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.     

Globular Clusters in Blue Compact Galaxies as Tracers of the Starburst History

Representing single stellar populations, globular clusters (GCs) arerelatively easy to model, thus providing powerful tools for studyingthe evolution of galaxies. This has been demonstrated for the bluecompact galaxy ESO338-IG04. GC systems in galaxies may be fossils ofstarbursts and mergers. Thus studies of GCs in the local universe mayadd to our understanding of the formation and evolution of galaxies andthe distant universe. This revised version was published online in July 2006 with corrections to the Cover Date.     

Merging Massive Star Clusters as Building Blocks of Dwarf Galaxies?

Recent spectroscopic observations of galaxies in the Fornax-Cluster reveal nearly unresolved ‘star-like’ objects with red-shifts appropriate to the Fornax-Cluster. These objects have intrinsic sizes of ≈ 100 pc and absolute B-band magnitudes in the range - 14 < M_B < -11.5 mag and lower limits for the central surface brightness μ_B ≥ 23 mag/arcsec² (Phillipps et al., 2001, Hilker et al., 1999), and so appear to constitute a new population of ultra-compact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar super-clusters (= clusters of star clusters; not to confuse with super stellar clusters (SSC)) by P. Kroupa (1998), which are rich aggregates of young massive star clusters (YMCs) that can form in collisions between gas-rich galaxies. Here we present the evolution of super-clusters in a tidal field. The YMCs merge on a few super-cluster crossing times. Super-clusters that are initially as concentrated and massive as Knot S in the interacting Antennae galaxies (Whitmore et al., 1999) evolve to merger objects that are long-lived and show properties comparable to the newly discovered UCDs. This revised version was published online in September 2006 with corrections to the Cover Date.     

Seyfert 2型星系中的星暴活动

星暴和活动星系核之间的联系是活动星系研究领域最重要、最活跃的研究课题之一。Seyfert星系由于距离较近、数目较多和相对低的核活动,已成为研究星暴和活动星系核之间联系的理想天体。综述了活动星系核中存在星暴的观测证据和Seyflert2型星系核区星暴活动的最新研究结果,着重讨论了存在两类Seyflert2型星系(一类是被遮挡的Seyfert1型星系,另一类是“真正”的Seyfert2型星系即不存在宽线区的Seyfert2型星系)的可能性．通过比较具有和不具有偏振宽线的Seyflert2型星系在红外、射电、光学和X射线光谱性质的差别,发现具有偏振宽线的Seyfert2型星系在本质上和Seyflert1型星系是同一类天体,差别只在于观测者视线方向的不同;而不具有偏振宽线的Seyfert2型星系是一些星系核活动较弱而星暴活动占主导的天体,这些星系从射电、红外、光学到硬X射线,都具有和星暴星系相似的性质。由于这些星系中核的吸积率将比Seyflert1型星系低近两个量级,因而它们很有可能是一些没有宽发射线区的Seyfert2型星系,即所谓的“真正”Seyflert2型星系。     

Searching for X-Ray Luminous Starburst Galaxies

The existence or otherwise of X-ray luminous star-forming galaxies has been an open question since the era of the Einstein satellite. Various authors have claimed the discovery of X-ray luminous star-forming galaxies but in many cases more careful spectroscopic studies of these objects have shown that many of them are in fact obscured AGN. In order to investigate the possibility that such a class of galaxies do exist, we have carried out a cross-correlation between optical and IRAS samples of galaxies which are known to contain large numbers of star-forming galaxies and catalogues of sources detected in X-ray surveys. The selection criteria for the optical follow-up observations was based on their X-ray and infrared (IRAS) colours and their X-ray luminosities. We note that this sample is by no means complete or uniformly selected and hence cannot be used for statistical studies; nevertheless, confirmation of the existence of such a class of objects would be an important step and would require us to understand the physical process responsible for such powerful X-ray emission. We have initiated an optical spectroscopic survey in order to obtain accurate spectroscopic classifications for all the objects which are claimed to be starburst galaxies. Here we present preliminary results from this survey. We have discovered a number of starburst galaxies with X-ray luminosities above ～ 10⁴¹ erg s^-1 (for H ₀=50 km s^-1 Mpc^-1).We investigate possible origins for the X-ray emission in individual cases.     

暗蓝星系计数与星暴矮星系族

本文采用演化星族综合方法，讨论了在低红移因短时间星暴过程而变亮的矮星系族。指出，它们的存在能够与目前暗星系计数和红移巡天结果相容，作为一个附加的星系族，它们较易演化为低面光度的星系而在邻域中探测不到，因此是解释暗星系计数和红移巡天结果的适当候选天体，值得对它们作进一步的研究。     

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.     

星暴星系：研究进展和热点问题

星暴星系是一类内部正在发生极为剧烈恒星形成的天体。介绍了星暴星系的研究历史及星暴星系的定义。综述了从射电波段到Ｘ射线波段星暴星系的光度光谱观察特征及其研究的最新进展。列出了星暴星系研究中存在的一些热点问题,包括星暴的触发机制,星暴时标,星族组分,恒星形成率,内红化以及星暴星系与活动星系核之间的关系等。最后,简述了可能有助于解决这些问题的观测手和理论方法。     

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.     

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.     

The Universe Evolution as Possible Mechanism of Formation of Galaxies and Their Clusters

The Kepler problem is studied in a space with the Friedmann-Lemaitre-Robertson-Walker metrics of the expanding universe. Cosmic evolution leads to decreasing energy of particles, causing free particles to be captured in bound states, so that the evolution of the universe can be treated as a possible mechanism of the formation of galaxies and clusters of galaxies. The cosmological model is considered where the evolution of the universe plays the role usually inscribed to cold dark matter.     

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.     

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.     

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.     

星系多次并合的解析模型

本文建立了星系集团多次并合所满足的一个方程组,并用数值求解得出了多次并合过程的一些物理性质．模型计算表明,那些初始较致密的星系集团,现在应正处于多次并合的过程之中,而它的数面密度却不因并合和半径收缩而增大．     

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.