Deficient Galaxies on the Outskirts of Virgo Recent Arrivals?

The presence on the Virgo cluster outskirts of spiral galaxies with gas deficiencies as strong as those of the inner galaxies stripped by the intracluster medium has led us to explore the possibility that some of these peripheral objects are not newcomers. A dynamical model for the collapse and rebound of spherical shells under the point mass and radial flow approximations has been developed to account for the amplitude of the motions in the Virgo I cluster (VIC) region. According to our analysis, it is not unfeasible that galaxies far from the cluster, including those in a gas-deficient group well to its background, went through its core a few Gyr ago. The implications would be: (1) that the majority of the H I-deficient spirals in the VIC region might have been deprived of their neutral hydrogen by interactions with the hot intracluster medium; and (2) that objects spending a long time outside the cluster cores might keep the gas deficient status without altering their morphology. Subject headings: galaxies: clusters: individual (Virgo) — galaxies: evolution — galaxies: ISM — galaxies: spiral 1. AN INTRIGUING POSSIBILITY WORTH EXPLORING A recent characterization of the large-scale 3D distribution of the neutral gas (H I) deficiency around the Virgo I Cluster (VIC) region by Solanes et al. (2002, hereafter Paper I) has shown that there are a significant number of galaxies with a dearth of atomic hydrogen at large Virgocentric distances. These peripheral gas-deficient objects, which can be observed both in the cluster front and in a probable background group well behind the cluster core, show gaseous deficiencies comparable in strength to those measured in the centers of Virgo and other rich galaxy clusters. One of the mechanisms that can most naturally account for the observed reduction in the interstellar gas content of cluster galaxies is the ram pressure ablation caused by the rapid motion of galaxies through the dense intracluster medium (ICM). There is now compelling evidence for the decisive participation of this process in the gaseous deficiencies of spirals located in the centers of rich clusters, either directly from observations (Giovanelli & Haynes 1985; Gavazzi & Jaffe 1987; Dickey & Gavazzi 1991) —including the discovery of shrunken gaseous disks (Cayatte et al. 1994; Bravo-Alfaro et al. 2000) and the finding that H I deficient spirals are on very eccentric orbits (Solanes et al. 2001)— or from theoretical studies that have checked the efficiency of this mechanism (Stevens, Acreman, & Ponman 1999; Quilis, Moore, & Bower 2000; Vollmer et al. 2001, to name only a few). In the outer cluster regions the low density of the intergalactic medium calls, in principle, for alternative gas removal mechanisms, such as gravitational tidal interactions. Indeed, observational evidence suggests that processes of this kind might have played an important role on the evolution of the galactic population in distant clusters (e.g., van Dokkum et al. 1999) due to favorable conditions for frequent low-relative velocity encounters among the galaxies in early epochs. Although in the VIC region low-relative-velocity galaxy-galaxy interactions may also be responsible for the gaseous deficiencies observed in some of the peripheral galaxies, it should not be forgotten that the dynamics of the Virgo region is dominated by large-scale nonHubble radial streaming motions. In this context, it is plausible that some galaxies at large Virgocentric distances are on very eccentric orbits that carry them right through the cluster center with high relative velocities and are therefore liable to have suffered a strong interaction with the ICM. One of the most influential studies of the Local Supercluster based on dynamical model calculations is the analysis by Tully & Shaya (1984) of the infall of galaxies in the Virgo Southern Extension (or Virgo II cloud) toward the VIC. The lumpy distribution of galaxies in space led these authors to predict a very irregular infall rate which would be responsible for the secular evolution of the mix of morphological types in the cluster. It was suggested that the formation of the cluster took place at an early epoch —when the universe was about one fourth of its present age (R. B. Tully 2002, private communication)— by a first generation of, probably, early-type galaxies. Afterwards, infall was reduced until very recently when the large spiral-rich Virgo II cloud has begun to fall into the cluster diluting the fraction of early-type systems. The fact that Tully & Shaya saw very few outwardly moving galaxies outside the 6 VIC circle supported their argument that most, or perhaps all, spirals and irregulars in Virgo, mostly supplied by the Virgo II cloud, were recent arrivals. All the findings of Tully & Shaya (1984) were based on a data set that contained a limited number of galaxies with relatively uncertain distance estimates. Now, with a much larger sample and more accurate distances, we provide evidence that a substantial number of galaxies, with a wide range of clustercentric 1 Departament d’Astronomia i Meteorologia, Universitat de Barcelona. Av. Diagonal, 647; 08028 Barcelona, Spain 2 Departament d’Enginyeria Informàtica i Matemàtiques, Universitat Rovira i Virgili. Av. Països Catalans, 26; 43007 Tarragona, Spain 3 CER d’Astrofísica, Física de Partícules i Cosmologia, Universitat de Barcelona. Av. Diagonal, 647; 08028 Barcelona, Spain 4 European Southern Observatory, Casilla 19001, Santiago 19, Chile 5 Observatoire de Paris-Meudon DESPA, F–92195 Meudon CEDEX, France 1