Abstract. Spiral galaxies that are deficient in neutral hydrogen are observed on the outskirts of the Virgo cluster. If their orbits have crossed the inner parts of the cluster, their interstellar gas may have been lost through ram pressure stripping by the hot X-ray emitting gas of the cluster. We estimate the maximum radius out to which galaxies can bounce out of a virialized system using analytical arguments and cosmological N-body simulations. In particular, we derive an expression for the turnaround radius in a flat cosmology with a cosmological constant that is simpler than previously derived expressions. We find that the maximum radius reached by infalling galaxies as they bounce out of their cluster is roughly between 1 and 2.5 virial radii. Comparing to the virial radius of the Virgo cluster, which we estimate from X-ray observations, these H I-deficient galaxies appear to lie significantly further away from the cluster center. Therefore, if their distances to the cluster core are correct, the H I-deficient spiral galaxies found outside of the Virgo cluster cannot have lost their gas by ram pressure from the hot intracluster gas.

The distances and H I contents of 161 spiral galaxies in the region of Virgo cluster are used to gain insight into the complicated structure of this galaxy system. Special attention has been paid to the investigation of the suggestion presented in an earlier work that some peripheral Virgo groups may contain strongly gas-deficient spirals.The three-dimensional galaxy distribution has been inferred from quality distance estimates obtained by averaging distance moduli based upon the Tully-Fisher relationship taken from eight published datasets previously homogenized, resulting in a relation with a dispersion of 0.41 mag. Previous findings that the spiral distribution is substantially more elongated along the line-ofsight than in the plane of the sky are confirmed by the current data. In addition, an important east-west disparity in this effect has been detected. The overall width-to-depth ratio of the Virgo cluster region is about 1 : 4, with the most distant objects concentrated in the western half. The filamentary structure of the spiral population and its orientation are also reflected by the H Ideficient objects alone. The H I deficiency pattern shows a central enhancement extending from ∼ 16 to 22 Mpc in line-of-sight distance; most of this enhancement arises from galaxies that belong to the Virgo cluster proper. However, significant gas deficiencies are also detected outside the main body of the cluster in a probable group of galaxies at line-of-sight distances ∼ 25-30 Mpc, lying in the region dominated by the southern edge of the M49 subcluster and clouds W ′ and W, as well as in various foreground galaxies. In the Virgo region, the H I content of the galaxies then is not a straightforward indicator of cluster membership.

Abstract.We have applied a relativistic Tolman-Bondi model of the Virgo cluster to a sample of 183 galaxies with measured distances within a radius of 8 degrees from M 87. We find that the sample is significantly contaminated by background galaxies which lead to too large a cluster mean distance if not excluded. The Tolman-Bondi model predictions, together with the HI deficiency of spiral galaxies, allows one to identify these background galaxies. One such galaxy is clearly identified among the 6 calibrating galaxies with Cepheid distances. As the TolmanBondi model predicts the expected distance ratio to the Virgo distance, this galaxy can still be used to estimate the Virgo distance, and the average value over the 6 galaxies is 15.4 ± 0.5 Mpc. Well-known background groups of galaxies are clearly recovered, together with filaments of galaxies which link these groups to the main cluster, and are falling into it. No foreground galaxy is clearly detected in our sample. Applying the B-band Tully-Fisher method to a sample of 51 true members of the Virgo cluster according to our classification gives a cluster distance of 18.0 ± 1.2 Mpc, larger than the mean Cepheid distance. Finally, the same model is used to estimate the Virgo cluster mass, which is M = 1.2 × 10 15 M within 8 degrees from the cluster center (2.2 Mpc radius), and amounts to 1.7 virial mass.

CCD observations of the gravitational lens system Q0957+561A,B in the BV RI bands are presented in this paper. The observations, taken with the 82 cm IAC-80 telescope, at Teide Observatory, Spain, were made from the beginning of 1996 February to 1998 July, as part of an on-going lens monitoring program. Accurate photometry was obtained by simultaneously fitting a stellar two-dimensional profile on each component by means of DAOPHOT software. This alternative method equals and even improves the results obtained with previous techniques. The final dataset is characterized by its high degree of homogeneity as it was obtained using the same telescope and instrumentation during a period of almost 3 years. The resulting delay, obtained with a new method, the δ 2 -test, is of 425 ± 4 days, slightly higher than the value previously accepted (417 days), but concordant with the results obtained by Pelt et al.

We calculate by means of the Press-Schechter formalism the density profile developed by dark-matter halos during accretion, i.e., the continuous aggregation of small clumps. We find that the shape of the predicted profile is similar to that shown by halos in high-resolution cosmological simulations. Furthermore, the mass-concentration relation is correctly reproduced at any redshift in all the hierarchical cosmologies analyzed, except for very large halo masses. The role of major mergers, which can cause the rearrangement of the halo structure through violent relaxation, is also investigated. We show that, as a result of the boundary conditions imposed by the matter continuously infalling into the halo during the violent relaxation process, the shape of the density profile emerging from major mergers is essentially identical to the shape the halo would have developed through pure accretion. This result explains why, according to high-resolution cosmological simulations, relaxed halos of a given mass have the same density profile regardless of whether they have had a recent merger or not, and why both spherical infall and hierarchical assembly lead to very similar density profiles. Finally, we demonstrate that the density profile of relaxed halos is not affected either by the capture of clumps of intermediate mass.Comment: 14 pages, 8 figures, accepted for publication in ApJ. Minor changes, matches the published version. To appear in the ApJ August 10, 2003 issu