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.     

The (sub)Structure of the Halo

The fossil record of the Milky Way indicates an evolution including periodic accretions of smaller galaxies and clusters, consistent with hierarchical models of galaxy formation. I discuss three observational programs that demonstrate that the phase space distribution of stars, clusters and dwarf galaxies in the Galactic halo contains degrees of substructure left by the débris of tidally disrupted stellar systems. This revised version was published online in July 2006 with corrections to the Cover Date.     

TheHI content of the Eridanus group of galaxies

The HI content of galaxies in the Eridanus group is studied using the GMRT observations and the HIPASS data. A significant HI deficiency up to a factor of 2–3 is observed in galaxies in the high galaxy density regions. The HI deficiency in galaxies is observed to be directly correlated to the local projected galaxy density, and inversely correlated to the line-of-sight radial velocity. Furthermore, galaxies with larger optical diameters are predominantly in the lower galaxy density regions. It is suggested that the HI deficiency in Eridanus is due to tidal interactions. In some galaxies, evidences of tidal interactions are seen. An important implication is that significant evolution of galaxies can take place in the group environment. In the hierarchical way of formation of clusters via mergers of groups, a fraction of the observed HI deficiency in clusters could have originated in groups. The co-existence of S0s and severely HI deficient galaxies in the Eridanus group suggests that tidal interaction is likely to be an effective mechanism for transforming spirals to S0s.     

The evolution of the galaxy red sequence in simulated clusters and groups

N -body/hydrodynamical simulations of the formation and evolution of galaxy groups and clusters in a Λ cold dark matter (ΛCDM) cosmology are used in order to follow the building-up of the colour–magnitude relation in two clusters and in 12 groups. We have found that galaxies, starting from the more massive, move to the red sequence (RS) as they get aged over times and eventually set upon a 'dead sequence' (DS) once they have stopped their bulk star formation activity. Fainter galaxies keep having significant star formation out to very recent epochs and lie broader around the RS. Environment plays a role as galaxies in groups and cluster outskirts hold star formation activity longer than the central cluster regions. However, galaxies experiencing infall from the outskirts to the central parts keep star formation on until they settle on to the DS of the core galaxies. Merging contributes to mass assembly until z ∼ 1, after which major events only involve the brightest cluster galaxies.
The emerging scenario is that the evolution of the colour–magnitude properties of galaxies within the hierarchical framework is mainly driven by star formation activity during dark matter haloes assembly. Galaxies progressively quenching their star formation settle to a very sharp 'red and dead' sequence, which turns out to be universal, its slope and scatter being almost independent of the redshift (since at least z ∼ 1.5) and environment.
Differently from the DS, the operatively defined RS evolves more evidently with z , the epoch when it changes its slope being closely corresponding to that at which the passive galaxies population takes over the star-forming one: this goes from z ≃ 1 in clusters down to 0.4 in normal groups.     

The role of E+A and post-starburst galaxies – I. Models and model results

Different compositions of galaxy types in the field in comparison to galaxy clusters as described by the morphology–density relation in the local universe are interpreted as a result of transformation processes from late- to early-type galaxies. This interpretation is supported by the Butcher–Oemler effect. We investigate E+A galaxies as an intermediate state between late-type galaxies in low-density environments and early-type galaxies in high-density environment to constrain the possible transformation processes. For this purpose, we model a grid of post-starburst galaxies by inducing a burst and/or a halting of star formation on the normal evolution of spiral galaxies with our galaxy evolution code galev . From our models, we find that the common E+A criteria exclude a significant number of post-starburst galaxies, and propose that comparing their spectral energy distributions leads to a more sufficient method to investigate post-starburst galaxies. We predict that a higher number of E+A galaxies in the early universe cannot be ascribed solely to a higher number of starburst, but is a result of a lower metallicity and a higher burst strength due to more gas content of the galaxies in the early universe. We find that even galaxies with a normal evolution without a starburst have an Hδ-strong phase at early galaxy ages.     

Galaxy evolution in clusters

The galaxy populations in present-day clusters are distinctly different from those of the field, indicating that environment plays a strong role in galaxy evolution. This review discusses some of the recent observations of moderate to high redshift clusters. A consistent picture of galaxy evolution in clusters appears to be emerging, which includes a population of galaxies which formed early in the cluster history, as well as field galaxies which have had their star formation truncated upon falling into the cluster potential. Galaxy interactions probably play an important role in exhausting star formation in some of these galaxies. However, there is significant variation in the populations of different cluster samples, with substantial evidence that some galaxies have their star formation terminated more gradually. This suggests that different mechanisms may dominate in different clusters, perhaps because of the recent merging history of the clusters. We also present a recent analysis of population gradients in clusters which suggests that the observed evolution in cluster populations is consistent with a scenario where changing infall rates drive the fraction of star forming galaxies in clusters, rather than a changing physical mechanism within the cluster. Thus, galaxy populations may provide a fundamental measure of the growth of large scale structure. This revised version was published online in July 2006 with corrections to the Cover Date.     

The inner jet of radio galaxy NGC 315 as observed with Chandra and the Very Large Array

We use semi-analytic techniques to study the formation and evolution of brightest cluster galaxies (BCGs). We show the extreme hierarchical nature of these objects and discuss the limitations of simple ways to capture their evolution. In a model where cooling flows are suppressed at late times by active galactic nucleus (AGN) activity, the stars of BCGs are formed very early (50 per cent at z ∼ 5, 80 per cent at z ∼ 3) and in many small galaxies. The high star formation rates in these high- z progenitors are fuelled by rapid cooling, not by merger-triggered starbursts. We find that model BCGs assemble surprisingly late: half their final mass is typically locked up in a single galaxy after   z ∼ 0.5  . Because most of the galaxies accreted on to BCGs have little gas content and red colours, late mergers do not change the apparent age of BCGs. It is this accumulation of a large number of old stellar populations – driven mainly by the merging history of the dark matter halo itself – that yields the observed homogeneity of BCG properties. In the second part of the paper, we discuss the evolution of BCGs to high redshifts, from both observational and theoretical viewpoints. We show that our model BCGs are in qualitative agreement with high- z observations. We discuss the hierarchical link between high- z BCGs and their local counterparts. We show that high- z BCGs belong to the same population as the massive end of local BCG progenitors, although they are not in general the same galaxies. Similarly, high- z BCGs end up as massive galaxies in the local Universe, although only a fraction of them are actually BCGs of massive clusters.     

Structure of the clusters of galaxies a 999, a 1016, and a 1142

The hierarchical structure of the clusters of galaxies A 999, A 1016, and A 1142, which are part of one supercluster, is investigated. The HTree method is used to determine that A 999 and A 1016 are a single, dynamically bound system consisting of two “cores” and a common field of galaxies. The composition of the “cores” includes almost all the E and S0 galaxies, the luminosity function of which is very nearly Gaussian. It is also established that the cluster A 1142 has a nonuniform structure, both in density distribution and in radial velocity distribution. The location of the maxima in density and in the radial velocity distribution correlates with the galaxies that are sources of radio and x-ray emission. Translated from Astrofizika, Vol. 43, No. 1, pp. 45-54, January–March, 2000.     

A unified model for the evolution of galaxies and quasars

We incorporate a simple scheme for the growth of supermassive black holes into semi-analytic models that follow the formation and evolution of galaxies in a cold dark matter-dominated Universe. We assume that supermassive black holes are formed and fuelled during major mergers. If two galaxies of comparable mass merge, their central black holes coalesce and a few per cent of the gas in the merger remnant is accreted by the new black hole over a time-scale of a few times 10⁷ yr. With these simple assumptions, our model not only fits many aspects of the observed evolution of galaxies, but also reproduces quantitatively the observed relation between bulge luminosity and black hole mass in nearby galaxies, the strong evolution of the quasar population with redshift, and the relation between the luminosities of nearby quasars and those of their host galaxies. The strong decline in the number density of quasars from z ∼2 to z =0 is a result of the combination of three effects: (i) a decrease in the merging rate; (ii) a decrease in the amount of cold gas available to fuel black holes, and (iii) an increase in the time-scale for gas accretion. The predicted decline in the total content of cold gas in galaxies is consistent with that inferred from observations of damped Ly α systems. Our results strongly suggest that the evolution of supermassive black holes, quasars and starburst galaxies is inextricably linked to the hierarchical build-up of galaxies.     

The K-band Hubble diagram for the brightest cluster galaxies: a test of hierarchical galaxy formation models

We analyse the K -band Hubble diagram for a sample of brightest cluster galaxies (BCGs) in the redshift range 0< z <1. In good agreement with earlier studies, we confirm that the scatter in the absolute magnitudes of the galaxies is small (0.3 mag). The BCGs exhibit very little luminosity evolution in this redshift range: if q ₀=0.0, we detect no luminosity evolution; for q ₀=0.5, we measure a small negative evolution (i.e., BCGs were about 0.5 mag fainter at z =1 than today). If the mass in stars of these galaxies had remained constant over this period of time, substantial positive luminosity evolution would be expected: BCGs should have been brighter in the past, since their stars were younger. A likely explanation for the observed zero or negative evolution is that the stellar mass of the BCGs has been assembled over time through merging and accretion, as expected in hierarchical models of galaxy formation. The colour evolution of the BCGs is consistent with that of an old stellar population ( z _for>2) that is evolving passively. We can thus use evolutionary population synthesis models to estimate the rate of growth in stellar mass for these systems. We find that the stellar mass in a typical BCG has grown by a factor ≃2 since z ≃1 if q ₀=0.0, or by factor ≃4 if q ₀=0.5. These results are in good agreement with the predictions of semi-analytic models of galaxy formation and evolution set in the context of a hierarchical scenario for structure formation. The models predict a scatter in the luminosities of the BCGs that is somewhat larger than the observed one, but that depends on the criterion used to select the model clusters.     

星系团的分层效应

对星系团各类分层效应的有关问题做了概要的评述,包括成员星系在位置空间和(或)速度空间中的形态分层、光度(质量)分层和元素丰度分层的表现形式和探测途径,分层效应可能的形成机制及其对星系和星系团的结构和演化的影响。     

Ram-pressure histories of cluster galaxies

Ram-pressure stripping can remove significant amounts of gas from galaxies that orbit in clusters and massive groups, and thus has a large impact on the evolution of cluster galaxies. In this paper, we reconstruct the present-day distribution of ram pressure and the ram-pressure histories of cluster galaxies. To this aim, we combine the Millennium Simulation and an associated semi-analytic model of galaxy evolution with analytic models for the gas distribution in clusters. We find that about one quarter of galaxies in massive clusters are subject to strong ram pressures that are likely to cause an expedient loss of all gas. Strong ram pressures occur predominantly in the inner core of the cluster, where both the gas density and the galaxy velocity are higher. Since their accretion on to a massive system, more than 64 per cent of galaxies that reside in a cluster today have experienced strong ram pressures of  >10⁻¹¹ dyn cm⁻²  which most likely led to a substantial loss of the gas.     

A VLA 20 and 90 centimetre radio survey of distant A-bell clusters with central cD galaxies

A radio survey, using the Very Large Array at 20 and 90 cm λ has been carried out in the direction of 46 distant Abell clusters (0.1 ≲ z ≲ 0.3) dominated by a cD galaxy (clusters classified to be Bautz-Morgan I type). A radio source coincident with the cD galaxy was detected in 16 clusters. We find that the radio luminosity function of the cD galaxies at 20cm λ, and below the luminosityP _1.4ghz ≲ 10^24.5 W Hz^-1, is similar to that of brightest ellipticals in less clustered environments. Above this luminosity, the cDs seem to have a higher probability of becoming radio sources. The effect of optical brightness on radio emission is shown to be the same for the two classes. No significantly large population of very-steep-spectrum sources with spectral index α >1.2 (flux density ∝ frequency^-α) was found to be associated with cD galaxies. A significant negative correlation is found between the radio luminosity of the cD galaxy and the cooling-time of the intra cluster medium near the galaxy. We also present evidence that the probability of radio emission from first-ranked galaxies is dependent upon their location relative to the geometrical centres of clusters and thus related to the morphological class and the evolutionary state of the clusters. We argue that both these effects are primarily caused by the dynamical evolution of these distant clusters of galaxies.     

Peculiar motions of galaxy clusters in the regions of the Corona Borealis,Bootes, Z 5029/A 1424, A 1190, A 1750/A 1809 superclusters of galaxies

We present results of the study of peculiar motions of 57 clusters and groups of galaxies in the regions of the Corona Borealis (CrB), Bootes (Boo), Z5029/A1424, A1190, A1750/A1809 superclusters of galaxies and the galaxy clusters located beyond massive structures (0.05 < z < 0.10). Using the SDSS (Data Release 8) data, a sample of early-type galaxies was compiled in the systems under study, their fundamental planes were built, and relative distances and peculiar velocities were determined. Within the galaxy superclusters, significant peculiar motions along the line of sight are observed with rms deviations of 652 ± 50 kms^?1—in CrB, 757 ± 70 kms^?1—in Boo. In the most massive A2065 cluster in the CrB supercluster, no peculiar velocity was found. Peculiar motions of the other galaxy clusters can be caused by their gravitational interaction both with A2065 and with the A2142 supercluster. It has been found that there are two superclusters projected onto each other in the region of the Bootes supercluster with a radial velocity difference of about 4000 kms^?1. In the Z 5029/A1424 supercluster near the rich Z5029 cluster, the most considerable peculiar motions with a rms deviations of 1366 ± 170 kms^?1 are observed. The rms deviations of peculiar velocities of 20 clusters that do not belong to large-scale structures is equal to 0 ± 20 kms^?1. The whole sample of the clusters under study has the mean peculiar velocity equal to 83 ± 130 kms^?1 relative to the cosmic microwave background.     

星系团的光学研究(Ⅰ):星系团表、大尺度分布、团的一般性质

星系团研究涉及面极广，此处仅简介光学研究的若干方面。全文分为两篇，本篇中介绍星系团表、星系团的大尺度分布、星系团的一般性质；第二篇中将介绍团星系的演化、环境与星系形态，并作简要的展望。     

Galaxy Evolution in Groups and Clusters: Workshop Summary

The study of galaxy clusters and groups covers a large range of physical scales, from cosmology to large scale structure, to individual galaxies. This workshop on the evolution of galaxies in clusters and groups provided a rich assembly of subjects. Brief summaries are given in some of these topics, including: the applications of clusters to cosmology, the evolution of cluster galaxies in the context of the Butcher-Oemler effect, the galaxy populations in clusters – from dwarfs to the cD galaxy, the interplay between cluster/group environment and the evolution of clusters, the infalling cluster members and the galaxy luminosity function. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of integrated photometric parameters of extragalactic star formation complexes and open star clusters in the Milky Way

A comparative analysis of the evolution of integrated photometric parameters for young star formation complexes in spiral and irregular galaxies and open star clusters in our Galaxy is performed. Extragalactic star formation complexes seen as giant H II regions and open star clusters in the Milky Way are shown to represent a single evolutionary sequence of objects at different stages of their evolution when the extinction is properly taken into account.     

The role of diffuse matter in galactic coronas

An upper limit to the gas content in the coronas of giant galaxiesM _gas<0.04M _cor has been calculated on the basis of observational data on the emission of clusters of galaxies in different wavelength ranges. An analysis of companion galaxies has revealed a correlation of their morphological types with the distances to central giant galaxies. A gasodynamical interpretation of this regularity is suggested and a possible picture of the evolution of the galactic coronas is presented.