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At least 883 records · Page 49
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NASA NTRS1985

Emission-line QSOs in the region of the Hercules cluster of galaxies

Slit spectrophotometry in the 305-855-nm range of 25 QSO candidates identified by slitless spectroscopy in two 1-deg-sq fields (one centered on Abell 2151 and one in a blank region) near the Hercules cluster of galaxies, obtained using image-dissector scanners on the 3-m Lick Observatory and 4-m KPNO telescopes at resolution 5-20 A during 1976-1979, are reported. The data are presented in tables and spectrograms and characterized in detail. In the cluster field, 14 candidates are confirmed as QSOs with z = 0.89-3.24; in the blank field, six are confirmed with z = 1.52-3.00. While the strongest feature of most spectra is found to be the Lyman alpha line, five QSOs have the 154.9-nm line of C IV and one the semiforbidden 190.9-nm line of C III as strong features. The wide variety of spectral profiles and absorption features in the higher-redshift QSOs and the lack of any QSOs with broad absorption troughs are discussed, and the use of these QSOs as probes for warm or cool gas in the Hercules cluster in Space Telescope Faint Object Spectrograph observations is suggested.

Burbidge, E. M.↗
NASA NTRS1985

Are galactic bulge X-ray burst sources leftovers of distupted globular clusters

Since approximately 1979, it is known that galactic bulge X-ray sources are low-mass binary systems, and that the X-ray bursters are a subset of the bulge sources. Grindlay and Hertz (1983) reported on the close proximity (in four of twelve cases) of an X-ray burst source and a normal star. The two authors believe that these alignments are not coincidental. They suggest that the normal star is surviving giant in a disrupted globular cluster core, of which the X-ray source would also be a member. The present investigation has the objective to show that Grindlay and Hertz made an error in their statistical analysis of a factor of approximately 10. It is, thus, believed that there is insufficient evidence for the conclusion reported by Grindlay and Hertz. On the basis of the present investigation, it is concluded that there exists at present no statistically significant observational evidence that X-ray bursters, presently located outside globular clusters, were formed in globular clusters.

Van Paradijs, J.↗
NASA NTRS1985

H I properties of dwarf irregular galaxies in the Virgo Cluster

A neutral hydrogen survey was carried out at Arecibo on 91 dwarf irregular galaxies, with morphological types Sdm through Im, in and around the direction of the Virgo Cluster. Only nine of these were found to be background galaxies, and 19 remain undetected, i.e., most of the candidate galaxies are indeed members of the Virgo Cluster or Supercluster. The distribution of positions and systemic velocities (compared with large spirals) shows no evidence for mass segregation. The H I depletion for dwarfs in the cluster core is only moderate, no more than for spirals. The magnitude-velocity width correlation is continous from spirls to dwarfs. Satistics on H I masses agree only partially with a simple stochastic star formation model.

Hoffman, G. L.↗
NASA NTRS1985

Clustered impacts - Experiments and implications

The characteristics of impact by clusters of projectiles are experimentally studied by launching grouped projectiles of aluminum shot, steel shot, iron filings, and sand. Cratering efficiency is considered as a function of a dimensionless parameter related to projectile size and impact velocity. The effects of different target and projectile densities on cratering efficiency are examined. Crater morphology is addressed by considering a typical example, reviewing the systematics between cluster dispersion and crater morphology for vertical impacts, and examining oblique angle impacts which have relevance for planetary secondary cratering processes. These results are compared with impacts by single bodies with different strengths. The evolution of the ejecta plume for clustered impacts is compared to that for single-body impacts for vertical and oblique impacts from 1.3 to 1.8 km/s. The experimental results are discussed in the context of planetary surface processes, emphasizing processes in an atmosphere-free environment and secondary impact cratering.

Schultz, P. H.↗
NASA NTRS1985

Ultraviolet studies of stars in the populous cluster NGC 2100 in the Large Magellanic Cloud

The ultraviolet energy distributions of the B stars in the LMC cluster NGC 2100 have been studied. The ultraviolet extinction law was studied for the LMC cluster. The T(eff)-luminosity diagrams were determined. The brightest stars in the cluster are evolved stars on the 'horizontal' supergiant branch. No P Cygni profiles indicating mass loss were recognized for these supergiants.

Bohm-Vitense, E.↗
NASA NTRS1984

Charging and hybridization in the finite cluster model

Cluster wavefunctions which have appropriate hybridization and polarization lead to reasonable properties for the interaction of an adsorbate with a solid surface. However, for Al clusters, it was found that the atomic change distribution is not uniform. The finite cluster size leads to changes not representative for an extended system. This effect appears to be dependent on the particular materials being studied; it does not occur in all cases.

Bauschlicher, C. W., Jr.↗
NASA NTRS1985

Long-slit spectroscopy of gas in the cores of X-ray luminous clusters

The results of long-slit spectroscopy obtained for the core regions of 14 clusters of galaxies are reported. The data are presented in detail. It is shown that the presence of optical emission is tied to the properties of the hot gas in the cluster and not to the morphology of the central galaxy or cluster, demonstrating that the optical systems are indeed formed by the cooling of hot gas. Cooling flows occur when the gas density exceeds a critical central value which corresponds to a cooling time scale which, it is argued, weakly favors low values of H(0). The kinematics of the gas flows are discussed. The excitation mechanisms, correlation of optical emission with radio properties, and upper limits on coronal line strengths from the hot gas are discussed.

Hu, E. M.↗
NASA NTRS1986

The X-ray spectrm of the Coma Cluster of galaxies

The HEAO 1 A-2 and OSO 8 observations of the Coma Cluster of galaxies in the 2-60 keV energy band are presented. The spectrum is not well fitted by an isothermal bremsstrahlung model with an Fe line. To model the spectrum more accurately, we have performed fits of polytropic models and find that the gas is well fitted by an equation of state with polytropic index ranging between 1.38 and 1.6. This analysis reduces the total cluster mass by a factor of 3-5 below the optically derived virial mass. Using the best-fitting parameters, the gas is found to be a significant fraction (25 percent) of the total cluster mass. The Fe K-alpha line blend is strongly detected, from which we infer 0.18-0.36 solar abundance in the X-ray emitting gas. The Fe K-beta line blend is also weakly detected. The spectrum is not consistent with the existence of a hard X-ray tail at the flux level reported by Bazzano el al. (1984). In addition, the HEAO I scanning mode data place strong constraints on possible contaminating sources in the region surrounding Coma.

Henriksen, M. J.↗
NASA NTRS1986

High-resolution CCD spectra of stars in globular clusters. I - Oxygen in M13

High-resolution (0.3 A) CCD spectra obtained at the 200 in. coude spectrograph have been analyzed for the abundances of O, Sc, Fe, and La in four stars in the globular cluster M13. Fe/H abundance is found to be = -1.6, as found by many other observers of this cluster. For three stars O/Fe abundance is found to be = +0.3 + or - 0.1, which is similar to O/Fe ratios in other globular clusters and metal-poor field stars. For star II-67, no oxygen line is visible at 6300 A and O/Fe abundance is found to be not greater than -0.4 (for a high carbon content) and not greater than -0.7 (for a low carbon content). The latter is more likely to be correct. Two possible explanations of the oxygen deficiency in II-67 are discussed: primordial deficiency, and CNO cycling at or above a temperature of 25,000,000 K.

Leep, E. M.↗
NASA NTRS1986

Self-regulated cooling flows in elliptical galaxies and in cluster cores - Is exclusively low mass star formation really necessary?

A self-consistent treatment of the heating by supernovae associated with star formation in a spherically symmetric cooling flow in a cluster core or elliptical galaxy is presented. An initial stellar mass function similar to that in the solar neighborhood is adopted. Inferred star-formation rates, within the cooling region - typically the inner 100 kpc around dominant galaxies at the centers of cooling flows in XD clusters - are reduced by about a factor of 2, relative to rates inferred when the heat input from star formation is ignored. Truncated initial mass functions (IMFs) are also considered, in which massive star formation is suppressed in accordance with previous treatments, and colors are predicted for star formation in cooling flows associated with central dominant elliptical galaxies and with isolated elliptical galaxies surrounded by gaseous coronae. The low inferred cooling-flow rates around isolated elliptical galaxies are found to be insensitive to the upper mass cutoff in the IMF, provided that the upper mass cutoff exceeds 2 M solar mass. Comparison with observed colors favors a cutoff in the IMF above 1 M solar mass in at least two well-studied cluster cooling flows, but a normal IMF cannot be excluded definitively. Models for NGC 1275 support a young (less than about 3 Gyr) cooling flow. As for the isolated elliptical galaxies, the spread in colors is consistent with a normal IMF. A definitive test of the IMF arising via star formation in cooling flows requires either UV spectral data or supernova searches in the cooling-flow-centered galaxies.

Silk, J.↗
NASA NTRS1986

An X-ray and optical study of the cluster of galaxies Abell 754

X-ray and optical data for A754 are used to study the relative distribution of the luminous and dark matter in this dense, rich cluster of galaxies with X-ray luminosity comparable to that of the Coma Cluster. A quantitative statistical comparison is made of the galaxy positions with the total mass responsible for maintaining the X-ray emitting gas in hydrostatic equilibrium. A simple bimodal model which fits both the X-ray and optical data suggests that the galaxies are distributed consistently with the projected matter distribution within the region covered by the X-ray map (0.5-1 Mpc). The X-ray and optical estimates of the mass in the central region of the cluster are 2.9 x 10 to the 14th and 3.6 + or - 0.5 x 10 to the 14th solar masses, respectively.

Fabricant, D.↗
NASA NTRS1986

Detection of a 'rotating' gas cloud 100 kiloparsecs in diameter surrounding 3C 275.1, a quasar at the center of a rich cluster of galaxies

The distorted radio quasar 3C 275.1 (z = 0.55) is the first QSO found to lie at the center of a rich cluster of galaxies. The quasar is surrounded by an extremely large elliptical nebulosity whose major axis exceeds 100 kpc. Spectroscopic observations of this nebulosity were obtained, with the spectrograph slit aligned along the nebulosity's major axis. The spectrum of the nebulosity in the visible is dominated by O II and O III forbidden emission lines, which yield a well-defined 'solid-body rotation curve' extending 40 kpc from the quasar nucleus, a rotation curve very different from those of normal spiral galaxies. The 3C 275.1 nebulosity is both the largest yet discovered around a quasar and the first in which organized large-scale motions (rotation?) have been identified. The nebulosity may be residue from multiple tidal interactions with cluster members, or it may be accumulated material from a 'cooling flow' of the type seen in low-redshift clusters which are X-ray sources.

Hintzen, P.↗
NASA NTRS1986

Stability and structure of metal clusters - Be(13) and Be(55)

Face-centered cubic (fcc) and hexagonally close-packed (hcp) structures are compared for Be(13) and Be(55) clusters. Both Be(13) and Be(55) prefer the fcc structure over the bulk hcp structure, but the energy difference per atom decreases for Be(55) relative to Be(13). The binding energy per atom, 1.3 eV for Be(55) and 0.8-0.9 eV for Be(13), reflects the greater total number of bonds in the larger cluster rather than a difference in bonding. The energies per bond are much more similar, in the range of 0.30-0.34 eV for both clusters. The size of the p-basis set used influences both stability and ionization potentials strongly.

Pettersson, Lars G. M.↗
NASA NTRS1986

Have burnt-out galaxies and galaxy clusters been detected?

It is shown here that an elevated rate of star formation may be responsible for burning out protogalaxies in the densest galaxy clusters, leading to the formation of candidate gravitational lenses and to an enhanced probability of close association of quasars in regions of apparently low galaxy density. Extreme tidal interactions occurring during the initial collapse of such clusters would stimulate the star formation and also could drive molecular clouds and stars onto radial orbits, fuelling massive black holes and leading to an enhanced frequency of occurrence of quasar pairs. It is argued that a fundamental bias exists against detecting luminous matter in the great galactic clusters.

Silk, Joseph↗
NASA NTRS1986

Head-on binary collisions in globular clusters

Collision probabilities between identical binaries of three different binding energies are computed for binaries in the cores of globular clusters. The more tightly bound a binary is, the smaller the chance is that it suffers a collision. It is found that a given binary in a globular cluster core probably does not suffer more than a very few collisions. If it is not broken up into single stars, it will become so tightly bound that collisions involving it are very rare. Since it is found that head-on collisions are so rare, the collisions which are probably more important in affecting the evolution of the cluster core are the long-range (large impact parameter) encounters.

Hoffer, James B.↗
NASA NTRS1987

X-ray halos in galaxies and clusters of galaxies - Theory

X-ray measurements provide an excellent method to determine the amount and distribution of the dark matter in clusters. Unfortunately, accurate temperature profiles, necessary to this method, are currently not available. However, if the intracluster gas is assumed to have a monotonically decreasing temperature, one finds that the dark matter is strongly concentrated to the cluster center, and has a mass which only exceeds the known baryonic mass by a factor of about three. On a second topic, cooling flows are shown to be a very common feature of cluster central and normal elliptical galaxies. The cooling gas is probably ultimately converted into low mass stars.

Sarazin, Craig L.↗
NASA NTRS1987

The evolution of cooling flows. I - Self-similar cluster flows

The evolution of a cooling flow from an initial state of hydrostatic equilibrium in a cluster of galaxies is investigated. After gas mass and energy are injected into the cluster at an early phase, the gas approaches hydrostatic equilibrium over most of the cluster and cooling becomes important in the dense central regions. As time passes, cooling strongly affects an increasing amount of gas. The effects of mass removal from the flow, the inclusion of magnetic or cosmic-ray pressure, and heat conduction are considered individually.

Chevalier, Roger A.↗
NASA NTRS1988

Catalog of open clusters and associated interstellar matter

The Catalog of Open Clusters and Associated Interstellar Matter summarizes observations of 128 open clusters and their associated ionized, atomic, and molecular iinterstellar matter. Cluster sizes, distances, radial velocities, ages, and masses, and the radial velocities and masses of associated interstellar medium components, are given. The database contains information from approximately 400 references published in the scientific literature before 1988.

Leisawitz, David↗