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.

We use the Galaxy And Mass Assembly (GAMA) survey to measure the local Universe mass dependent merger fraction and merger rate using galaxy pairs and the CAS structural method, which identifies highly asymmetric merger candidate galaxies. Our goals are to determine which types of mergers produce highly asymmetrical galaxies, and to provide a new measurement of the local galaxy major merger rate. We examine galaxy pairs at stellar mass limits down to M * = 10 8 M ⊙ with mass ratios of <100:1 and line of sight velocity differences of ∆V < 500 km s −1 . We find a significant increase in mean asymmetries for projected separations less than the sum of the individual galaxy's Petrosian 90 radii. For systems in major merger pairs with mass ratios of <4:1 both galaxies in the pair show a strong increase in asymmetry, while in minor merger systems (with mass ratios of >4:1) the lower mass companion becomes highly asymmetric, while the larger galaxy is much less affected. The fraction of highly asymmetric paired galaxies which have a major merger companion is highest for the most massive galaxies and drops progressively with decreasing mass. We calculate that the mass dependent major merger fraction is fairly constant at ∼ 1.3 − 2% between 10 9.5 < M * < 10 11.5 M ⊙ , and increases to ∼ 4% at lower masses. When the observability time scales are taken into consideration, the major merger rate is found to approximately triple over the mass range we consider. The total co-moving volume major merger rate over the range 10 8.0 < M * < 10 11.5 M ⊙ is (1.2 ± 0.5) × 10 −3 h 3 70 Mpc −3 Gyr −1 . Key words: galaxies: general -galaxies: evolution -galaxies: interactions -galaxies: statistics ⋆ E-mail: kcasteels@gmail.com (KRVC) 2 K. R. V. Casteels et al.

High-resolution N-body simulations of hierarchical clustering in a wide variety of cosmogonies show that the density profiles of dark matter halos are universal, with low mass halos being denser than their more massive counterparts. This mass-density correlation is interpreted as reflecting the earlier typical formation time of less massive objects. We investigate this hypothesis in the light of formation times defined as the epoch at which halos experience their last major merger. Such halo formation times are calculated by means of a modification of the extended Press & Schechter formalism which includes a phenomenological frontier, ∆ m , between tiny and notable relative mass captures leading to the distinction between merger and accretion. For ∆ m ∼ 0.6, we confirm that the characteristic density of halos is essentially proportional to the mean density of the universe at their time of formation. Yet, proportionality with respect to the critical density yields slightly better results for open universes. In addition, we find that the scale radius of halos is also essentially proportional to their virial radius at the time of formation.We show that these two relations are consistent with the following simple scenario. Violent relaxation caused by mergers rearranges the structure of halos leading to the same density profile with universal values of the dimensionless characteristic density and scale radius. Between mergers, halos grow gradually through the accretion of surrounding layers by keeping their central parts steady and expanding their virial radius as the critical density of the universe diminishes.

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

We show that the two basic assumptions of the model recently proposed by Manrique and coworkers for the universal density profile of cold dark matter (CDM) halos, namely that these objects grow inside out in periods of smooth accretion and that their mass profile and its radial derivatives are all continuous functions, are both well understood in terms of the very nature of CDM. Those two assumptions allow one to derive the typical density profile of halos of a given mass from the accretion rate characteristic of the particular cosmology. This profile was shown by Manrique and coworkers to recover the results of numerical simulations. In the present paper, we investigate its behavior beyond the ranges covered by present-day N -body simulations. We find that the central asymptotic logarithmic slope depends crucially on the shape of the power spectrum of density perturbations: it is equal to a constant negative value for power-law spectra and has central cores for the standard CDM power spectrum. The predicted density profile in the CDM case is well fitted by the 3D Sérsic profile over at least 10 decades in halo mass. The values of the Sérsic parameters depend on the mass of the structure considered. A practical procedure is provided that allows one to infer the typical values of the best NFW or Sérsic fitting law parameters for halos of any mass and redshift in any given standard CDM cosmology.

This is the rst paper of a series of two devoted to develop a practical method to describe the growth history of bound virialized objects in the gravitational instability scenario without resorting to N-body simulations. Here we present the basic tool of this method, \the con uent system formalism", which allows us to follow the ltering evolution of peaks in a random Gaussian eld of density uctuations. This is applied to derive the theoretical mass function of objects within the peak model framework. Along the process followed for the derivation of this function, we prove that the Gaussian window is the only one consistent with the peak model ansatz. We also give a well justi ed derivation of the density of peaks with density contrast upcrossing a given threshold in in nitesimal ranges of scale and correct this scale function for the cloud-in-cloud e ect. Finally, we characterize the form of the mass vs. scale and the critical overdensity vs. collapse time relations which are physically consistent with the peak model in an Einstein-de Sitter universe with density eld endowed with di erent power spectra. The result is a fully justi ed semianalytical mass function which is close to the Press & Schechter (1974) one giving good ts to N-body simulations. But the interest of the con uent system formalism is not { 2 { merely formal. It allows us to distinguish between accretion and merger events, which is essential for the detailed modelling of the clustering process experienced by objects.

In two previous papers a semi-analytical model was presented for the hierarchical clustering of halos via gravitational instability from peaks in a random Gaussian field of density fluctuations. This model is better founded than the extended Press-Schechter model, which is known to agree with numerical simulations and to make similar predictions. The specific merger rate, however, shows a significant departure at intermediate captured masses. The origin of this was suspected as being the rather crude approximation used for the density of nested peaks. Here, we seek to verify this suspicion by implementing a more accurate expression for the latter quantity which accounts for the correlation among peaks. We confirm that the inclusion of the peak-peak correlation improves the specific merger rate, while the good behavior of the remaining quantities is preserved.