice no major modiﬁcationAppendix B in the evolution of the donor and the accreted gas.4. Accretion scenario 4.2. General properties4.1. Relevant parameters and numerical simulations 4.2.1. Morphology of the polar ringAs was the case for the ﬁrst scenario, some numerical In Table 5, we describe the result of each run.tests have proved that inner parameters of both galax- An example of formation of a polar ring by tidal ac-ies (disk, bulge and halo masses, disk radius) are not the cretion is shown of Fig. 6 (run C1). As in the mergingmost relevant. The most important parameters, given in scenario, tidal accretion accounts for a large variety of po-Table 4, are the gas content (mass and radial extent) of lar structures. The main property of a polar ring, its meaneach galaxy, and orbital parameters, which are: radius R, may be smaller as well as larger than the host
F. Bournaud & F. Combes: Formation of polar ring galaxies 9 Run mG host G rhost mG donor G rdonor V Θ Φ Rm a b c d e f g h i C1 0 - 0.3 3.0 200 90 90 5 C2 0 - 0.3 3.0 300 20 90 5 C3 0 - 0.3 3.0 350 90 60 5 C4 0 - 0.2 3.0 200 90 90 5 C6 0.2 1. 0.3 1.5 150 80 80 5 C7 0 - 0.2 2.5 200 90 50 6 C8 0 - 0.4 2.5 200 90 50 6 C10 0 - 0.3 2.5 250 70 70 6 C11 0 - 0.35 5.0 230 80 80 6 C12 0 - 0.5 6.0 250 65 65 8 C16(e) 0 - 0.3 5.0 175 90 90 6 C17(e) 0 - 0.3 5.0 150 90 75 4 C18(e) 0 - 0.2 4.0 250 90 75 8 C19(e) 0 - 0.3 5.0 175 40 25 4.5 C20(e) 0 - 0.2 4.0 250 25 40 5 C21(e) 0 - 0.2 4.0 250 10 10 5 C22 0 - 0.2 4.0 250 45 20 5 C23 0 - 0.3 4.0 200 70 55 7 C24 0.25 1. 0.3 4.0 200 65 55 6 C25 0 - 0.3 4.0 200 80 65 5 C26 0.3 1.4 0.3 4.0 200 65 65 5 C27 0 - 0.5 4.0 100 90 90 0Table 4. Run parameters. Units: 109 M⊙ , kpc, km.s−1 a (e): elliptical host galaxy b host gas mass fraction c host gas radial extent, in units of stellar radius d donor gas mass fraction e donor gas radial extent, in units of stellar radius f relative velocity before the encounter g angle between the donor disk plane and the host equatorial plane h angle between the donor orbital plane and the host equatorial plane i minimal radius between the host center and the donor center, in units of the host optical radiusgalaxy optical radius, as already shown by Reshetnikov & – only a small fraction of the donor is accretedSotnikova (1997). They also found that the host dark haloinﬂuences the value of R, which some of our simulations This led Iodice (2001) and Iodice et al. (2002a) to sayconﬁrm. As well as the merging scenario, accretion pro- that rings formed by accretion would be signiﬁcantlyduces warped rings and rings with inner spiral structures lighter than host galaxies, unlike some observed rings as; transient helicoidal features resulting in polar rings are in NGC 4650A. Actually, the fraction of accreted gas mayalso observed. reach 40% of the donor gaseous mass (run C11), provided Just after their formation, rings have a small radial that the gas extent in the donor is large enough (gas atextent: ∆R/R varies from 10% to 35%. ∆R seems at this large radii is not strongly gravitationally bound) and thestage smaller than in the merging scenario. Yet, in run donor inner rotation is favorable to accretion: the accretedC27, the merging of the donor galaxy with the host galaxy, mass can be large in the case of direct resonance (run C3)after the accretion of the polar ring, disturbs the ring, while no gas is accreted in run C4 (retrograde resonance).and makes ∆R/R raise from 25% to 55% (see Fig. 7 and Moreover, if the donor contains gas outside this stellardiscussion in Sect. 6.2.2). More generally, rings may be disk, accretion can be achieved from the donor to the hostdisturbed by interactions with other galaxies, which will even if the donor is more massive than the host (for in-have a similar eﬀect: even if rings have a small radial ex- stance run C8 where the donor mass is 4 times larger thantent just after their formation in this scenario, they may the host mass, and accretion still occurs from the donorget a larger radial extent later. to the host). Finally, with large donor masses and large The last noticeable point concerns the mass of polar accreted fractions, the ring masses are sometimes of therings. One could a priori think that: same order as the host mass (see Fig. 8). An observational example of a galaxy that is accreting a large gas mass from – for gas to be accreted from the donor by the host a more massive galaxy is NGC 646 (Danziger & Schuster galaxy, the donor must be less massive than the host 1974, Horellou & Booth 1997).
10 F. Bournaud & F. Combes: Formation of polar ring galaxiesFig. 6. Particle distribution in run C1. Blue: host (stars) – Red: donor (star) – Green: victim (gas and stars formedafter the ring in this gas).4.2.2. Other components fraction is larger than 25% before the accretion event, and still larger than 20% 1 Gyr after the ring formation (seeIn the merging scenario, we have shown that the host also Sect. 6.3). During the formation of the polar ring,galaxy had to contain little gas. Here, the host galaxy accreted matter is placed directly around the ring radiusmay be gas rich: in runs C24 and C26, the host gas mass
F. Bournaud & F. Combes: Formation of polar ring galaxies 11 from forming, even if present only at radii smaller than the ﬁnal ring radius R. Another important diﬀerence to the ﬁrst scenario is that stars are generally not accreted from the donor, for they form a warm, non-dissipative, medium. The frac- tion of accreted stars in our simulations is always smaller than 10% and nearly always smaller than 5%. Moreover, the few accreted stars join the polar ring via the same tidal tail as accreted gas. Thus, no stellar halo is expected around the polar ring, contrary to the merging scenario (see Sect. 3.2.2). 4.2.3. Stability and inclination of polar rings The stability concern does not rule out the accretion sce- nario : polar structures are as stable as in the merging scenario (some rings were simulated over 10 Gyrs), except some highly inclined rings that are dissolved in a few Gyrs (see Sect. 6.3). The relevant point is not that rings formedFig. 7. Run C27: the merging of a part of the donor galaxy by accretion are less stable due to their formation history,with the host galaxy, after the formation of the polar ring, but that they are sometimes more inclined. In the mergingmakes the radial extent of the polar ring raise to 55%, so scenario, the ﬁnal ring was closer to the polar axis thanthat this structure could be regarded as a polar disk. the initial victim galaxy, because of the infall of polar gas inside the center of the equatorial system during the ring formation. In the accretion scenario, we do not observe such a systematic large gas infall, yet the ring orientation is still diﬀerent from the initial value of Θ. The likely ex- planation is that only a part of gas is found in the polar ring: some gas is sometimes found in the host galaxy (but this eﬀect is not as large and as general as in the merging scenario), some gas remains in the donor, and some gas is dispersed at large radii from both galaxies. The polar ring gas has only a part of the initial angular momentum of the donor gas, thus is not compelled to be parallel to the donor initial disk. For instance, this explains why two nearly parallel disks, or disks on a nearly equatorial orbit, may result in a nearly polar ring: such unexpected situa- tions are observed in runs C19,20,22. In these runs, initial parameters are far from ideal values for the formation of a polar structures, but the ﬁnal ring is not far from polar. In run C22, the ring is about 25 degrees from polar, while initially Θ = 45 degrees. This allows this ring to be stable (it has been followed in the simulation during 4 Gyrs after its formation).Fig. 8. Run C11: a very massive polar ring is formed by With the same inclination (i.e. less than 25 degreestidal accretion, the ring mass is about 80% of the host from the polar axis), rings are as stable as in the mergingmass. The ring is slightly warped in its outer regions. scenario, and the stability concern does not seem to be closely linked to the formation mechanism. In the accre- tion scenario, we ﬁnd in addition more inclined rings. This will be discussed in Sect. 6.3.R, thus gas present in the host galaxy at radii larger orsmaller than R will not disturb the ring formation, for 4.3. Constraints on parametersboth gaseous component will not meet. In the mergingscenario, polar gas oscillates at diﬀerent radii because of We will see in Sect. 5.2 that parameters cannot be re-the large change in the central mass (see Appendix B) and garded as independent in this scenario. Thus it is not asalways encounters the intruder gas, so that equatorial gas easy to determine a limit on each parameter as it was inat some radii is much more able to prevent the polar ring the merging scenario. However, doing this is not required
12 F. Bournaud & F. Combes: Formation of polar ring galaxiesin the forthcoming comparison of scenarios. We will sim- The simulation parameter choices that we have madeply need to know in Sect. 5.2 that constraints on param- (see Table 1) may seem arbitrary: for instance, the valueeters in this scenario are signiﬁcantly smaller than in the of Θ is varied in more runs than the value of Φ, and somemerging scenario: situations have not been studied (for instance Θ = 45 and Φ = 45 degrees in the same run). The statistics based – we observe the formation of polar rings with values on simulations may then seem biased, but is actually not. of Θ and Φ smaller than in the merging scenario, as The presented runs have been used only to delimit the explained in Sect. 4.2.3 range of parameters for which a polar ring is formed in – relative velocities and distance between galaxies can agreement with observations. From the obtained range of be rather large without preventing a massive ring from values for each parameter, we can determine the probabil- being accreted (for instance run C11) ity represented by these combined ranges, and derive the – host galaxy may contain a signiﬁcant fraction of gas statistics. (runs C24, C26) ; this will be largely discussed in Sect. 6.3. Before the ring formation, the host galaxy may even be gas rich and contain gas outside its stel- 5.2. Probability of the accretion scenario lar disk (run C26). In the merging scenario, we have established independentThe three categories of parameters (gas content of the constraints on each parameter, in order to determine thehost galaxy, orientation of galaxies, relative velocities and probability that a polar ring is formed during a mergingradius) are thus submitted to smaller constraints than in event. This cannot be done so easily for the accretion sce-the merging scenario. This will now be the issue of the nario, for:ﬁrst comparison of both scenarios. – the frequency of galaxy interactions is not known,5. Probability of each scenario and abundance of while we had been able to ﬁx an upper limit to the polar rings frequency of major mergers. – the parameters for the accretion scenario are not in-5.1. Probability of the merging scenario dependent. For instance, the sense of rotation of theIn Sect. 3.3, we established in what conditions the merging donor is crucial, so that two perpendicular disks on aof two spiral galaxies results in a polar ring. We now de- polar orbit may form no ring in some situations, whilerive the associated probability: we assume that two spiral two parallel disks on a nearly equatorial orbit may formgalaxies collide and merge, and determine the probability a polar ring in some other situations.that this event forms a polar ring: As parameters cannot be regarded as independent, deriv- – the probability that Θ > 50 degrees is 0.45 ing a dependable probability for this scenario would re- – the probability that Φ > 45 degrees is 0.29 quire us to vary all the relevant parameters in the same – the probability that Rcoll < 0.5 is 0.25 time, while we had varied one parameter after the other – the constraint on the gas content is applied to HI ob- for the merging scenario. A large number of simulations servations of spiral galaxies (Broeils & Rhee, 1997): we would then be required, but deriving a precise probability select randomly two galaxies, and check whether one of for the accretion scenario is not required for our study, them may be a victim galaxy, i.e. contain three times for previous results allow us to qualitatively compare the more gas then the other one, or have a twice as large probability of both scenarios: we have simply shown that gaseous disk. The probability that a randomly chosen polar rings can form by accretion in a large range of pa- pair of galaxies is a possible intruder-victim pair, with- rameters, with values of Θ and Φ that are far from their out regarding which one is the victim, is 0.3. ideal values. The constraints are smaller than for the merg- The probability that two merging spirals form a PRG ing scenario: two perpendicular disks have been observedis then 0.25 × 0.29 × 0.3 × 0.45 = 1%. to form a polar ring by tidal accretion, while they never The constraint on the relative velocity is more diﬃcult form a polar ring according to the merging scenario.to convert into probability. Yet, it seems reasonable to ad- Not only are the constraints on parameters smaller,mit that a given galaxy has undergone at most 5 merging but also tidal interaction events (that may lead to merg-events with spiral galaxies (the so-called “major merg- ers or not) are much more frequent than galaxy collisionsers”). Merging events with dwarf galaxies are ignored, for leading to mergers, for the radius between galaxies maythey would not account for the large masses of the ob- be much larger and a large relative velocity does not pre-served polar rings. The adopted value of 5 major mergers vent tidal accretion of much matter (we have obtainedis a large upper limit, and at each merging event, the prob- 40% of accreted gas in run C11, with V = 230 km.s−1ability that a polar ring forms is 1%. Finally, the proba- and Rm = 6). Thus, the probability that a possible hostbility that a possible host galaxy (lenticular or early-type galaxy has acquired a polar ring by tidal accretion is sig-spiral) has acquired a polar ring during a merging event niﬁcantly larger than the probability that it has acquiredis at most 5%. a polar ring during a merger event.
F. Bournaud & F. Combes: Formation of polar ring galaxies 135.3. Abundance of polar rings and comparison of The remaining properties may discriminate between sce- scenarios narios: they are the existence of gas-rich host galaxies, of inclined rings, and the possible presence of a stellar haloFrom their atlas of PRGs, Whitmore et al. (1990) estimate around the polar ring (Sect. 3.2.2). We now compare thesethat 4.5% of the possible host galaxies have acquired a properties in simulations and observations.polar ring, and say that this estimate is uncertain by abouta factor 3. According to the merging scenario, polar rings are sta- 6.2. NGC 4650Able features, so that no polar rings would have disap- 6.2.1. Photometrypeared. 1.5% to 13.5% of the possible host galaxies shouldthen have a polar ring. This is obviously a lower limit, for The main photometric diﬀerence between the numericalmany rings may remain unknown. On the other hand, we results of both scenarios is the presence of a stellar halo inhave shown that at most 5% of the possible host galaxies which the polar ring is embedded according to the merg-have acquired a polar ring during a merging event. Thus, ing scenario. This stellar halo is the remnant of the stellaras 5 is in between 1.5 and 13.5, we can claim that: component of the victim galaxy, which is dispersed dur- ing the merger: only gas settles in a polar plane, due to – the probability of the merging scenario is large enough its dissipative nature. According to the accretion scenario, to account for all the observed polar rings only gas is accreted, for it is colder than stars and is gen- – the probability of the merging scenario would become erally present at larger radii in the donor galaxy. Nearly too small, if many polar rings have not been detected no stars are accreted from the donor galaxy, even if the yet. donor galaxy has a massive stellar disk. Obviously, stars We have not determined a probability for the accretion are found in the polar ring because of star formation, butscenario, but explained that it was much more probable no stellar halo is obtained. Testing the presence of a stellarthan the merging scenario. Thus: halo around a PRG may then give insight into its history. We have generally observed in simulations of merging – the probability of the accretion scenario is large enough events that most of the stellar mass of the victim settles to account for all the observed polar rings inside the ring radius R. More precisely, we have noticed – the probability of the accretion scenario would still that in 80% of our simulations, more than 50% of the be large enough, if many polar rings have not been victim’s stellar mass is observed inside the disk radius R. detected yet. The large probability of the accretion We use deep HST observations of NGC 4650A scenario even implies that many polar rings have not (Gallagher et al. 2002) and try to detect a stellar halo. been detected yet, for they are for instance not massive Optical observations have already shown some stars rather enough. far from the polar ring plane (see Fig. 12 in Arnaboldi et al. 1997), yet it seems premature to attribute them to a Probability concerns do not allow us to rule out one of stellar halo, for HI gas (that belongs to the polar ring) isthe scenarios. Even if the probability of the merging sce- observed at the same locations (see Fig. 2 in Arnaboldi etnario is much smaller than the probability of the accretion al. 1997). Deep HST observations also ﬁnd stars at largescenario, only discoveries of many new PRGs, that would radii, but a detailed analysis is required to decide whetherincrease the observed fraction of polar rings, would weaken they belong to the faintest regions of the polar ring andthis scenario. the host galaxy or form a third component. Iodice et al. (2002a) have shown that the structure of6. Comparison with observations NGC 4650A is well described, even at large radii, by a two-component model. This suggests that no stellar halo6.1. General considerations is required to account for the observed luminosity. Yet, we still do not know if really no halo is present, or if the haloIn Sect. 1, we mentioned several observational properties is too faint to be detected.of polar rings. Most of them are reproduced by both for- To estimate this, we build a luminosity model thatmation scenarios: accounts for the host galaxy and the polar ring. The model – most host galaxies are early-type, which is explained for the host galaxy is the same model as in Iodice et al. by the constraint on the gas content of host galaxies, (2002a). We call µ the surface luminosity of the polar ring, related to the fact that two orthogonal gaseous systems obtained after the subtraction of the host galaxy model. cannot be found at the same radius. The model for the ring is of the form: – polar rings may be very massive (x − xc (z)) – polar rings have various radii µmodel (x, z) = f (z) exp − (3) ∆x – most polar rings are nearly polar – most polar rings(even all rings in the merging scenario) where z denotes the polar axis and x denotes the axis per- are very stable and may be observed after several Gyrs pendicular to the polar ring plane. Similarities between – polar rings show various inner morphologies rings and rotating disks (Iodice 2001, 2002a) suggest that
14 F. Bournaud & F. Combes: Formation of polar ring galaxiesthe polar ring may have an exponential proﬁle perpendic-ular to its plane (as is the case on Fig. 3). Yet, no reasonimplies that the polar ring radial proﬁle f (z) should beexponential, because the ring is at least partly depletedinside its radius R, xc (z) would be zero if the ring wasexactly polar. It is not the case, and the ring is warped,too, so that we deﬁne for each z: xµ(x, z)dxxc (z) = (4) µ(x, z)dxGiven that the exact radial proﬁle of the ring can hardlybe deduced from observations (the ring is seen edge-on),the most natural choice for f (z) is simply:f (z) = µ(xc (z)) (5) Fig. 9. Proﬁle of the polar ring perpendicular to its plane,One free parameter, ∆x, is then ﬁtted to the HST image. observed at the ring radius R, and result of our two- This model does not study the ring radial distribution, component model: at this radius, the host luminosity isbut aims at exploring whether an exponential thickness negligible, and the ring shows an exponential thickening.of the ring accounts for the observed luminosity: if it is No stellar halo, that would be thicker than the ring, isthe case, matter observed far from the polar plane would detected.belong to the faintest regions of the polar ring, if it isnot the case, a third component (a stellar halo) would beformed by this matter. the result of a merging event, the victim galaxy should The model ﬁts observations well. For instance, at the have been a LSB galaxy.ring radius R, an exponential thickening of the polar ring This does not totally rule out the merging scenario.is really close to the observed proﬁle (see Fig. 9). No stellar Yet, the accretion scenario was already most likely to oc-halo then seems required, at least with the quality of our cur. A supplementary constraint to the merging scenarioobservations: no robust systematic deviation is observed is now that one of the invoked galaxies should have beenbetween our model and the real luminosity. More precisely, an LSB. This constraint is not present in the accretion sce-if a stellar halo is present, it is below the noise: its surface nario, for no stars are accreted, and no stellar halo aroundluminosity is smaller than |µ(x, z) − µmodel (x, z)|. The rel- the polar ring is expected. Thus, the probability that aevant quantity is then: merging event has resulted in NGC 4650A is strongly re- duced, so that NGC 4650A is much likely to be the result √ |µ(x, z) − µmodel (x, z)| dxdz (6) of a tidal accretion event. x2 +z 2 <RIt indicates the mass of the stellar halo inside the ring 6.2.2. Polar disk or polar ring?radius. As mentioned before, this mass is more than halfof the stellar mass of the victim galaxy is nearly all our Iodice (2001), and Iodice et al. (2002a) have proposed thatsimulations. We then obtain that the stellar mass of the the polar structure of NGC 4650A is more likely a diskvictim galaxy before the merging event should have been than a ring. Both shapes are not easy to discriminate, forsmaller than 3.6 · 109 M⊙ . the polar structure is seen edge-on. Iodice et al. (2002a) The polar ring nowadays contains 8 · 109 M⊙ of HI gas presented this as an argument in favor of the merging sce-(Arnaboldi et al. 1997) and 4 · 109 M⊙ of stars (Iodice nario. It is true that the radius of polar rings just afteret al. 2002a). The total mass of the polar ring is the their formation is slightly larger in the merging scenario,gaseous mass of the victim galaxy: stars in the polar ring according to N-body simulations. Yet, as explained by theare formed after the ring formation. In fact, the gaseous radial model presented in Appendix B, the formation ofmass of the victim galaxy may even be larger, if some gas the polar structure really results in a ring, not in a disk.is dispersed or converted into stars during the collision, Exceptionally, as noticed by Bekki (1998) and conﬁrmedbefore the ring formation. Then, the victim galaxy should by run A4, very low velocity collisions could result in po-have contained before the collision at the most 3.6 · 109 lar disks, provided that the relative velocity of collidingM⊙ of stars and at least 12 · 109 M⊙ of gas. It was thus galaxies of about 30 or 40 km.s−1 : with such a strongcharacterized by: supplementary constraint, and also the constraint on the victim stellar content found just below, the probability of – a gas mass fraction larger than 75% the merging scenario really becomes marginal. Thus, the – a visible mass larger than 15 · 109 M⊙ polar structure of NGC 4650A is bound either to be a ringSuch a gas mass fraction is found in both dwarf and low and not a disk, or to be a polar disk resulting of a previoussurface brightness (LSB) galaxies. As the total mass ex- polar ring, while it is very unlikely that a polar disk hascludes dwarf galaxies, we conclude that if NGC 4650A is formed directly around this host galaxy.
F. Bournaud & F. Combes: Formation of polar ring galaxies 15 Indeed, we have noticed in Sect. 4.2.1 (see also Fig. 7) mass, and the distribution of dark matter). This selectionthat the accretion scenario forms rings with moderate ra- eﬀect makes most polar ring appear nearly polar, yet in-dial extents, but that these rings may get a large radial clined rings exist, as in NGC 660, or also in ESO 235-G58extent, or even become disk-like structures, when they are (Iodice 2001), which requires an accretion mechanism.disturbed after their formation. The fact that NGC 4650A We have shown that when a polar ring forms in amay have a polar disk, or at least a very extended ring, galaxy merger, a signiﬁcant fraction of gas falls insideis thus not an argument against one of the formation sce- the host galaxy during the ring formation. Yet, it formsnarios: both scenarios may result in ring with large radial a small, central structure, in which gas is quickly con-extent, or even disks. verted into stars, and that never replenishes the whole host disk with gas. Moreover, the intruder cannot be a gas-rich galaxy: even if the ring radius R is larger than6.2.3. Surrounding galaxies the radius of the intruder, polar gas would have large ra-NGC 4650A belongs to a galaxy group. We have selected dial motion making its radius change and cross equatorial“surrounding galaxies” of NGC 4650A from the LEDA gas, which would prevent the ring from forming. Thus,database: such galaxies are assumed to be distant from either the gas of the intruder or the gas of the victim can-NGC 4650A of less than 45 minutes, and have radial not produce a gas-rich host galaxy. In a tidal accretionvelocity larger than 2507 and lower than 3307 km.s−1 event, the problem is solved: polar gas is directly placed(NGC 4650A radial velocity is 2907 km.s−1 ). Foreground around the ring radius R, so that it may never encounterand background galaxies are rejected. For these 15 galax- the gas of the host galaxy, provided that the ring radiusies, we ﬁnd a radial velocity dispersion of 200 km.s−1 . If R is larger than the host gas component radius, which iswe assume isotropic velocities in the group of galaxies, the the case in NGC 660. Constraints on the gas content ofmean relative velocity of two colliding galaxies in a head- the host galaxy are weaker in the accretion scenario thanon collision would be 400 km.s−1 , which makes a major in the merging scenario, and the gas-rich host galaxy ofmerger resulting from a head-on collision very unlikely: NGC 660 may result only from a tidal accretion event.two colliding galaxies are bound not to merge in this re- Finally, both characteristics of NGC 660 (inclined ring,gion of the Universe. gas-rich host galaxy) cannot be reproduced by our simu- NGC 4650A and the spiral galaxy NGC 4650 are at the lations of merging events. On the contrary, they are insame radial distance (the radial velocity of NGC 4650 is agreement with the results of the accretion scenario. On2909 km.s−1 ). The real distance between both galaxies is Fig. 10, we show the result of a simulation of accretion, inonly about 6 times the optical radius of NGC 4650. A tidal which the polar ring is highly inclined (42 degrees) and theaccretion event may then have form the polar ring from host galaxy is gas-rich (run C26). This numerical result isNGC 4650, and may easily explain why the ring is massive, quite similar to NGC 600. The ring mass is here 30% ofsince NGC 4650 is much more massive than NGC 4650A the visible mass, and the polar ring is observed duringhost galaxy. NGC 4650 is thus a candidate donor galaxy. 2.5 Gyrs in a nearly spherical dark halo: this ring is un- stable but needs time to dissolve. Van Driel et al. (1995) have found that the ring mass in NGC 660 was even more6.3. NGC 660 important (40% of the visible mass), so that the polar structure of NGC 660 will probably persist during someThe main characteristic of NGC 660 is that the ring ex- Gyrs after its formation.tends to about 45 degrees from perpendicular (Whitmoreet al. 1990, Combes et al. 1992). This polar ring is thus As noticed by Iodice (2001), most polar rings arelikely to be unstable, even if self-gravity may stabilize it if nearly polar. This could a priori be explained only by themassive enough (Arnaboldi & Sparke 1994). The second merging scenario. Actually, this could also be explained bynoticeable feature is the presence of gas in the host galaxy: the accretion scenario; if the host potential is ﬂat enoughthe host gas content in typical of a late-type spiral: Van in most PRGs, inclined rings will be dissolved quickly,Driel et al. (1995) have found MHI /L0 = 0.3M⊙/L⊙,B in and will rarely be observed. So, the fact that polar rings Bthe host disk, and detected a large mass of molecular gas are nearly polar does not argue for the merging scenario,in the disk. at least until the shape of galactic potentials is known. Accretion may form many nearly polar rings, and some According to numerical simulations of the merging sce- more inclined structures, rarely observed for they are lessnario, no ring is inclined of more than 25 degrees from the stable, and also as they are confused with barred galaxiespolar axis. The ring of NGC 660 is maybe unstable, but (see Iodice 2001 for ESO 235-G58).has formed and is observed, which is not reproduced in themerging scenario. The fact is that in the merging scenario,very inclined rings are not observed to be unstable, but are 6.4. NGC 2685observed not to form at all. The accretion scenario formsrings with larger inclinations. Inclined rings are rarely ob- NGC 2685, the Helix galaxy, has been studied in detail byserved, for they are probably unstable and disappear in Eskridge & Pogge (1997). Observations of a large metal-a few Gyrs (but the stability depends on the exact ring licity, color indices, and molecular gas were claimed to ar-
16 F. Bournaud & F. Combes: Formation of polar ring galaxiesFig. 10. Run C26 : gas+stars (left) – gas (right). The polar ring mass is 30% of the host mass, the ring is 42 degreesfrom polar, and the host is gas-rich.gue against the idea that the ring forms from tidally cap- be observed at radii larger than R, as well as gas at radiitured dwarf galaxies, and that the polar ring would be a smaller than R as is observed in NGC 660.long-lived self-gravitating structure instead. Yet, we haveshown that the accretion scenario does not require thedonor to be a dwarf galaxy, and that a massive donor can 7. Polar rings and dark matterform a massive (thus self-gravitating) polar ring; more- 7.1. Dark halos of polar ring galaxiesover we also show that most polar rings formed by tidalaccretion are long-lived. Thus, the results of Eskridge & 7.1.1. Dark matter around donor galaxiesPogge (1997) do not seem to argue against the accretionscenario, but only to require that the donor was rather The formation of a polar ring according to the accretionmassive and the ring is stable. scenario is based on the development of tidal tails, which is An important feature of NGC 2685 is the presence inﬂuenced by the potential of the donor galaxy (Dubinskiof an outer, equatorial ring. The host galaxy is not gas- et al. 1996). As shown by Dubinski et al. (1999), somedepleted, as is the case for NGC 660, but contains gas distributions of dark matter may inhibit the formation ofat radii larger than the polar ring (Schinnerer & Scoville tidal tails. This could prevent the accretion scenario from2002). occurring, or make it much less frequent. However, tidal We have already explained that the presence of large tails around interacting galaxies are frequently observed inamount of gas in the host galaxy is not accounted for by the Universe. Thus, it seems that the results of Dubinski etthe merging scenario. In the case of NGC 2685, the host al. (1996, 1999) should be interpreted in the following way:galaxy is not really gas-rich, thus its gas content is not in observations of tidal tails provides some constraints oncontradiction with the merging scenario, provided that the dark halo shapes, and dark distributions that prevent tidalvictim disk has contained enough gas (see Sect. 3.3). Yet, tails from forming are in fact rare, so that the accretionpolar gas would sweep every radius before the polar ring scenario actually occurs.forms (see Fig. B.1, B.3), which would compel equatorialgas to join the polar ring (as observed in run B13), in 7.1.2. Dark matter around polar ring galaxiesthe same way as a larger quantity of equatorial gas wouldmake the polar gas fall inside the equatorial plane. An In our simulations of the merging scenario, when bothouter equatorial ring and a polar ring – more generally galaxies are initially embedded in spheroidal halos, theequatorial and polar gas – cannot coexist after a merging ﬁnal dark halo is spheroidal, and its ﬂattening is most ofevent: one of the gaseous components (equatorial/polar) the time smaller than E4. The dark-to-visible mass ratiowill join the other one, depending on their relative mass. is never larger than in preexisting spiral galaxies.Instead, two orthogonal rings may result from an accretion According to the accretion scenario, when galaxies areevent, provided that they have not the same radius: the initially embedded in spheroidal halos, the ﬁnal halo isrelevant aspect of this scenario is that polar gas is directly directly related to the preexisting halo of the host galaxy,placed around the ring ﬁnal radius R. Polar gas will then slightly more spherical because of the tidal interaction.deplete the equatorial disk from its gas around R, but will The dark-to-visible mass ratio in PRGs is again not largernot encounter gas at other radii. An outer ring may then than in spiral galaxies.
F. Bournaud & F. Combes: Formation of polar ring galaxies 17 Iodice et al. (2003) have shown that polar ring veloc- dissolved. Only tidal accretion can produce such inclinedities are abnormally large, when compared to the Tully- structuresFisher relation for spiral galaxies. The dark halo could We ﬁnally conclude that at least most PRGs – and pos-then be either ﬂattened along the polar ring, or spheroidal sibly all of them – are the result of tidal accretion events.and very massive: the dark-to-visible mass ratio inside the The two most recent studies of the distribution of darkoptical radius would be as large as 3.5. Yet, velocities ob- matter in PRGs (Combes & Arnaboldi 1996, Iodice et al.served in host disk are not abnormally large (e.g. Combes 2003), based on two diﬀerent approaches, conclude that& Arnaboldi 1996), which led Iodice et al. (2003) to rule dark halos are ﬂattened towards polar rings in most PRGs.out spherical, very massive, halos. Our simulations con- This is fully compatible with the accretion scenario, andﬁrm that the dark-to-visible mass ratio in PRGs cannot suggests that some dark matter is cold molecular gas.be generally larger than 1 (i.e. not larger than in spiralgalaxies), which provides another argument against verymassive dark halos. For this reason, only dark structures Appendix A: Models for galaxiesthat are ﬂattened towards polar rings account for obser- A.1. Disksvational results of Iodice et al. (2003), as already found byCombes & Arnaboldi (1996) in the case of NGC 4650A. We model stellar and gaseous disks with Toomre-Kuzmin Dark halos ﬂattened towards polar rings may be form disks, and a vertical distribution:during accretion events, provided that some dark matter is zcold molecular gas (Pfenniger & Combes 1994, Pfenniger ρ(r, z) = σ(r)sech2 (A.1) h0et al. 1994). Under this hypothesis, the ideas that polarrings are issued of tidal accretion events and that dark where σ(r) is the surface density of the disk and h0 = 1halos of PRGs are ﬂattened towards most polar rings are kpc. The initialization of particles velocities for the diskthen compatible. On the other hand, when no dark matter is described in Bournaud & Combes (2002).is cold-gas, most halos are spherical or slightly ﬂattenedtowards the host galaxies, and rarely toward the polar A.2. Dark halos, bulges, and elliptical galaxiesring; which is not supported by observations (Iodice et al.2003). Halos are ﬁrst built as Plummer spheres and given an isotropic velocity dispersion corresponding to an isother- mal virialized system. We sometime use ﬂattened oblate8. Conclusion structures of axis ratio e: the vertical position of each par- ticle is then reduced by factor e from the initial PlummerOur numerical simulations have conﬁrmed that both sce- sphere. The anisotropy in the ﬁnal system should thennarios are rather robust, and possibly result in realistic verify (Becquaert & Combes 1997):polar structures. The accretion scenario appears to bemuch more likely than the merging scenario: there are 2(1 − e2 ) √ 1 − arcsin(e)/estrong constraints for a merging event to result in a PRG. ev = 1− 1−e2 √ (A.2)However, the merging scenario is robust enough to account arcsin(e)/e − 1 − e2for the fraction of PRG that have been detected until now. where ev is deﬁned by: Comparison of numerical results and observationalproperties tend to indicate that polar rings are actu- 2 2 2 vzally the consequence of tidal accretion events. Identifying vx = vy = (A.3) 1 − e2 vdonor galaxies (when they have not merged with the hostafter ring formation) would be a conﬁrmation, yet nu- where (x, y) is the disk plane, and z the direction of themerical simulations indicate that no obvious link remains minor axis of the oblate dark halo. The kinetic energy isbetween the donor and the ring, even only 1 or 2 Gyrs af- computed to keep the system virialized, velocities are thenter the ring formation. Deep photometric studies, aiming distributed to particles as if the system was isothermal,at detecting the stellar remnant of the victim disk in the and oriented in space according to Eq. A.3. The scale-merging scenario, may bring another conﬁrmation that length of dark halo is computed to insure a ﬂat rotationPRGs are the result of tidal accretion events. curve in the outer disk. We also notice that the accretion scenario may form Bulges are built in the same manner as dark halos.massive rings, as the ring of NGC 4650A. The donor is They are generally assumed to be E5 spheroidal systems.not required to be less massive than the future host, but Elliptical galaxies are ﬁrst built as E0 systems accord-could be more massive, and a large fraction of the donor ing to the model of Jaﬀe (1983). They are then convertedgas could be accreted in the polar ring. Our simulations to oblate systems and virialized, exactly as was done foralso suggest that most polar rings are long-lived struc- dark halos. Prolate systems have not been considered, fortures, stabilized by self-gravity, independently of the for- the shape of the host galaxy is not a crucial point in ourmation scenario. Some inclined rings exist, as NGC 660. study, and most host galaxies are disk galaxies and notSuch rings are likely to be unstable, but need time to be ellipticals.