Central Regions Of Clusters Research Articles

A Preliminary Study of the Orion Nebula Cluster Structure and Dynamics

We use optical and near-infrared star counts to explore the structure and dynamics of the Orion Nebula Cluster (ONC). This very young (<1 Myr) cluster is not circularly symmetric in projection but is elongated north-south in a manner similar to the molecular gas distribution in the region, suggesting that the stellar system may still reflect the geometry of the protocluster cloud. Azimuthally averaged stellar source counts compare well with simple spherically symmetric, single-mass King cluster models. The model fits suggest that the inner Trapezium region should be regarded as the core of the ONC, not as a distinct entity as sometimes advocated. We estimate that the core radius of the cluster is 0.16-0.21 pc and that the central stellar density approaches 2 × 10^4 stars pc^(-3). Adopting the stellar velocity dispersion from published proper-motion studies, virial equilibrium would require a total mass within about 2 pc of the Trapezium of ~4500 M_☉, slightly more than twice the mass of the known stellar population and comparable to the estimated mass in molecular gas projected onto the same region of the sky. If ≳ 20% of the remaining molecular gas is converted into stars, thus adding to the binding mass, given that the present stellar population alone has a total energy close to zero, the ONC is likely to produce a gravitationally bound cluster. The ONC also exhibits mass segregation, with the most massive (Trapezium) stars clearly concentrated toward the center of the cluster and some evidence for the degree of central concentration to decrease with decreasing mass down to 1-2 M_☉, as would be expected for general mass segregation. Given the extreme youth of the stars compared with the estimated range of collisional relaxation times, the mass segregation is unlikely to be the result of cluster relaxation. Instead, we suggest that the mass segregation reflects a preference for higher mass stars to form in dense, central cluster regions.

Mass Functions &amp; Stellar Populations of Globular Clusters

I present a summary of results from various HST photometric studies of the dense central regions of Galactic globular clusters that my collaborators and I have carried out over the last 6 years. The dataset includes short exposures of 47 Tuc, M15, M3 and M13 obtained with the aberrated Planetary Camera-I (PC-I) and F555W (“V”) and F785LP (“I”) filters, as well as post-refurbishment Wide Field Planetary Camera 2 (WFPC2) snapshots of the post core collapse clusters M15, M30, and NGC 6624 in F336W (“U”), F439W (“B”), and V. Recently, a very deep, doubly oversampled PC-I U image of the core of 47 Tuc, and accompanying B and V images, have also been analyzed. In addition, we have carried out extensive checks of incompleteness and photometric error with the help of multiband image simulations that mimic the relevant characteristics of the HST PC-I and WFPC2 images: empirical point spread function, crowding effects based on a realistic density profile and stellar luminosity function (LF), noise, undersampling, A/D saturation, etc..

CO mapping of the central regions of the globular clusters ω Cen and 47 lue

CO mapping of the central regions of the globular clusters ω Cen and 47 lue

Properties of the Monoceros R2 Stellar Cluster

view Abstract Citations (140) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Properties of the Monoceros R2 Stellar Cluster Carpenter, John M. ; Meyer, Michael R. ; Dougados, Catherine ; Strom, Stephen E. ; Hillenbrand, Lynne A. Abstract We present an extensive study of the stellar population of an embedded cluster in the MonR2 molecular cloud based upon a wide field (\about 15\arcmin x 15\arcmin) J, H, and K band mosaic, deep near-infrared imaging at J, H, K, and \NBL\ bands of the central cluster region, and spectroscopic observations of 34 stars. By comparing the properties of the MonR2 cluster with other star forming regions, we ultimately hope to learn how the properties and formation of stars of various masses are related to the local physical conditions. The K band star counts indicate that the MonR2 cluster extends over a \about 1.1 pc x 2.1 pc area with a FWHM cluster size of \about 0.38 pc. Within this region the cluster contains \about 309 stars brighter than m_K = 14.5\M\ and \more than or equal to 475 stars over all magnitudes with a central stellar volume density of \about 9000 stars pc(-3) . We have further explored the properties of the cluster by using the spectroscopic and photometric data to construct an extinction-limited sample of 115 stars in the central 0.77 pc x 0.77 pc region of the cluster that is designed to contain all stars with \AV\ <= 11.3\M\ and stellar masses >= 0.1 M\sun. As a lower limit, 62% of the stars in this sample contain a near-infrared excess at K and/or L band. The K band excess fraction may be as high as 72% if the accretion characteristics of the stars in the MonR2 cluster are similar to stars in Taurus-Auriga. An initial reconnaissance of the stellar mass function suggests that the ratio of high to low mass stars in the extinction-limited sample is consistent with the value expected for a Miller-Scalo IMF. We do not find compelling evidence for mass segregation in the extinction-limited sample for stellar masses \less than or equal to 2 M\sun, although the most massive star (\about 10 M\sun) in the cluster appears to be forming near the cluster center. The properties of the MonR2 cluster are similar to other rich young clusters in the solar neighborhood, such as NGC 2024 and the Trapezium. Publication: The Astronomical Journal Pub Date: July 1997 DOI: 10.1086/118465 Bibcode: 1997AJ....114..198C full text sources ADS | data products SIMBAD (911) CDS (1) Related Materials (1) Erratum: 1997AJ....114.1275C

Are there binary galaxies in clusters?

We investigate a sample of 73 rich clusters containing in total 7754 galaxies and find, after a thorough assessment of the projection effects, that 6 per cent of the galaxies are part of a ‘binary’ system. Apparent ‘binaries’ are defined as nearest-neighbour pairs for which the velocity difference ΔV of the galaxies does not exceed 300 km s−1 and the maximum separation AR is less than 0.1 h−1 Mpc. This results in 1514 pairs. Monte Carlo simulations show that 1047 of these must be accidental, leaving 467 binaries. These binaries cannot be identified individually, but their properties can be studied statistically. The binaries found in this way are dynamically ‘soft’, i.e. ΔV is less than the global σV of the cluster. Despite this, they only seem to avoid the inner 0.4 h−1 Mpc of clusters, suggesting that outside a radius of 0.4 h−1 Mpc the velocity dispersion must locally be lower than the global value of σV that we measure in projection, or that many of the found pairs are unbound. There is a significant correlation between the presence of binaries and the detection of substructure with the Dressier & Shectman test. Binaries in binary-rich clusters seem to be directly associated with the low-σV subclusters for which the DS-test is sensitive. The fraction of binaries in the 19 binary-rich clusters is nearly as high (≈ 12 per cent) as in the population of fore- and background galaxies (‘interlopers’) that is normally removed from the cluster surveys (≈14.5 per cent). In the remaining 54 clusters the binary fraction is considerably lower (≈3 per cent). Depending on the assumed form of the orbits of the galaxies in a binary, the average mass of the binaries lies in the range of 2–5 × 10 11h−1 M⊙, while the average M/L-ratio lies in the range of 20–40 h M⊙/L⊙ for circular orbits to about 45–100 h M⊙/L⊙ for eccentric orbits, which is consistent with the results for field binaries. Monte Carlo models of the binaries suggest that the orbits are likely to have low eccentricities. Both the masses and the M/L-ratios of binaries are larger inside 1.2 h−1 Mpc than outside, which argues against the tidal stripping of galaxy haloes in the central regions of clusters. We have compared our Monte Carlo methods with recent N-body models of clusters which incorporate galaxy formation in a crude way. These models reproduce the main statistical properties of the binaries. When applied to the models, our statistical method detects about 75 per cent of the bound pairs of galaxies that satisfy our selection criteria for binaries. In comparison with the binaries in the data, the masses of bound pairs in these models are systematically larger by a factor of 6. This prevents more detailed comparisons between the data and the models.

A weak lensing survey in the fields of z 1 luminous radio sources

In this paper we present weak lensing observations of the fields around eight z ∼ 1 luminous radio sources. These data are used to search for the lensing signatures of mass structures that either are physically associated with the radio objects, or are foreground systems projected along the line of sight. The radio sources were all the subjects of deep imaging with WFPC-2 on the Hubble Space Telescope, providing high-quality shape information on large numbers of faint galaxies around these sources. Statistical analysis of the coherent shear field visible in the shapes of the faint galaxies indicates that we have detected a weak lensing signal close to one of the targets, 3C 336 at z = 0.927, with a high level of confidence. A second, independent WFPC-2 observation of this target reinforces this detection. Our results support the earlier suggestion of weak lensing in this field by Fort et al. using ground-based data. Using limits on the likely redshift distribution of the faint galaxies used m our analysis, we convert our observed lensing signal into an estimate of the mass of the cluster around 3C 336. We also combined the shear distributions in the remaining seven fields to improve our sensitivity to weak shear signals from any structure typically associated with these sources. We find no detectable signal, and estimate an upper limit on the maximum shear allowed by our observations. We conclude that while some z ∼ 1 luminous radio sources are found in the central regions of rich clusters, the majority do not lie within the central regions of massive collapsed structures at these epochs. The upper limit on the mass of any structure around a typical radio source is similar to that expected of a group or poor cluster. Our data also limit the lensing contribution of foreground structures, although this is sensitive to the assumed offset between the dominant source of shear and the line of sight to the radio source. We suggest that further lensing observations of distant radio sources and their host environments may allow the cluster Lx-mass relationship to be mapped at high z. This is crucial for interpreting the results of the next generation of deep X-ray surveys, and thus for providing a strong observational constraint on the redshift evolution of the cluster mass function out to z = 1.

Star cluster evolution, dynamical age estimation and the kinematical signature of star formation

We use N-body integration to follow the evolution of clusters of 200 binary systems with different initial half mass radii $R_{0.5}$. We also simulate single-star clusters. All clusters evolve according to the same $n(t)$ curve, where $n(t)$ is the number density of stars in the central 2~pc sphere at time $t$. $n(t)$ and the lifetime are independent of (i) the inital proportion of binaries and (ii) the initial $R_{0.5}$. Mass segregation measures the dynamical age of the cluster. The proportion of binaries in the central cluster region is a sensitive indicator of the initial cluster concentration. If most stars form in binaries in a typical embedded cluster which is located at the edge of a giant molecular cloud, then we estimate that at most about 10~per cent of all pre-main sequence stars achieve near escape velocities from the molecular cloud. The large ejection velocities resulting from close encounters between binary systems imply a `halo' distribution of young stars over large areas surrounding star forming sites which is expected to have a significantly reduced binary proportion and a significantly increased proportion of stars with depleted circumstellar disks. We compare the time dependent model single star and system luminosity function in the central cluster region with the observational Hyades and Pleiades luminosity functions and find no evidence for different dynamical properties of stellar systems at birth in the Hyades, Pleiades and Galactic field stellar samples. The observed proportion of binary stars in the very young Trapezium Cluster is consistent with the early dynamical evolution of a cluster with a very high initial stellar number density.

Stellar population in early-type galaxies: Further evidence for environmental influences

Multifiber and long-slit spectra have been used to study the spectral properties of early-type galaxies in a variety of environments, from the centers of dense clusters to relatively isolated situations. A system of spectral indicators, defined in the wavelength interval λλ4000-4400 A, has been used to analyze the integrated spectra. For five of these spectral indices, systematic differences are found between the integrated spectra of early-type galaxies in the centers of dense clusters and those of E/S0's inhabiting relatively low- density environments. In each spectral index diagram, there is a clear separation between early-type galaxies in low-density environments and metal-rich galactic globular clusters, with the E/S0's in high-density environments situated roughly midway between these two extremes. From the behavior of the galaxies and globular clusters in the spectral index diagrams, constraints can be placed on the nature of the stellar population differences between early-type galaxies in high- and low- density environments. The most striking difference is that the luminosity-weighted mean spectral type of the evolved stars (i.e., giant branch and horizontal branch stars) in E/S0's in low-density environments is inferred to be considerably later type and more metal-enriched than in E/S0's in the central regions of dense clusters. On the other hand, the overall integrated spectral type (i.e., the luminosity-weighted mean spectral type of all of the stars in a galaxy) of early-type galaxies does not vary with environment. To reconcile this apparent conflict, we propose that a substantial intermediate-age population is present in the early-type galaxies found in low-density environments, a population that is considerably reduced or altogether lacking in the early-type galaxies in dense clusters. Hence we conclude that early-type galaxies in dense clusters have experienced a truncated star formation and chemical enrichment history when compared to their counterparts in lower-density surroundings.

Hot gas in the cold dark matter scenario: X-ray clusters from a high-resolution numerical simulation

A new, three-dimensional, shock-capturing hydrodynamic code is utilized to determine the distribution of hot gas in a standard cold dark matter (CDM) model of the universe. Periodic boundary conditions are assumed: a box with size 85 h(exp -1) Mpc having cell size 0.31 h(exp -1) Mpc is followed in a simulation with 270(exp 3) = 10(exp 7.3) cells. Adopting standard parameters determined from COBE and light-element nucleosynthesis, sigma(sub 8) = 1.05, omega(sub b) = 0.06, and assuming h = 0.5, we find the X-ray-emitting clusters and compute the luminosity function at several wavelengths, the temperature distribution, and estimated sizes, as well as the evolution of these quantities with redshift. We find that most of the total X-ray emissivity in our box originates in a relatively small number of identifiable clusters which occupy approximately 10(exp -3) of the box volume. This standard CDM model, normalized to COBE, produces approximately 5 times too much emission from clusters having L(sub x) is greater than 10(exp 43) ergs/s, a not-unexpected result. If all other parameters were unchanged, we would expect adequate agreement for sigma(sub 8) = 0.6. This provides a new and independent argument for lower small-scale power than standard CDM at the 8 h(exp -1) Mpc scale. The background radiation field at 1 keV due to clusters in this model is approximately one-third of the observed background, which, after correction for numerical effects, again indicates approximately 5 times too much emission and the appropriateness of sigma(sub 8) = 0.6. If we have used the observed ratio of gas to total mass in clusters, rather than basing the mean density on light-element nucleosynthesis, then the computed luminosity of each cluster would have increased still further, by a factor of approximately 10. The number density of clusters increases to z approximately 1, but the luminosity per typical cluster decreases, with the result that evolution in the number density of bright clusters is moderate in this redshift range, showing a broad peak near z = 0.7, and then a rapid decline above redshift z = 3. Detailed computations of the luminosity functions in the range L(sub x) = 10(exp 40) - 10(exp 44) ergs/s in various energy bands are presented for both cluster central regions and total luminosities to be used in comparison with ROSAT and other observational data sets. The quantitative results found disagree significantly with those found by other investigators using semianalytic techniques. We find little dependence of core radius on cluster luminosity and a dependence of temperature on luminosity given by log kT(sub x) = A + B log L(sub x), which is slightly steeper (B = 0.38) than is indicated by observations. Computed temperatures are somewhat higher than observed, as expected, in that COBE-normalized CDM has too much power on the relevant scales. A modest average temperature gradient is found, with temperatures dropping to 90% of central values at 0.4 h(exp -1) Mpc and 70% of central values at 0.9 h(exp -1) Mpc. Examining the ratio of gas to total mass in the clusters normalized to Omega(sub B) h(exp 2) = 0.015, and comparing with observations, we conclude, in agreement with White (1991), that the cluster observations argue for an open universe.

Cluster-size dependence of alloying behavior in gold clusters

Cluster-size dependence of alloying behavior in nm-sized atom clusters has been studied by transmission electron microscopy, using clusters in the Au-Cu system. It was revealed that occurrence of rapid spontaneous alloying becomes more difficult with increasing cluster size. In gold clusters of approximately 4 nm in the mean size, a rapid dissolution of copper atoms took place and homogeneously mixed Au-Cu alloy clusters were formed. In gold clusters of approximately 10 nm in the mean size, rapid alloying of copper took place only at a shell-shaped region beneath the free surface of individual clusters and pure gold was retained at the central region of clusters. In gold clusters of approximately 30 nm in the mean size, no rapid alloying of copper was induced. The ease with which spontaneous alloying takes place is discussed in terms of the lattice softening in atom clusters.

Semiempirical limits on the thermal conductivity of intracluster gas

A semiempirical method for establishing lower limits on the thermal conductivity of hot gas in clusters of galaxies is described. The method is based on the observation that the X-ray imaging data (e.g., Einstein IPC) for clusters are well described by the hydrostatic-isothermal beta model, even for cooling flow clusters beyond about one core radius. In addition, there are strong indications that noncooling flow clusters (like the Coma Cluster) have a large central region (up to several core radii) of nearly constant gas temperature. This suggests that thermal conduction is an effective means of transporting and redistributing the thermal energy of the gas. This in turn has implications for the extent to which magnetic fields in the cluster are effective in reducing the thermal conductivity of the gas. Time-dependent hydrodynamic simulations for the gas in the Coma Cluster under two separate evolutionary scenarios are presented. One scenario assumes that the cluster potential is static and that the gas has an initial adiabatic distribution. The second scenario uses an evolving cluster potential. These models along with analytic results show that the thermal conductivity of the gas in the Coma Cluster cannot be less than 0.1 of full Spitzer conductivity. These models also show that high gas conductivity assists rather than hinders the development of radiative cooling in the central regions of clusters.

Predicted anisotropy in the arrival directions of cosmic gamma rays at 1014 eV

A more detailed analysis than hitherto is made of the generation of ultra high energy gamma rays in the Universe. The new situation takes a model in which very energetic cosmic rays are accelerated, mainly in the central regions of galaxy clusters, and allows the cosmic rays to diffuse outward with an energy-dependent diffusion coefficient. The particles generate gamma rays by virtue of interactions with the microwave background radiation; these gamma rays cascade through extragalactic space and those with energies of order 1014 eV should be detectable by the new extensive air shower arrays soon to come into service.

When did cores of clusters of galaxies form?

view Abstract Citations (17) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS When Did Cores of Clusters of Galaxies Form? Maoz, Eyal Abstract There is now much observational evidence which indicates that the formation of clusters of galaxies is still an ongoing process. However, in the central region of clusters, there often resides a dense and relaxed component which appears to have formed earlier than its surroundings. Assuming the gravitational instability picture, what is the place of these structures in the sequence of creation? We improve the usual procedure for determining the time of breaking away from the general expansion, and show that the redshift of turnaround, Z_t_, depends on a single parameter α which combines directly observed quantities: recession velocity, central velocity dispersion, and angular size of the core. The developed estimator is totally independent of the value of H_0_. However, it involves a numerical coefficient which varies with the model of collapse. Thus, even with complete and exact observational data, our lack of knowledge of the correct collapse model prevents Z_t_ from being sharply determined. Results are obtained for structures with various properties and for various choices of the cosmological density parameter {OMEGA}_0_. A typical case is the central component of the Coma Cluster, for which we get 4.1 <= Z_t_ <= 8.5 for {OMEGA}_0_ = 0.2 or 2.4 <= Z_t_ <= 5.0 for {OMEGA}_0_ = 1. It also turns out that unless protoclusters have collapsed in substantially different ways, the epoch of protocluster turnaround is correlated with the velocity dispersion in the cluster center. The existence of highly nonlinear mass perturbations of the order of 10^13.5 +/- 0.5^ M_sun_ at such moderate redshifts might have had considerable influence on galaxy evolution. Publication: The Astrophysical Journal Pub Date: August 1990 DOI: 10.1086/169058 Bibcode: 1990ApJ...359..257M Keywords: Cosmology; Galactic Clusters; Galactic Evolution; Astronomical Models; Computational Astrophysics; Gravitational Collapse; Red Shift; Astrophysics; COSMOLOGY; GALAXIES: CLUSTERING; GALAXIES: FORMATION; GALAXIES: INTERGALACTIC MEDIUM full text sources ADS | data products SIMBAD (3) NED (1)

CCD photometry of the Large Magellanic Cloud cluster NGC 1866 - Observational results

view Abstract Citations (60) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CCD Photometry of the Large Magellanic Cloud Cluster NGC 1866: Observational Results Brocato, E. ; Buonanno, R. ; Castellani, V. ; Walker, A. R. Abstract The first part of this paper is devoted to a new CCD investigation of stars in the field of the LMC cluster NGC 1866. Two main frames have been analyzed, one covering the cluster's central region and the other covering an area about 10' from the cluster's center, and data has been collected for 2207 objects. On this basis, a C-M diagram has been derived, which shows a well-developed blue sequence reaching a limiting magnitude of about V = 21 mag together with the already known sequence of the cluster's He-burning giants. A new feature in our data is represented by the evidence for a second group of less luminous giants clumping around V = 19 mag, which we interpret as field He-burning giants. The second part of the paper is devoted to a comparison of these data with theoretical calculations. On the basis of new evolutionary tracks we find that the composition Y = 0.28, Z = 0.02 already used by Becker and Mathews offers an adequate fitting of the observed range in color spanned by cluster He giants. Theoretical isochrones for this composition have been produced and are compared with the observed distribution of cluster stars. A good agreement with the observed C-M diagram is achieved for a cluster age of the order of τ = 100 Myr, whereas field stars appear to be significantly older, with ages of about 500 Myr or more. The distribution of cluster stars is studied, and both the luminosity function of the blue sequence (BS) and number of He giants relative to BS stars are exanined. We find that both these cluster characteristics can be reproduced with a Salpeter slope for the initial mass function, overcoming previous difficulties raised by suggestions of a serious disagreement between theory and observations in this cluster. The Appendix reports data for the evolutionary models and for the corresponding isochrones, covering both the H- and He-burning phases. Publication: The Astrophysical Journal Supplement Series Pub Date: September 1989 DOI: 10.1086/191363 Bibcode: 1989ApJS...71...25B Keywords: Astronomical Photometry; Globular Clusters; Magellanic Clouds; Stellar Evolution; Charge Coupled Devices; Color-Magnitude Diagram; Giant Stars; Star Distribution; Tables (Data); Astrophysics; CLUSTERS: GLOBULAR; GALAXIES: MAGELLANIC CLOUDS; PHOTOMETRY; STARS: EVOLUTION full text sources ADS | data products SIMBAD (17) CDS (2) Related Materials (1) Catalog: 1996yCat..20710025B

The distribution of mass for spiral galaxies in clusters and in the field

view Abstract Citations (9) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Distribution of Mass for Spiral Galaxies in Clusters and in the Field Forbes, Duncan A. ; Whitmore, Bradley C. Abstract A comparison is made between the mass distributions of spiral galaxies in clusters and in the field using Burstein's mass-type methodology. Both Hα emission-line rotation curves and more extended H I rotation curves are used. The fitting technique for determining mass types used by Burstein and coworkers has been replaced by an objective χ^2^ method. Mass types are shown to be a function of both Hubble type and luminosity, contrary to earlier results. The present data show a difference in the distribution of mass types for spiral galaxies in the field and in clusters, in the sense that mass type I galaxies, where the inner and outer velocity gradients are similar, are generally found in the field rather than in clusters, as Burstein and coworkers previously reported. This can be understood in terms of the results of Whitmore, Forbes, and Rubin, who find that the rotation curves of galaxies in the central region of clusters are generally falling, while the outer galaxies in a cluster and field galaxies tend to have flat or rising rotation curves. Publication: The Astrophysical Journal Pub Date: April 1989 DOI: 10.1086/167326 Bibcode: 1989ApJ...339..657F Keywords: Galactic Clusters; Galactic Structure; Mass Distribution; Spiral Galaxies; Galactic Rotation; H Alpha Line; Luminosity; Astrophysics; GALAXIES: CLUSTERING; GALAXIES: INTERNAL MOTIONS; GALAXIES: STRUCTURE full text sources ADS | data products SIMBAD (101) NED (98)

Evolution of N-body open clusters

Realistic n-body simulations of dynamical evolution of open clusters are discussed and compared with observations. Most of the models have 1000 bodies with initial masses following a power-law mass function of slope α = – 2.75 and mean mass 0.5 M⊙. Stellar evolution and mass loss are considered in detail. Neutron stars or white dwarfs (depending on the initial stellar mass) are generated by instantaneous changes in individual masses, when stars reach the end of their main sequence life. Close approaches between particles are treated by a two-body regularization technique that allows one to follow binary evolution in detail. Two types of tidal perturbation are considered: a smooth linearized galactic tidal field, simulated assuming that the clusters move in a circular orbit at 10 kpc from the galactic centre; and transient shocks, simulated by encounters with extended interstellar clouds of different mass spectrum, density and space concentration. Excellent agreement is found between the distribution of lifetime for galactic clusters and the life of the models. The combined action of evolutionary stellar mass loss and binaries (for clusters with 1000 bodies and a realistic mass function) is enough to arrest the core collapse. Tidal heating shapes the halo of the cluster and produces a distinctive density and velocity distribution. Encounters with standard clouds do not alter the life-time of the clusters; only giant molecular clouds produce a catastrophic disruption. Effects of mass segregation and preferential escape of light stars can account for the observed depletion of low luminosity stars in the better studied central region of clusters.

Ultra-high-energy cosmic rays from clusters of galaxies

It is proposed that the highest-energy cosmic rays are produced in galaxies in the central regions of clusters. The particles diffuse outwards under the influence of randomly directed intergalactic magnetic fields. Many of the particles detected at the Earth come from the Virgo cluster but if the diffusion coefficient is high enough other, more distant clusters, will also contribute significantly. The model gives a natural explanation of the measured spectral shape and anisotropy above about 1018 eV; its requirements for cosmic-ray energy in cluster sources are not excessive if, as seems likely, the production spectrum is comparatively flat (j(E) varies as tildeE- gamma with gamma in the region of 2.1 to 2.2).

Gravitational encounters in rich clusters of galaxies

view Abstract Citations (51) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gravitational encounters in rich cluster of galaxies. Strom, S. E. ; Strom, K. M. Abstract Gravitational encounters in dense clusters may significantly alter the structural properties of elliptical galaxies. Observational data presented in this paper suggest that ellipticals located near the dense central regions of spiral-poor and cD clusters of galaxies may lose up to 50% of their initial mass, while their characteristic sizes may decrease by a factor of 1.5 or more. These results, when compared with numerical simulations of galaxy encounters, lend support to the view that tidal stripping of halo stars is a plausible cause of these changes. The observable properties of E galaxies that are located in lower-density spiral-rich clusters appear to be relatively homogeneous and show no evidence of the effects of gravitational encounters. Publication: The Astrophysical Journal Pub Date: November 1978 DOI: 10.1086/182801 Bibcode: 1978ApJ...225L..93S Keywords: Elliptical Galaxies; Galactic Clusters; Galactic Structure; Gravitational Effects; Digital Simulation; Halos; Spiral Galaxies; Astrophysics; Clusters of Galaxies:Galaxy Encounters; Elliptical Galaxies: Structure full text sources ADS | data products SIMBAD (6)

Existence and amount of intergalactic dust

The existence of intergalactic dust has been proved by the following observational facts: the decrease of the numbers of distant galaxies and clusters of galaxies behind the central regions of near clusters of galaxies; the different distributions of RR Lyrae stars and galaxies near ι Microscopii (Hoffmeister's cloud); the dependence of colour excesses of galaxies on supergalactic coordinates as well as on the surface density of bright galaxies; the colour index vs redshift correlation of quasistellar objects. The densities of intergalactic dust are estimated to be between 5×10−30 g cm−3 (near the centers of clusters of galaxies) and 2×10−34 g cm−3 (in general intergalactic space). The grains may be formed either in the early phases of the Universe (25<z<50) or may be expelled from galaxies by the radiation pressure. The most effective destruction process seems to be the evaporation by soft cosmic rays.

The bright streaks in the H? disk chromosphere

Evidence is presented demonstrating the existence of a type of chromospheric structure in the form of bright streaks. These are extensions across the solar disk of elongated bright mottles which originate in the central regions of clusters of mottles. They are best observed on good filtergrams at Hα ± 0.5 A through comparison with filtergrams at other positions on the line profile. Their length can be as much as 200 sec of arc. The bright streaks appear to be predominantly horizontal loop structures, while the well-known spicules are mainly vertical structures. A bright streak may be well defined or rather diffuse along its length, and many of them are accompanied by darker boundaries or envelopes. It is usual to find a loop of bright streak bridging the central areas of two mottle clusters. It seems that the observed pattern in the space between the chromospheric network at Hα ± 0.5 A results partly from the interactions of the bright streaks of different stages of evolution traversing the area in different directions.