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At least 973 records · Page 54
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NASA NTRS1990

Gravitational lens models of arcs in clusters

It is now well established that the luminous arcs discovered in clusters of galaxies, in particular those in Abell 370 and Cluster 2244-02, are produced by gravitational lensing of background sources. The arcs are modeled and constraints are placed on the distribution of the mass in the clusters and the shape and size of the sources. The models require, as expected, a large amount of dark matter in the clusters and a mass-to blue-light ratio for the cluster which exceeds 100 solar mass/solar luminosity and could be as high as 1000 solar mass/solar luminosity depending on cosmological parameters and the distribution of the dark matter. Furthermore, it is found that in the case of the arc in A370 the dark matter must have a different distribution than the luminous galaxies, while for the arc in Cl 2244 the dark matter can have a distribution similar to that of the light matter (galaxies) or a separate distribution.

Bergmann, Anton G.↗
NASA NTRS1984

Studies of X-ray clusters of galaxies/intergalactic plasmas

Intergalactic plasmas were investigated from both an observational and theoretical point of view. A multiobject spectrometer, the MX spectrograph was used to obtain detailed dynamical information on clusters of galaxies; this information was then compared with X ray emission from hot gas in these clusters. Several spectra of galaxies are presented, and data reduction of the spectra was discussed. The existence of quasar winds in Seyfert galaxies and the interaction between such a wind and the interstellar medium also were considered.

Scott, J. S.↗
NASA NTRS1980

SAS 3 and related observations of Abell 2029

X-ray and radio observations of the cluster of galaxies show that the central cD galaxy dominates the emissions. Resolution of the X-ray source shows that its extent is significantly greater than that of the profile of surface brightness of the cD galaxy. Dressler's model of most of the cluster mass with very high M/L(v) is similarly extended. The optically dark mass may provide low-energy X-ray emission of the coronae of 5 x 10 to the 16th stars with very high M/L(v), but in the 2-10 keV range the data are consistent with current theoretical predictions for hot intracluster gas.

Johnson, H. M.↗
NASA NTRS1986

Dark matter near the sun

The amount of dark matter in the disk of the Galaxy at the solar position is determined by comparing the observed distributions of tracer stars with the predictions obtained from different assumptions of how the unseen matter is distributed. The major uncertainties, observational and theoretical, are estimated. For all the observed samples, typical models imply that about half of the mass in the solar vicinity must be in the form of unobserved matter. The volume density of unobserved material near the sun is about 0.1 solar mass/cu pc; the corresponding column density is about 30 solar masses/cu pc. This, so far unseen, material must be in a disk with an exponential scale height of less than 0.7 kpc. All the existing observations are consistent with the unseen disk material being in the form of stars not massive enough to burn hydrogen. It is suggested that the unseen material that is required to hold up the rotation curves of galaxies and to satisfy the virial theorem for clusters of galaxies might also be in the form of low-mass stars.

Bahcall, J. N.↗
NASA NTRS1990

Constraints on large-scale clustering from the autocorrelation properties of the X-ray background

Expectations for the angular correlation function of intensity fluctuations of the extragalacitc 2-10 keV X-ray background are discussed in relation to the two-point spatial correlation functions, xi(r), of X-ray sources. A simple analytic formula for the amplitude of intensity correlations, Gamma(theta), holding in the low-redshift, small-separation approximation, is derived. The HEAO 1 A-2 upper limits on Gamma(theta) have been exploited to derive constraints on the local xi(r) functions of rich clusters of galaxies, of AGNs, and on the AGN-cluster cross-correlation, as well as on evolution of the correlation functions with cosmic time. X-ray data are found to be compatible with the Bahcall and Soneira estimates of xi(cluster-cluster) as well as with the results of recent studies of quasar clustering.

De Zotti, G.↗
NASA NTRS1993

A study of the x ray environment of radio galaxies

We are currently working on a program to use extensive x-ray and radio databases to investigate the relationship between extended radio emission and environment in clusters of galaxies. The radio galaxy morphology is determined using VLA imaging and the x-ray properties are determined from Einstein IPC images. This study is motivated by the hypothesis that the key to understanding radio galaxies lies in the local environment. To test this hypothesis we have studied the detailed relationship between galaxy radio emission and the x-ray morphology of their parent clusters. In this pilot study we have used 35 radio sources found in 27 clusters. We have determined the position angle of the x-ray and radio emission, and x-ray and radio luminosities. The x-ray position was taken to be the position of peak flux of the subclump containing the radio galaxy. The radio position was taken to be the position of the galaxy. We do not find a correlation between the x-ray and radio source position angle. This remains true when the sample is divided into subsamples according to radio morphology (wide angle tail, twin jet, narrow angle tail galaxies). We find a weak correlation between the radio source luminosity and the x-ray luminosity. We have computed the distance from the radio galaxy position to the center of the x-ray clump. We find a mean distance from the x-ray clump center of 0.16 Mpc for the radio galaxies in this sample. The mean distance to the nearest clump of x-ray emission is typically half the distance to the optical cluster center. We thus find strong evidence that radio galaxies are located very close to clumps of x-ray emission. These subclumps are not always affiliated with the central cluster x-ray emission. This supports our hypothesis that x-ray emission may provide a key to understanding radio galaxy morphology. We find evidence that radio galaxies occur in clusters that contain prominent substructures. Radio galaxies may thus provide an added diagnostic of the cluster dynamical state.

Rhee, George F.↗
NASA NTRS2000

The Heavy Element Abundance in Groups of Galaxies

Over the past few years we have analyzed a sample of clusters observed by the Advanced Spacecraft for Cosmology Astrophysics (ASCA) X-ray satellite. We performed spatially resolved X-ray spectroscopy of a sample of 18 relaxed clusters of galaxies with gas temperatures below 4 keV. The spectral analysis was done using ASCA/SIS (Solid state Imaging Spectrometer) data combined with imaging data from ROSAT/PSPC (German acronym for X-ray satellite/Position Sensitive Proportional Counter) and Einstein/IPC (Imaging Proportional Counter) observations. We derived temperature profiles using single-temperature fits for all of the clusters in the sample, and also corrected for the presence of cold gas in the center of so-called 'cooling flow' clusters. For all of the clusters in the sample we derived Si and Fe abundance profiles. For a few of the clusters we also were able to derive Ne and S abundance profiles. We compared the elemental abundances derived at similar overdensities in all of the clusters in the sample. We also compared element mass-to-light ratios for the entire sample. We concluded that the preferential accretion of low entropy, low abundance gas into the potentials of groups and cold clusters can explain most of the observed trends in metallicity. In addition, we discussed the importance of preheating of the intracluster medium by Type II supernovae on the cluster scaling relations.

David, Laurence↗
NASA NTRS1990

The kinematics of Abell clusters

Velocity histograms, galaxy positions, and velocity dispersions are presented for 69 nearby Abell clusters. The shape of the cumulative distribution for R = 1 or more clusters does not match the predictions of standard CDM models for any biasing parameter b. The only models consistent with the median and maximum dispersions of the sample are those of b about 1.6-2.0 and in which clusters are identified in three dimensions. Velocity dispersions of high-redshift systems appear fundamentally different from those of nearby clusters. The median dispersion of the subset of 25 cD clusters is similar to that of non-cD systems. A substantial fraction of the cD galaxies have velocities significantly different from the mean of their parent clusters. Three of these have 4 s or more measured redshifts. These last two results suggest that the formation of cD galaxies is dominated by the local, rather than global, cluster environment.

Zabludoff, Ann I.↗
NASA NTRS1972

Extragalactic X-ray sources.

The extended X-ray sources associated with the Virgo, Perseus, and Coma clusters of galaxies can best be explained as a result of the Compton scattering of relativistic electrons (generated in the radio sources which are present in each of these clusters) on the microwave background radiation. The same physical process involving microwave infrared or optical photons generated in the nuclei of active galaxies may account for the more compact X-ray sources. Thermal bremsstrahlung of hot gas in clusters of galaxies is a less likely source of X-rays.

Brecher, K.↗
NASA NTRS1994

Imaging of 16 distant EMSS clusters with z greater than or equal to 0.2 and L(subx,44) greater than or equal to 4: New arcs and first consequences

Medium-deep V, R, and I observations of a subsample of 16 clusters of galaxies in the Experimental Mobile Satellite System (EMSS) sample defined from the Einstein satellite data are presented. Three new cases of clusters with giant luminous arcs (length/width greater than 10, V less than 22) have been discovered together with six new arcs (4 less than l/w less than 10, V less than 22), and we propose that these arcs are new examples of gravitational lensing of background galaxies by the rich clusters. Combined with giant luminous arcs already observed in the EMSS sample, there is a total of six giant luminous arcs observed in the sample of 16 clusters. This high rate of success in finding arcs added to their axis ratio distribution is consistent with very small core radii (less than 100 kpc) for the dark matter distribution in rich clusters, and rules out models in which dark matter follows the X-ray gas.

Le Fevre, O.↗
NASA NTRS1988

On hierarchical solutions to the BBGKY hierarchy

It is thought that the gravitational clustering of galaxies in the universe may approach a scale-invariant, hierarchical form in the small separation, large-clustering regime. Past attempts to solve the Born-Bogoliubov-Green-Kirkwood-Yvon (BBGKY) hierarchy in this regime have assumed a certain separable hierarchical form for the higher order correlation functions of galaxies in phase space. It is shown here that such separable solutions to the BBGKY equations must satisfy the condition that the clustered component of the solution has cluster-cluster correlations equal to galaxy-galaxy correlations to all orders. The solutions also admit the presence of an arbitrary unclustered component, which plays no dyamical role in the large-clustering regime. These results are a particular property of the specific separable model assumed for the correlation functions in phase space, not an intrinsic property of spatially hierarchical solutions to the BBGKY hierarchy. The observed distribution of galaxies does not satisfy the required conditions. The disagreement between theory and observation may be traced, at least in part, to initial conditions which, if Gaussian, already have cluster correlations greater than galaxy correlations.

Hamilton, A. J. S.↗
NASA NTRS2000

Extragalactic Radio Jets and Intergalactic Medium 2

During the mid 1990s, the ROSAT satellite provided the highest resolution X-ray imaging, with useful X-ray spectroscopy, and excellent sensitivity. ROSAT"s PSPC and HRI instruments gave us our first high-quality data on the AGN phenomenon in nearby galaxies - the central theme of this research project. Initially this project concentrated on separating the AGN-related component of the X-ray emission from thermal radiation from the surrounding atmospheres in radio galaxies. There was ample reason to believe that this separation would be possible, although earlier work had taken the view that the X-ray emission from radio galaxies is either wholly AGN-related (and hence of sub-arcsec scale), or wholly from the galaxy and cluster atmospheres (and hence of scales - 10 arcsec or more in the low-redshift radio galaxies that we chose to study). First with the PSPC, and then with the HRI, we proved that a wide range of AGN core X ray emissivities could be found, and that the generic radio galaxy produces both AGN-related and atmosphere-related X-radiation. We demonstrated that there is a close relationship between the core X-ray power and the core radio power, and found instances in which the hot atmospheres of the galaxies must be participating in cooling flows, and other cases where those atmospheres are relatively stable. In some cases, there is a clear active relationship between the extended radio emission and the X-ray atmospheres: thus in NGC 326, we interpret the distorted radio structure as evidence of buoyancy as the radio plasma rises through the cluster atmosphere. In 3C 449, we can see that the radio plasma is displacing the X-ray emitting gas. In a further case (NGC 1265), we hoped that the motion of a radio galaxy through a cluster atmosphere would be apparent though the wake that it might establish - unfortunately, other structure in the atmosphere of the Perseus cluster tends to dominate our X-ray image, and no useful limits could be placed on the strength of the wake. In the light of so-called "unified" models of AGN, it is expected that the low-power radio galaxies studied under these programmes are the unbeamed counterparts of BL Lac objects. Accordingly, we predicted that BL Lac objects, as a class, should show X-ray halos from atmospheres similar to those associated with radio galaxies. The difficulty with studying this is that BL Lac objects tend to be X-ray bright, and the halos then vanish under the wings of the point response function. We found a BL Lac with less of a dynamic range problem, mapped it with the ROSAT HRI, and duly found an atmosphere (Hardcastle et al. 1999). Unfortunately for unification models, this atmosphere is exceptionally dense, so that it must be participating in an intense cooling flow. The implications of this for unification models are far from clear, and further work is under way.

Birkinshaw, Mark↗