Ram Pressure Stripping and Galaxy Orbits: the Case of the Virgo Cluster
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Ram pressure stripping and galaxy orbits: The case of the Virgo cluster B. Vollmer Max-Planck-Institut f¨ur Radioastronomie, Bonn, Germany and Observatoire de Paris, Meudon, France. V. Cayatte, C. Balkowski Observatoire de Paris, DAEC, UMR 8631, CNRS et Universit´e Paris 7, F-92195 Meudon Cedex, France. and W.J. Duschl1 Institut f¨ur Theoretische Astrophysik der Universit¨at Heidelberg, Tiergartenstraße 15, D-69121 Heidelberg, Germany. ABSTRACT We investigate the role of ram pressure stripping in the Virgo cluster using N-body simulations. Radial orbits within the Virgo cluster’s gravitational potential are modeled and analyzed with respect to ram pressure stripping. The N-body model consists of 10 000 gas cloud complexes which can have inelastic collisions. Ram pressure is modeled as an additional acceleration on the clouds located at the surface of the gas distribution in the direction of the galaxy’s motion within the cluster. We made several simulations changing the orbital parameters in order to recover different stripping scenarios using realistic temporal ram pressure profiles. We investigate systematically the influence of the inclination angle between the disk and the orbital plane of the galaxy on the gas dynamics. We show that ram pressure can lead to a temporary increase of the central gas surface density. In some cases a considerable part of the total atomic gas mass (several 108 M ) can fall back onto the galactic disk after the stripping event. A quantitative relation between the orbit parameters and the resulting Hi deficiency is derived containing explicitly the inclination angle between the disk and the orbital plane. The comparison between existing Hi observations and the results of our simulations shows that the Hi deficiency depends strongly on galaxy orbits. It is concluded that the scenario where ram pressure stripping is responsible for the observed Hi deficiency is consistent with all Hi 21cm observations in the Virgo cluster. Subject headings: ISM: clouds { ISM: kinematics and dynamics { Galaxies: cluster: individual: Virgo Cluster { Galaxies: evolution { Galaxies: interactions { Galaxies: ISM { Galaxies: kinematics and dynamics 1. Introduction 1also at: Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, D-53121 Bonn, Germany Since Gunn & Gott (1972) have introduced the concept of ram pressure stripping, which can affect galaxies moving inside the Intracluster Medium (ICM) of a galaxy cluster, this mechanism has 1 been invoked to explain different observational In the local universe, the existence of a large phenomena as the HI deficiency of spiral galaxies number of post starburst galaxies in the Coma in clusters (Chamaraux et al. 1980 , Bothun et al. cluster seems to indicate a triggering of the star 1982, Giovanelli & Haynes 1985) or the lower star formation activity by effects which are related to formation activity of cluster spiral galaxies (e.g. the cluster environment (Bothun & Dressler 1986, Dressler et al. 1999, Poggianti et al. 1999). Caldwell et al. 1993). Caldwell & Rose (1997) Some other mechanisms have been proposed af- observed an enhanced number of post starburst fecting spiral galaxies in clusters, among which: galaxies within cluster environments. On the con- trary, Zabludoff et al. (1996) found that interac- galaxy–galaxy interactions (galaxy “harass- tions with the cluster environment, in the form of • ment”; Moore et al. 1996, 1998) the intracluster medium or cluster potential, are not essential for ”E+A” formation. HST images galaxy–cluster gravitational interaction (Byrd • of the Coma starburst and post starburst galax- & Valtonen 1990, Valluri 1993) ies (Caldwell, Rose, & Dendy 1999) revealed that All these kinds of interaction have in common that star formation takes mainly place in the inner disk they can in principle remove the neutral gas from of these galaxies and is suppressed in the outer a galaxy inducing a decrease of its star formation disk. If the starburst is related to the cluster en- activity. vironment, ram pressure stripping can naturally account for these characteristics. The strength of these interactions depends cru- cially on galaxy orbits. Radial orbits allow galax- The best place to study the gas removal due ies to go deeper into the cluster potential where to ram pressure is the Virgo cluster as it is the their velocity increases considerably and where the closest cluster which can be observed in great de- galaxy density and the density of the ICM is sub- tail. Most of the spiral galaxies seem to have stantially higher. Dressler (1986) analyzed Gio- entered the cluster only recently (within several Gyr, Tully & Shaya 1984). About half of them vanelli & Haynes (1985) data showing that Hi de- ficient spiral galaxies are on radial orbits. Numer- became Hi deficient (Giovanelli & Haynes 1985). ical simulations (Ghigna et al. 1998) showed that Their Hi disk sizes are considerably reduced (van galaxy halos evolving within the cluster settle on Gorkom & Kotanyi 1985, Warmels 1988, Cayatte isotropic orbits with a median ratio of pericentric et al. 1990, 1994). Concerning the stellar con- to apocentric radii of 1:6. tent, their intrinsic color indices are not signif- icantly different from field galaxies of the same The recent analysis of a spectroscopic catalog morphological type (Gavazzi et al. 1991, Gavazzi of galaxies in ten distant clusters (Dressler et al. et al. 1998). But despite the Hi deficiency, cluster 1999, Poggianti et al. 1999) has shown that the galaxies do not show a reduced CO content (Ken- galaxy populations of these clusters are charac- ney & Young 1989, Boselli et al. 1997) neither terized by the presence of a large number of post a reduced infrared luminosity (Bicay & Giovanelli starburst galaxies. Poggianti et al. (1999) con- 1987). This means that the neutral gas was re- cluded that the most evident effect due to the moved without affecting the molecular component cluster environment is the quenching of star for- and without changing colors very much. The only mation rather than its enhancement. They found interaction cited above producing this signature is two different galaxy evolution timescales in clus- ram pressure stripping. ters. (i) A rapid removal of the gas ( 1Gyr). (ii) A slow transformation of morphology∼ (several Very few simulations have been done to quan- Gyr). They stated that the mechanism responsi- tify ram pressure stripping using Eulerian hydro- ble for the fast gas removal without changing the dynamic (Takeda, Nulsen, & Fabian 1984, Gaetz, galaxy’s morphology is very probably ram pressure Salpeter & Shaviv 1987; Balsara, Livio, & O’Dea stripping. Furthermore, Couch et al. (1998), using 1994) or SPH codes (Tosa 1994; Abadi, Moore, HST data, claimed that there are fewer disturbed & Bower 1999). We use a sticky particles model galaxies than predicted by the galaxy harassment in which each particle represents a cloud complex model. They suggested that ram pressure strip- with a given mass dependent radius. The viscos- ping by the hot ICM truncates star formation. ity of the clumpy ISM is due to inelastic collisions 2 1 2 1 between the particles. The effect of ram pressure (2πG)− vrotR− (Binney & Tremaine 1987): is modeled as an additional acceleration on the Σ 2 1 = 2 (1) particles located in the direction of the galaxy’s gas vrotR− ρICMvgalaxy , motion. where Σ =gas is the stellar/gas surface den- ∗ This article is devoted to the question if ram sity, ρICM is the ICM density, vrot is the ro- pressure stripping can account for the observed tation velocity, and vgalaxy is the galaxy’s ve- Hi deficiency, Hi distributions, and velocity fields locity within the cluster. We assume Σgas = 21 2 1 of spiral galaxies in the Virgo cluster. Further- const = 10 cm− and vrot = 150 km s− . min more we investigate possible star formation mech- The minimum ram pressure pram to affect the anisms related to a ram pressure stripping event. atomic gas disk can be determined by setting We first discuss radial galactic orbits within the R = RHI = 15 kpc. For our model galaxy this min 3 1 2 cluster gravitational potential (Section 2). The leads to pram 500 cm− (km s− ) .Fig.1 model simulating the neutral gas content of an in- shows the galaxy∼ velocity as a function of the falling galaxy is described in Section 3, followed ICM density for constant values of the ram pres- 2 by the detailed description of the simulations (Sec- sure pram = ρICM vgalaxy. The area below the tion 4). The evolution of the stripped gas in the solid line corresponds to the part of the parame- 2 3 1 2 intracluster medium is investigated in Section 5. ter space where ρICMvgalaxy <500 cm− (km s− ) , In Section 6 we give the results of these simula- i.e. where ram pressure is not effective. The curves 2 tions and discuss them in Section 7. We confront correspond to a values of ρICMvgalaxy=1000, 2000, 3 1 2 observations of various quantities of spiral galax- 5000, 10000 cm− (km s− ) that we used in our ies with the knowledge acquired with the help of simulations. For the simulations we varied the in- the simulations (Section 8) and give possible ex- clination angle Θ between the disk and the orbital planations for their dynamical and physical state plane: Θ=0o,20o,45o,90o. With this defini- (Section 9 and 10). The summary and conclusions tion of the inclination angle, Θ=0o means edge-on are given in Section 11. stripping and Θ=90o face-on stripping. For our 1 We adopt a velocity of 1150 km s− for the model, this represents the physically most mean- Virgo cluster (Huchra 1988) and a distance of ingful definition and has the advantage that it 17 Mpc. leads to simple fitting functions of our numerical results. 2. Galaxy orbits We took a β-model (Cavaliere & Fusco-Femiano 1976) to describe the total mass density and the The hot intracluster medium (ICM) of density 2 ICM density: ρICM exerts a ram pressure: pram = ρICM vgalaxy 2 on a galaxy, which moves with a velocity v r 3 β galaxy ρ = ρ 1+ − 2 (2) through it.