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Dark Energy Domination in the Virgocentric Flow A&A 520, A104 (2010) Astronomy DOI: 10.1051/0004-6361/201014912 & c ESO 2010 Astrophysics Dark energy domination in the Virgocentric flow A. D. Chernin1,2, I. D. Karachentsev3,O.G.Nasonova3,P.Teerikorpi1,M.J.Valtonen1, V. P. Dolgachev2, L. M. Domozhilova2, and G. G. Byrd4 1 Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 21500 Piikkiö, Finland e-mail: [email protected] 2 Sternberg Astronomical Institute, Moscow University, Moscow 119899, Russia 3 Special Astrophysical Observatory, Nizhnii Arkhys 369167, Russia 4 University of Alabama, Tuscaloosa, AL 35487-0324, USA Received 3 May 2010 / Accepted 1 June 2010 ABSTRACT Context. The standard ΛCDM cosmological model implies that all celestial bodies are embedded in a perfectly uniform dark energy background, represented by Einstein’s cosmological constant, and experience its repulsive antigravity action. Aims. Can dark energy have strong dynamical effects on small cosmic scales as well as globally? Continuing our efforts to clarify this question, we now focus on the Virgo Cluster and the flow of expansion around it. Methods. We interpret the Hubble diagram from a new database of velocities and distances of galaxies in the cluster and its envi- ronment, using a nonlinear analytical model, which incorporates the antigravity force in terms of Newtonian mechanics. The key parameter is the zero-gravity radius, the distance at which gravity and antigravity are in balance. Results. 1. The interplay between the gravity of the cluster and the antigravity of the dark energy background determines the kine- matical structure of the system and controls its evolution. 2. The gravity dominates the quasi-stationary bound cluster, while the antigravity controls the Virgocentric flow, bringing order and regularity to the flow, which reaches linearity and the global Hubble rate at distances ∼>15 Mpc. 3. The cluster and the flow form a system similar to the Local Group and its outflow. In the velocity-distance diagram, the cluster-flow structure reproduces the group-flow structure with a scaling factor of about 10; the zero-gravity radius for the cluster system is also 10 times larger. Conclusions. The phase and dynamical similarity of the systems on the scales of 1−30 Mpc suggests that a two-component pattern may be universal for groups and clusters: a quasi-stationary bound central component and an expanding outflow around it, caused by the nonlinear gravity-antigravity interplay with the dark energy dominating in the flow component. Key words. Local Group – galaxies: clusters: individual: Virgo cluster – dark matter – dark energy 1. Introduction For R > 10−15 Mpc, a roughly linear velocity-distance relation may be seen in the flow, making it resemble the global Hubble Early studies of the motion of our Galaxy (reviewed by Huchra expansion. But strong deviations from the linear relation exist at 1988) led to the discovery of the peculiar velocity of the Local smaller distances, not surprisingly: 1) the linear expansion flow, Group towards the Virgo cluster. To illustrate this, imagine V = HR, is the property of the universe beyond the cosmic “cell the distance to Virgo to be 17 Mpc and the Hubble constant ≥ −1 −1 of uniformity” ( 300 Mpc); 2) the Virgo cluster is a strong lo- H0 = 72 km s Mpc , then the expected cosmological ve- −1 cal overdensity; 3) the matter distribution around the cluster is locity of the cluster is 1224 km s . The observed velocity is highly nonuniform up to the distance of about 30 Mpc. about 1000 km s−1. The difference reflects our peculiar velocity of about 220 km s−1 towards Virgo. Besides the natural deviations, we have here an example of a This estimate of the retarded expansion (the so-called “Virgo paradox realized by Sandage (1986, 1999). He saw a mystery in infall”) views the universal expansion as existing not only on the fact that the local expansion proceeds in a quite regular way. “truly cosmological” scales ∼1000 Mpc, but also on local scales Moreover, the rate of expansion is similar, if not identical, to the of ∼10 Mpc only weakly disturbed. Observational and theoreti- global Hubble constant (Ekholm et al. 2001;Thimetal.2003; cal aspects of the Virgo infall were studied e.g. by Silk (1974, Karachentsev 2005; Karachentsev et al. 2002a, 2003b; Whiting 1977), Peebles (1976), Hoffman & Salpeter (1982), Sandage 2003). Originally, the Friedmann theory described the expan- (1986), Teerikorpi et al. (1992), and Ekholm et al. (1999, 2000). sion of a smooth, uniform self-gravitating medium. But the lin- ear cosmological expansion was discovered by Hubble where it should not be: in the lumpy environment at distances <20 Mpc. 1.1. The Virgocentric flow The Virgo infall is a particular feature of what is known as 1.2. The local relevance of dark energy the Virgocentric flow: hundreds of galaxies (our Galaxy among them) are receeding away from the Virgo cluster. Recent data Soon after the discovery of dark energy (Riess et al. 1998; (Karachentsev & Nasonova 2010) show that the flow veloci- Perlmutter et al. 1999) on global scales, we suggested (Chernin ties range from nearly zero around 5−8 Mpc from the cluster et al. 2000;Chernin2001; Baryshev et al. 2001; Karachentsev center to about 2000 km s−1 at the distances of about 30 Mpc. et al. 2003a; Chernin et al. 2004; Teerikorpi et al. 2006)that Article published by EDP Sciences Page 1 of 7 A&A 520, A104 (2010) dark energy with its omnipresent and uniform density (repre- sented by Einstein’s cosmological constant) may provide the dy- namical background for a regular quiescent expansion on local scales, resolving the Hubble-Sandage paradox. Our key argu- ment came from the fact that the antigravity produced by dark energy is stronger than the gravity of the Local Group at dis- tances larger than about 1.5 Mpc from the group center1. The present paper extends our work from groups to the scale of clusters, focusing on the Virgo cluster. 2. The phase space structure Recent observations of the Virgo cluster and its vicinity permit a better understanding of the physics behind the visible structure and kinematics of the system. Fig. 1. Hubble diagram with Virgocentric velocities and distances for 2.1. The dataset 761 galaxies of the Virgo Cluster and the outflow (Karachentsev & Nasonova 2010). The two-component pattern is outlined by bold dashed The most complete list of data on the Virgo Cluster and ∼ the Virgocentric flow has been collected by Karachentsev & lines. The zero-gravity radius RZG 10 Mpc is located in the zone between the components. The broken line is the running median used Nasonova (2010). These include distance moduli of galax- by K & N to find the zero-velocity radius R0 = 6−7 Mpc. Two lines = − − ies from the Catalogue of the Neighbouring Galaxies ( CNG, from the origin indicate the Hubble ratios H = 72 km s 1 Mpc 1 and −1 −1 Karachentsev et al. 2005) and also from the literature with the HV = 65 km s Mpc . Two curves show special trajectories, corre- best measurements prefered. Distances from the tip of the red sponding to the parabolic motion (E = 0; the upper one) and to the giant branch (TRGB) and the Cepheids are used from the CNG minimal escape velocity from the cluster potential well (the lower one). together with new TRGB distances (Karachentsev et al. 2006; The flow galaxies with the most accurate distances and velocities (big Makarov et al. 2006; Mei et al. 2007). For galaxy images in dots) occupy the area between these curves. two or more photometric bands obtained with WFPC2 or ACS cameras at the Hubble Space Telescope, the TRGB method yields distances with an accuracy of about 7% (Rizzi et al. distances. The zero-velocity radius gives the upper limit of the 2007). The database includes also data on 300 E and S0 galaxies size of the gravitationally bound cluster, and the diagram shows ≥ from the surface brightness fluctuation (SBF) method by Tonry that the Virgocentric flow starts at R R0 and extends at least et al. (2000) with a typical distance error of 12%. The total up to 30 Mpc. sample contains the velocities and distances of 1371 galaxies The zero-velocity radius has been often used for estimating within 30 Mpc from the Virgo cluster center. Especially inter- the total mass M0 of a gravitationally bound system. According esting is the sample of 761 galaxies selected to avoid the ef- to Lynden-Bell (1981) and Sandage (1986), the spherical model Λ= fect of unknown tangential (to the line of sight) velocity compo- with 0 leads to the estimator nents. The velocity-distance diagram for this sample taken from − M = (π2/8G)t 2R3. (1) Karachentsev & Nasonova (2010)isgiveninFig.1. 0 U 0 With the age of the universe tU = 13.7 Gyr (Spergel et al. 2007), 2.2. The zero-velocity radius and the cluster mass Karachentsev & Nasonova (2010) find the Virgo cluster mass 14 M0 = (6.3 ± 2.0) × 10 M. This result agrees with the virial 14 The zero-velocity radius within the retarded expansion field mass Mvir = 6 × 10 M estimated by Hoffman & Salpeter 14 around a point-like mass concentration means the distance where (1982)and∼4 × 10 M of Valtonen et al. (1985) and Saarinen the radial velocity relative to the concentration is zero. In the & Valtonen (1985). Teerikorpi et al. (1992) and Ekholm et al. ideal case of the mass concentration at rest within the expanding (1999, 2000) found that the real cluster mass might be from 1 15 Friedmann universe this is the distance where the radial pecu- to 2 the virial mass, or (0.6−1.2) × 10 M.
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