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NASA NTRS1988

X-ray emission lines from cooling flows

Techniques employing individual X-ray line strengths to estimate the rate at which gas cools in the intracluster medium are discussed. It is demonstrated that departures from ionization equilibrium in the cooling plasma are not important at X-ray temperatures. Nonrestrictive upper limits are obtained for four cooling flow clusters. The results favor the existence of substantial cooling flows and argue against the dominance of conduction or other heat sources.

Canizares, Claude R.↗

NASA NTRS1990

Einstein observations of the Hydra A cluster and the efficiency of galaxy formation in groups and clusters

The Einstein imaging proportional counter observations of the poor cluster of galaxies centered on the radio galaxy Hydra A are examined. From the surface brightness profile, it is found that the X-ray-emitting gas in the Hydra A cluster must be condensing out of the intracluster medium at a rate of 600 solar masses/yr. This is one of the largest mass deposition rates observed in a cluster of galaxies. The ratio of gas mass to stellar mass is compared for a variety of systems, showing that this ratio correlates with the gas temperature.

David, L. P.↗

NASA NTRS1990

H I absorption toward cooling flows in clusters of galaxies

An H I survey of 14 cooling flow clusters and two noncooling flow clusters was conducted, and H I absorption features were detected against the nuclear radio continuum sources of two cooling flow dominant (CFD) galaxies, 2A 0335 + 096 and MKW3s. The absorption features are broad and redshifted with respect to the stellar absorption-line velocity of the CFDs by 90-225 km/s. This indicates that the H I is falling onto, and is probably gravitationally bound to, the CFDs. The kinematics of the H I clouds suggest a possible kinematic link between the warm and cold phases of the intracluster medium. The clouds are orders of magnitude smaller in radius and mass and larger in density than Galactic H I clouds. The detected CFDs have mass-accretion rates that are about 2.5 times larger than the CFDs that were not detected.

Mcnamara, Brian R.↗

NASA NTRS1990

Stripping of gas and dust from the elliptical galaxy M86

Past observations of the x ray morphology of M86 have revealed that the galaxy is experiencing ram-pressure stripping due to its large velocity (1500 km s(-1)) relative to the intracluster medium of Virgo (Forman et al. 1979, Fabian, Schwartz, and Forman 1980). Observations indicate that the x ray emitting gas in the plume of M86 is still being produced from the continual heating of gas and dust stripped from nearer the galaxy's center. Researchers obtained two-dimensional Infrared Astronomy Satellite (IRAS) images of M86 which have revealed that there are two spatially separated regions of emission, one at 60 microns and the other at 100 microns of the IRAS wavebands. The 100 microns emission, presumably from cool dust (at approximately 20 K), appears to be located near the center of the galaxy together with HI (detected by Bregman, Roberts and Giovanelli 1988), while the 60 microns emission appears to lie more than 3 arcminutes away from the optical center in a direction slightly south of the center of the plume. Optical images produced by scanning U.K. Schmidt plates, reveal asymmetric isophotal contours along the major axis of the galaxy (first reported by Nulsen and Carter in 1987, which they propose as excess emission due to star formation). This excess optical emission is co-incident with the direction of the 60 micron infra-red emission.

White, D. A.↗

NASA NTRS1990

NGC 4438: Ram pressure sweeping of a tidally disrupted galaxy

NGC 4438 is the highly HI deficient peculiar spiral in the center of the Virgo cluster. Observations are given of the neutral hydrogen emission obtained with the Very Large Array (VLA) in the D-array configuration. These observations map out the total HI as determined from single dish measurements, and show the hydrogen to be confined to a region about one third the size of the optical disk and displaced to the side of the galaxy opposite M87. The hydrogen content of the galaxy is over an order of magnitude less than that expected for a galaxy of its type. The data suggest that the HI deficiency is a result of ram pressure stripping of the gas in the outer regions of the galaxy by the hot intracluster medium after being tidally perturbed.

Hibbard, J. E.↗

NASA NTRS1990

Clues to galaxy activity from rich cluster simulations

New simulations of rich cluster evolution are used to evaluate the first infall hypothesis of Gunn and Dressler - the idea that the enhanced fraction of active galaxies seen in high redshift clusters is due to a one-time burst of star formation triggered by the rapid rise in external pressure as a galaxy plows into the hot intracluster medium (ICM). Using three-dimensional simulations which contain both baryonic gas and collisionless dark material, local static pressure histories for test orbits of galaxies are generated and a simple trigger threshold based on dP/dt/P sub ISM is applied to define an active fraction of the population. The results lend qualitative and quantitative support to the first infall interpretation.

Evrard, August E.↗

NASA NTRS1991

X-ray emission from poor clusters of galaxies

The X-ray, optical, and radio properties of a sample of 25 poor clusters of galaxies were investigated on the basis of published observations. The X-ray emission from poor clusters was classified into morphological types depending on whether they contain a dominant galaxy or not, as was done by Jones and Forman (1984) for rich clusters. It was found that 13 of the clusters could be classified as D-type clusters, where the peak coincides with a dominant galaxy. For the five T-type clusters in this group, it was found that radio sources are detected almost three times as often as in D-type clusters, indicating that the production of radio sources in poor clusters may be dependent on the shape of the cluster potential well. The optical properties of seven clusters that were not detected at X-ray wavelengths are indistinguishable from those of the X-ray detected clusters, indicating that all poor clusters have a hot intracluster medium.

Price, Rob↗

NASA NTRS1991

Multifrequency VLA observations of PKS 0745 - 191 - The archetypal 'cooling flow' radio source?

Ninety-, 20-, 6- and 2-cm VLA observations of the high-radio-luminosity cooling-flow radio source PKS 0745 - 191 are presented. The radio source was found to have a core with a very steep spectrum (alpha is approximately -1.5) and diffuse emission with an even steeper spectrum (alpha is approximately -1.5 to -2.3) without clear indications of the jets, hotspots, or double lobes found in the other radio sources of comparable luminosity. It is inferred that the energy to power the radio source comes from the central engine, but the source's structure may be heavily influenced by the past history of the galaxy and the inflowing intracluster medium. It is shown that, while the radio source is energetically unimportant for the cluster as a whole, it is important on the scale of the cooling flow. The mere existence of cosmic rays and magnetic fields within a substantial fraction of the volume inside the cooling radius has important consequences for cooling-flow models.

Baum, S. A.↗

NASA NTRS1992

Gamma rays from clusters of galaxies

The possibility that Coma-type radio halos are powered by secondary electrons produced during the decay of energetic charge pions is examined. These pions are generated by the collision of cosmic ray protons with thermal ions in the intracluster medium. A calculation of both the cosmic ray concentration and the spectrum needed to power the halo in the Coma cluster is presented. The gamma-ray flux is predicted and it is shown that this model can be tested with the Gamma Ray Observatory Energetic Gamma Ray Experiment Telescope.

Vestrand, W. Thomas↗

NASA NTRS1993

CO deficiency in galaxies of the Fornax cluster?

There is ample observational evidence that cluster galaxies are different from those in the field. Interaction with the hot intracluster medium affects the morphology of the galaxies, their gaseous content and possibly their star-formation activity. Tidal encounters between galaxies also play an important role. The atomic component has been investigated in detail for several clusters, among them our neighbor Virgo. With the Swedish-ESO 15 m telescope, we have observed in the 12CO(1-0) transition the 23 brightest spirals and lenticulars of the Formax cluster.

Horellou, Cathy↗

NASA NTRS1993

The detection of large amounts of cool, x ray absorbing gas in distant clusters of galaxies. What does this mean?

We present an x-ray spectral study of 12 distant (z = 0.17-0.54) rich clusters of galaxies observed with the Einstein Observatory Imaging Proportional Counter. These x-ray spectral data show evidence for substantial excess absorptions beyond those expected in the galaxy, indicating the presence of large amounts of x-ray absorbing cool gas in these distant clusters. The mean value of the excess absorptions corresponds to an absorbing gas column density approximately greater than 10(exp 21)/sq cm. We calculate the x-ray luminosities of the clusters with observed fluxes only in the 0.8-3.5 keV band where the fluxes are less effected by the absorptions, and use the temperature-to-luminosity correlation (known only for nearby clusters) to estimate the temperatures of the hot intracluster medium (ICM) in the distant clusters. These temperature estimates, together with the spectral fits, provide further constraints on the column densities in the individual clusters. For the cluster CL 0016+16, the lower limit on the column density is found to be 8 x 10(exp 20)/sq cm at the 99 percent confidence limit. We also show that the ratio of the temperature obtained from the spectral fit to the temperature expected from the correlation tends to decrease with increasing look-back time, indicating possible temperature evolution of the hot ICM in the recent past. The inclusion of this evolutionary effect further increases the absorptions required in fitting the spectra.

Wang, Qingde↗

NASA NTRS1993

Can cluster environment modify the dynamical evolution of spiral galaxies?

Over the past decade many effects of the cluster environment on member galaxies have been established. These effects are manifest in the amount and distribution of gas in cluster spirals, the luminosity and light distributions within galaxies, and the segregation of morphological types. All these effects could indicate a specific dynamical evolution for galaxies in clusters. Nevertheless, a more direct evidence, such as a different mass distribution for spiral galaxies in clusters and in the field, is not yet clearly established. Indeed, Rubin, Whitmore, and Ford (1988) and Whitmore, Forbes, and Rubin (1988) (referred to as RWF) presented evidence that inner cluster spirals have falling rotation curves, unlike those of outer cluster spirals or the great majority of field spirals. If falling rotation curves exist in centers of clusters, as argued by RWF, it would suggest that dark matter halos were absent from cluster spirals, either because the halos had become stripped by interactions with other galaxies or with an intracluster medium, or because the halos had never formed in the first place. Even if they didn't disagree with RWF, other researchers pointed out that the behaviour of the slope of the rotation curves of spiral galaxies (in Virgo) is not so clear. Amram, using a different sample of spiral galaxies in clusters, found only 10% of declining rotation curves (2 declining vs 17 flat or rising) in opposition to RWF who find about 40% of declining rotation curves in their sample (6 declining vs 10 flat or rising), we will hereafter briefly discuss the Amram data paper and compare it to the results of RWF. We have measured the rotation curves for a sample of 21 spiral galaxies in 5 nearby clusters. These rotation curves have been constructed from detailed two-dimensional maps of each galaxy's velocity field as traced by emission from the Ha line. This complete mapping, combined with the sensitivity of our CFHT 3.60 m. + Perot-Fabry + CCD observations, allows the construction of high-quality rotation curves. Details concerning the acquisition and reduction procedures of the data are given in Amram. We present and discuss our preliminary analysis and compare them with RWF's results.

Amram, P.↗

NASA NTRS1992

What we learn about cooling flows through the study of the 10,000 K gas in clusters

The physical properties of the emission-line nebulae associated with central dominant galaxies in clusters are explored within the context of standard cooling-flow models of the hot ICM. It is pointed out that the properties of the 10,000 K gas are inconsistent with simple theories in which this gas has accreted out of the hot ICM in thermal instabilities within a standard cooling flow atmosphere. Several possible ramifications for our understanding of the properties of clusters and the hot intracluster medium are discussed.

Baum, Stefi A.↗

NASA NTRS1993

X-ray spectra of 12 distant clusters of galaxies

An X-ray spectral analysis of 12 distant clusters of galaxies observed on-axis with the Einstein Observatory imaging proportional counter is presented. Statistically, X-ray spectral data of the highest red shift clusters in the sample are inconsistent with an optically thin plasma emission model plus absorptions only in the Milky Way. Inclusions of excess absorptions in the clusters significantly improve the spectral fits to the data, indicating the possible presence of large amounts of X-ray absorbing cool gas in some of the distant clusters. By using X-ray luminosities determined from observed fluxes only in the 0.8-3.5 keV band and the temperature-to-luminosity correlation for nearby clusters, the estimated temperatures of the hot intracluster medium in these distant clusters constrain the absorption columns in the clusters. The data also show possible positive temperature evolution of the hot ICM with time which, if present, would further increase the absorptions required in fitting the spectra.

Wang, Qingde↗

NASA NTRS1993

When clusters collide - A numerical Hydro/N-body simulation of merging galaxy clusters

A 3D numerical simulation of two merging clusters of galaxies, using a hybrid Hydro/N-body code, is presented. The hydrodynamics of the code is solved by an Eulerian finite difference method. Initial results disclose that the X-ray emission of the dominant cluster becomes elongated and broadened; heating occurs at the core of the dominant cluster as a result of multiple shocks, and high velocity gas motions within the intracluster medium. It is predicted that clusters which have undergone recent mergers and do not have cooling flows will have high peculiar gas velocities and that the shocks and turbulence generated during the merger may power cluster-wide radio halos. Prolonged high-velocity gas motions through the dominant cluster core possibly play a major role in the formation and shaping of wide-angle tailed radio sources associated with central dominant galaxies. The N-body component of the simulation reveals the subcluster to be dispersed as it passes through the dominant cluster.

Roettiger, Kurt↗

NASA NTRS1993

An X-ray method for detecting substructure in galaxy clusters - Application to Perseus, A2256, Centaurus, Coma, and Sersic 40/6

We use the moments of the X-ray surface brightness distribution to constrain the dynamical state of a galaxy cluster. Using X-ray observations from the Einstein Observatory IPC, we measure the first moment FM, the ellipsoidal orientation angle, and the axial ratio at a sequence of radii in the cluster. We argue that a significant variation in the image centroid FM as a function of radius is evidence for a nonequilibrium feature in the intracluster medium (ICM) density distribution. In simple terms, centroid shifts indicate that the center of mass of the ICM varies with radius. This variation is a tracer of continuing dynamical evolution. For each cluster, we evaluate the significance of variations in the centroid of the IPC image by computing the same statistics on an ensemble of simulated cluster images. In producing these simulated images we include X-ray point source emission, telescope vignetting, Poisson noise, and characteristics of the IPC. Application of this new method to five Abell clusters reveals that the core of each one has significant substructure. In addition, we find significant variations in the orientation angle and the axial ratio for several of the clusters.

Mohr, Joseph J.↗

NASA NTRS1993

ROSAT observations of clusters with wide-angle tailed radio sources

The goal of these ROSAT PSPC pointed observations was to understand the nature of X-ray emission associated clusters that contain luminous wide-angle tailed (WAT) radio sources identified with the centrally dominant cluster galaxies. These 500 kpc diameter radio sources are strongly affected by confinement and interaction with the intracluster medium. So, a complete picture of the origin and evolution of these radio sources is not possible without detailed X-ray observations which sample the distribution and temperature of the surrounding hot gas. Two WAT clusters have been observed with the ROSAT PSPC to date. The first is Abell 2634 which contains the WAT 3C 465 and was approved for observations in AO-1. Unfortunately, these observations were broken into two widely separated pieces in time. The first data set containing about 9000 sec of integration arrived in mid-March, 1992. The second data set containing about 10,500 sec arrived just recently in early April (after a first tape was destroyed in the mail). The second cluster is 1919+479 which was approved for observations in AO-2. These ROSAT data arrived in October 1992.

Burns, Jack O.↗

NASA NTRS1994

Searching for cluster magnetic fields in the cooling flows of 0745-191, A2029, and A4059

We have performed sensitive polarimetric radio observations with the Very Large Array (VLA) of three galaxies: PKS 0745-191, PKS 1508+059, and PKS 2354-350, embedded in x-ray cooling flow clusters. High sensitivity, multifrequency maps of all three, along with spectral index and Faraday rotation measure (RM) maps of PKS 1508+059 and PKS 2354-350 are presented. For PKS 1508+059 and PKS 2354-350 models of the electron density of the intracluster medium (ICM) have been used to set lower limits of 0.1 and 2.7 microG, respectively, on the magnetic field in the ICM based on the observed RMs. In an x-ray selected sample of cooling flow clusters with an associated radio source, 57% (8/14) are found to have absolute RMs in excess of 800 radians/sq m. This sample includes the three sources of this study and all the other high RM sources found to date at zeta less than 0.4. These facts are consistent with the high RM phenomenon being produced by magnetic fields associated with the relatively dense, hot x-ray gas in cooling flow clusters.

Taylor, Gregory B.↗