Evolution of Clusters of Galaxies: Mass Stripping from Galaxies and Growth of Common Halos

We investigated the evolution of clusters of galaxies using self-consistent $N$-body simulations in which each galaxy was modeled by many particles. We carried out simulations for about 20 cases using different initial conditions. In all simulations, clusters were initially in virial equilibrium. We found that more than half of the total mass escaped from individual galaxies within a few crossing times of the cluster, and that a diffuse halo was formed. The growth rate of the common halo depended on the size of individual galaxies only weakly. The stripping of the mass from galaxies was mainly due to the interaction of galaxies, not due to the effect of the tidal field of the cluster potential. The amount of stripped mass was larger for galaxies in the central region than for those in the outer region, since the interactions were more frequent in the central region. As a result, a positive correlation between the distance from the center and the mass of the galaxy developed. The volume-density profile of the common halo is expressed as $\rho\propto r^{-1}$ in the central region. This mass distribution is consistent with the mass distribution in clusters estimated using X-ray observations.Comment: 12 pages with 12 figures; accepted for publication in PAS

Collisional Evolution of Galaxy Clusters and the Growth of Common Halos

We investigated the dynamical evolution of clusters of galaxies in virial equilibrium using Fokker-Planck models and self-consistent N-body models. In particular, we focused on the growth of a common halo, which is a cluster-wide halo formed by matter stripped from galaxies, and the development of a central density cusp. The Fokker-Planck models include the effects of two-body gravitational encounters both between galaxies and between galaxies and common halo particles. The effects of tidal mass stripping from the galaxies due to close galaxy-galaxy encounters and accompanying dissipation of the orbital kinetic energies of the galaxies were also taken into account in the Fokker-Planck models. We find that the results of the Fokker-Planck models are in excellent agreement with those of the N-body models regarding the growth of the common halo mass and the evolution of the cluster density profiles. In the central region of the cluster, a shallow density cusp, approximated by $\rho (r) \propto r^{-\alpha}$ ($\alpha \sim$ 1), develops. This shallow cusp results from the combined effects of two-body relaxation and tidal stripping. The cusp steepness, $\alpha$, weakly depends on the relative importance of the tidal stripping. When the effect of stripping is important, the central velocity dispersion decreases as the central density increases and, consequently, a shallow ($\alpha <2$) cusp is formed. In the limit of no stripping, usual gravothermal core collapse occurs, i.e. the central velocity dispersion increases as the central density increases with a steep ($\alpha >2$) cusp left. We conclude from our consideration of the origin of the cusp demonstrated here that shallow cusps should develop in real galaxy clusters.Comment: revised, 21 pages, 16 figures, to appear in PASJ, 54, No.1 (2002

Structural Evidence for Environment-Driven Transformation of the Blue Galaxies in Local Abell Clusters - A85, A496, and A754

We compare V-band structural properties of cluster and field galaxies, to test whether much of the current cluster membership resulted from the transformation of infalling spirals into red early types. We use 140 galaxies from 3 z<0.6 Abell clusters (A85,A496,A754 - spectroscopic membership) with blue colors, and compare them to 80 field galaxies with similar colors and luminosities. Our previous work (McIntosh et al. 2004, ApJ submitted) shows that the blue galaxies in these local clusters are a recently infalling population that has yet to encounter the dense core. We quantify galaxy internal structure from 2D B/D decompositions using GIM2D. We observe structural differences between blue galaxies in clusters, compared to the field. Most blue cluster members are physically smaller and fainter than their field counterparts. At a matched size and luminosity, recent arrivals are smoother in appearance, yet their total light is as disk-dominated as in normal field spirals. Moreover, 50% of blue cluster galaxies have blue cores or globally blue color profiles, in contrast with field spirals that have typically red color gradients. Without environment dependent evolution outside of cluster cores, we expect blue disk galaxies in clusters and the field to have similar morphologies, sizes, and color gradients. We show conclusively that galaxy properties do reflect the environment in which the galaxy is found. The data show that the transformation of accreted galaxies is not confined to the dense cluster core. The overall properties of bluer cluster galaxies are best explained by environment-driven transformation of accreted field spirals, and our results suggest that the processes that govern color and morphological evolution occur separately. (abridged)Comment: Accepted for publication in ApJ, 23 pages, Latex using emulateapj5.sty and onecolfloat.sty (included), 15 figures, version with full resolution Figs. 2 and 5 at http://www.astro.umass.edu/~dmac/Papers/transf.hires.ps Substantial revisions of Introduction and Discussion; extensive minor changes throughout paper following recommendations by referee; conclusions unchanged except for removal of pre-processing interpretatio

Benefits of group compassion-focused therapy for treatment-resistant depression: A pilot randomized controlled trial

Major depression is one of the most common mental health problems worldwide. More than one-third of patients suffer from treatment-resistant depression (TRD). In this study, we explored the feasibility of group compassion-focused therapy (CFT) for TRD using a randomized controlled trial with two parallel groups. Eighteen participants were randomly allocated to the intervention group (CFT and usual care) and control group (usual care alone) and a participant in each group withdrew. Participants in the intervention group received a 1.5-h session every week for 12 weeks. The effects of the intervention on the participants’ scores were calculated using a linear mixed model. There was a larger reduction in their depressive symptoms and fears of compassion for self and a greater increase in their compassion for self compared to the control group participants. The reliable clinical indices showed that in the CFT (intervention) group, three of nine participants recovered (33%), two improved (22%), two recovered but non-reliably (22%), and the condition of two remained unchanged (22%). These findings indicate adequate feasibility of group CFT for TRD in Japanese clinical settings