Structure and Dynamics of Clusters and the HI Content of Spiral Galaxies, The

Abstract

This thesis is focused on the implication of de dynamics of clusters on the HI content of galaxies. Target of this study has been the Virgo cluster region.<br/><br/>We have first inferred the three-dimensional galaxy distribution of the Virgo cluster using Tully-Fisher distance estimates from eight published datasets. The spiral distribution is found to be very elongated along the line of sight. This filamentary structure is also reflected by the HI-deficient objects. We found a central enhancement in the HI distribution, arising from galaxies that belong to the Virgo cluster proper. However, significant gas deficiencies were also detected outside the main body of the cluster in a probable group of galaxies at distances <i>circa</i> 25 to 30 Mpc, as well as in various foreground galaxies. <br/><br/>The presence on the Virgo cluster outskirts of spiral galaxies with gas deficiencies as strong as those of the inner galaxies likely stripped by the intracluster medium (ICM) has led us to explore the possibility that some of these peripheral objects were not newcomers. A dynamical model for the collapse and rebound of spherical shells was developed. According to our analysis, it is not unfeasible that galaxies far from the cluster went through its core a few Gyr ago. In order to further investigate this possibility, we have estimated the maximum radius out to which galaxies can bounce out of a virialized system, using both cosmological <i>N</i>-body simulations and analytical arguments. We have shown that material that falls into a cosmological structure cannot bounce out beyond <i>circa</i>2.5 virial radii. This would explain the presence of stripped galaxies up to <i>circa</i>2 virial radii, which is more than twice the extension of the ICM revealed by the X-ray maps. After estimating the virial radius and mass of the Virgo cluster from X-ray observations, the peripheral HI-deficient galaxies appear to lie significantly farther away from the cluster centre. Therefore, it does not seen plausible that these galaxies have crossed the cluster core if their distance estimates are accurate. <br/><br/>The velocity field drawn by the Virgo cluster has been further studied by comparing the positions of both spiral and elliptical galaxies in velocity-distance plots with simulated diagrams obtained from cosmological <i>N</i>-body simulations. We have found that <i>circa</i>20% relative Tully-Fisher distance error is consistent with the great majority of the galaxies. We have shown, moreover, that distance errors may lead to an incorrect fitting of infall models that can generate significant errors in the distance and, specially, in the mass estimates of clusters. <br/><br/>In addition, we have investigated in detail the feasibility of four possibilities for the origin of the dearth of neutral gas in the outlying HI-deficient Virgo spirals: 1) large relative distance errors so these galaxies have passed through its core and seen their gas removed by ram pressure stripping; 2) tidal interactions with other galaxies; 3) recent mergers with smaller galaxies; and 4) an erroneous determination of their gas content (e.g. S0/a's misclassified as Sa's).<br/><br/>Finally, the cosmological <i>N</i>-body simulations of dark matter halos have also been used to study the effects of non-sphericity, substructure and streaming motions on the structure and internal kinematics of clusters recovered from the Jeans analysis. We found that the virial mass, concentration parameter and velocity anisotropy of the halos can be reproduced satisfactory independently of the halo shape, although the viral mass tends to be underestimated, the concentration parameter overestimated, while the recovered anisotropy is typically more tangential that the true one. The mass, concentration and velocity anisotropy of halos are recovered with better precision when their mean velocity profiles are near zero.