Gas in Groups and Clusters of Galaxies

Groups and clusters contain a large fraction of hot gas which emitsX-ray radiation. This gas yields information on the dynamical stateand on the total mass of these systems. X-ray spectra show that heavyelements are present in the gas. As these metals must have beenproduced in the cluster/group galaxies and later transported into thegas, the metallicity is a good tracer for the transport processes. Severalpossible processes, that transport gas from the small potential wellsof the galaxies into the clusters and groups, are discussed.     

Mass of Galaxies in Pairs

We have compared the frequency distribution of the dynamical observedquantity log (V _z ² r _p), for a sample of 46 pairs of elliptical galaxies, to the distribution of this quantity obtained from numerical simulations of pairs of galaxies. From such an analysis, where we have considered the structure of the galaxies and its influence in the orbital evolution of the system, we have obtained the characteristic mass and the mass-luminosity ratio for the sample. Our results show that the hypothesis of point-mass in elliptical orbits is, for this sample, an approximation as good as the model that takes into account the structure of the galaxies. The statistical method used here gives an estimate of a more reliable mass, it minimizes the contamination of spurious pairs and it considers adequately the contribution of the physical pairs. We have obtained a characteristic mass to the 46 elliptical pairs of 1.68 × 10¹² ± 7.01 ×10¹¹ M_⊙ with M/L = 17.6 ± 7.3 (H ₀ = 60 km s^-1Mpc^-1). This revised version was published online in July 2006 with corrections to the Cover Date.     

Starburst Galaxies in Clusters

The nature of the starburst phenomenon in galaxies is investigated using a narrow-band colour system designed to study colour evolution in distant clusters. Work on zero redshift, luminous far-IR galaxies, calibrated by starburst models, demonstrates the usefulness of this colour system in isolating starburst from normal star-forming colours, and also demonstrates a strong correlation with far-IR colours despite reddening effects. The same colour system applied to distant clusters finds that a majority of the faint blue cluster population are starburst dwarf galaxies, probably the progenitors of the current population of dwarf ellipticals in nearby clusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Orientation of Galaxies in Clusters

A detailed analysis of the geometry of galaxies in clusters has been undertaken in both two and three dimensional space. The procedure was applied to the three Abell clusters A1644, A548 E and A548 W. No significant alignment trend of galaxies in clusters confirmed in three dimensional space was found. This result is consistent with the mixed dark matter model MDM of galaxy formation. The result is also consistent with Peebles suggestion that protogalaxies acquired angular momentum from tidal torques exerted by their neighbours in the early universe. The amount of angular momentum predicted by this mechanism could be described by a single dimensionless papameter λ≈ 0.8. N-body experiments have shown that λ has approximately normal distribution with mean value about 0.07 and with a standard deviation of about 0.03. From the analyic fit to flat rotation curves it is found that tidal torque can provide the observed angular momentum if the mean collapse factor is about 20 and the mean halo-to-disk mass ratio is of order 10. The MDM model does not predict any systematic relation between the tidal torques among the halos of galaxies and the large scale structures such as the clusters, superclusters of galaxies and giant voids in between. Further work in this area is still required. This revised version was published online in July 2006 with corrections to the Cover Date.     

Distribution of Galaxies in 10 Clusters

The number-density distributions for further 10 clusters of galaxies were derived by counting galaxies on the red Palomar Sky Survey prints. For 9 clusters the radial number-density distribution and the radial cumulative galaxy distribution were calculated.     

Cosmic Rays and Clusters of Galaxies

The presence of a significant population of relic relativistic electrons - created at an early epoch of the Universe - has been invoked to explain the diffuse EUV emission excess observed in a number of galaxy clusters. While the postulated inverse Compton scattering of the 3° K background radiation by cosmic ray electrons might indeed be utilized as an important diagnostic tool for the physical nature of the intracluster cosmic rays, it is shown here that continuous generation plus reacceleration would be necessary if the conditions on the observed energy spectral distribution and energy supply rate are to be met in the case of clusters with large radio halos. This revised version was published online in July 2006 with corrections to the Cover Date.     

Non-thermal Emission in Clusters of Galaxies

The most evident signature for the presence of non-thermal elements (magnetic fields and relativistic electrons) in clusters of galaxies is given by the presence of diffuse radio regionspresent so far in a limited number of objects. We review the recent discoveries of new spectral components by EUVE and BeppoSAX in the spectra of some clusters of galaxies that allow to better determine the non-thermalquantities by relating radio and X-ray data. In collaboration with: M. Orlandini, G. Brunetti, L. Feretti, G. Giovannini, P. Grandi, & G. Setti.     

Two-fluid Dynamics in Clusters of Galaxies

We develop a theoretical formulation for the large-scale dynamics of galaxy clusters involving two spherical ‘isothermal fluids‘ coupled by their mutual gravity and derive asymptotic similarity solutions analytically. One of the fluids roughly approximates the massive dark matter halo, while the other describes the hot gas, the relatively small mass contribution from the galaxies being subsumed in the gas. By properly choosing the self-similar variables, it is possible to consistently transform the set of time-dependent two-fluid equations of spherical symmetry with serf-gravity into a set of coupled nonlinear ordinary differential equations (ODEs). We focus on the analytical analysis and discuss applications of the solutions to galaxy clusters.     

Turbulence and Magnetic Fields in Clusters of Galaxies

We consider turbulence generated by galaxies moving transonically through the intracluster gas. We show that neither the gravitational drag nor the gas stripping from the galaxies are able, by themselves, to generate turbulence at a level required to feed the dynamo in the intracluster gas. Some implications for cluster radio halos are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of Starburst Galaxies in Merging Clusters

A significant fraction of clusters of galaxies are observed to have substructure, which implies that merging between clusters and subclusters is a rather common physical process in cluster formation. It still remains unclear how cluster merging affects the evolution of cluster member galaxies. We report the results of numerical simulations that show the dynamical evolution of a gas-rich, late-type spiral in a merger between a small group of galaxies and a cluster. The simulations demonstrate that the time-dependent tidal gravitational field during merging excites non-axisymmetric structure of the galaxy, subsequently drives efficient transfer of gas to the central region and finally triggers a secondary starburst. This result provides a close physical relationship between the emergence of starburst galaxies and the formation of substructure in clusters. We accordingly interpret post-starburst galaxies located near substructure of the Coma Cluster as one observational example indicating the global tidal effects of group–cluster merging. Our numerical results further suggest a causal link between the observed excess of blue galaxies in distant clusters and the cluster virialization process through hierarchical merging of subclusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

宇宙常数和星系团

在Ｘ射线观测提高了星系团质量和半径测量精度的新形势下，本文讨论了宇宙常数不等于零的平坦宇宙冷暗物质结构形成模型，利用球对称扰动区在宇宙常数不为零时的动力学方程的解，估计了星系团形成红移与宇宙常数是否为零的关系，计算了星系团质量函数随红移的演化．计算表明，红移为零时星系团的数密度基本上由谱参数Γ决定．若假定星系团质量只有２０％的不确定性，可限定Γ的适用范围约为０．１５－０．３２．高红移星系团的数密度观测不仅有可能对宇宙常数是否为零作出鉴别，而且当精度够高时还能对宇宙物质密度的大小作出限制     

Photometric Properties of Clusters of Galaxies

We present CCD photometry of 16 Abell clusters and one cluster candidate found in POSS-II field 861. The images were taken at the 0.9 m Telescope at Cerro Tololo, in the g, r and i filters of the Gunn–Thuan system. We tested the idea proposed by Garilli et al. (1996) that there is a population of unusually red galaxies which could be associated with either the field or clusters. Garilli et al. (1996) suggest that these galaxies have anomalously red colours, but we find that these objects are all near the limiting magnitude of the images (20^m<r<22^m) and have colours that are consistent with those expected for stars or field galaxies at z∼0.7. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the Mass Discrepancy in Clusters of Galaxies

The change of the slope in the luminosity function of the members of the Coma cluster near M ≈ — 21 is interpreted as an evolutionary effect. Originally there should have existed many luminous objects now being evolved to invisible masses in form of “black holes”. If the faint part of the luminosity function is extrapolated beyond M ≈ — 21 the observed discrepancy in the mass of the cluster determined by the virial theorem and by summing up the individual masses of the galaxies vanishes. The comparison of the hypothetical frequency of the black holes with the estimated numbers of the strong extragalactic radio sources encourages us to the assumption of a relation between the two groups of objects.     

Quasars in Regions of Rich Clusters of Galaxies

Certain characteristics of quasars observed in the regions of three rich clusters of galaxies are investigated. It is shown that the luminosity functions constructed for samples of quasars from the regions of the Virgo and Fornax clusters are similar to each other and show a very sharp maximum at a stellar magnitude of 18^m.0-18^m.3. The maximum of the luminosity function for quasars from the region of the Coma cluster (A 1656) is shifted toward fainter magnitudes and lies at 21^m. This effect is explained by the affiliation of the majority of the quasars to the respective clusters. It is shown that the absolute stellar magnitude of these quasars is –13^m, which corresponds to the luminosity of dwarf galaxies of low luminosity. It is suggested that the local quasars are formed in two ways: by ejection from galactic nuclei and in quasar associations without a parent galaxy, directly from protostellar matter.     

Investigating Clusters of Galaxies with Planck and Herschel

With the Planck and Herschel satellite missions of the European Space Agency, the far-infrared and submillimeter window will offer new investigation tools toward clusters of galaxies in the distant Universe. These are the Sunyaev Zel'dovich (SZ) effect of the cosmic microwave background and the thermal emission of dust grains. The power of the SZ effect is such that Planckis expected to discover thousands of new clusters at redshifts larger than 0.2, where only a few tens are known today. The dust can be present at large scale in the intracluster medium, and we show that even at very low abundances it is able to be a major cooling agent for the whole cluster. However the dominating dust emission will be that of the background infrared star forming galaxies. In all cases, the data processing of space borne sensitive submillimeter observations of clusters of galaxies such as the one that Planck and Herschel will provide, will require a very carefull combined analysis of the SZ effect and dust thermal emission.     

The Number-diameter Relation of Galaxies

Contrary to the number-magnitude relation, the number-angular diameter relation is influenced by the luminosity function. The number-diameter counts by DODD et al. (1975) are in agreement with the normal luminosity function and a homogeneous space distribution without a local density excess. Almost all the faint small (<18 arcsec) galaxies may be first-ranked cluster members.