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ABSTRACT: Eighteen of the 23 MKW and AWM poor clusters now have 10 or more measured redshifts within 1.5h−1 Mpc of the central galaxy; 11 clusters have at least 20 measured redshifts. Based on the 21 clusters for which we have sufficient velocity information, the median velocity scale is 336 km s−1, a factor of two smaller than found for rich clusters. Several of the poor clusters exhibit complex velocity distributions due to the presence of nearby clumps of galaxies. We check on the velocity of the dominant galaxy in each poor cluster relative to the remaining cluster members. Significantly high relative velocities of the dominant galaxy are found in only 4 of 21 poor clusters, 3 of which we suspect are due to contamination of the parent velocity distribution. Several statistical tests indicate that the D/cD galaxies are at the kinematic centers of the parent poor cluster velocity distributions. Mass‐to‐light ratios for the 13 of the 15 poor clusters for which we have the required data are in the range 50≤M/LB(0)≤200 M☉/L☉. © 1995 American Institute of Physics.
AIP Conference Proceedings. 06/1995; 336(1):221-230.
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ABSTRACT: Most studies of correlations between X-ray and optical properties of galaxy clusters have used the largest samples of data available, regardless of the morphological types of clusters included. Given the increasing evidence that morphology is related to a cluster's degree of dynamical evolution, we approach the study of X-ray and optical correlations differently. We evaluate the relationship between velocity dispersion and temperature for a limited set of galaxy clusters taken from Bird (1994), which all possess dominant central galaxies and which have been explicitly corrected for the presence of substructure. We find that $\sigma _r \propto T^{0.61 \pm 0.13}$. We use a Monte Carlo computer routine to estimate the significance of this deviation from the $\sigma _r \propto T^{0.5}$ relationship predicted by the virial theorem. We find that the simulated correlation is steeper than the observed value only 4\% of the time, suggesting that the deviation is significant. The combination of protogalactic winds and dynamical friction reproduces nearly exactly the observed relationship between $\sigma _r$ and $T$. Comment: 27 pages, LaTeX, requires aasms.sty and epsf.tex; accepted for publication in The Astrophysical Journal. 2 PostScript figures, as well as PostScript version of complete paper, available by anonymous ftp from ftp://kula.phsx.ukans.edu/
05/1995;
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ABSTRACT: We have obtained a 12.5 ks image of the Hercules Cluster, A2151, with the ROSAT PSPC. Comparison of the optical and X-ray emission coincides with the highest-density peak in the distribution, and is bimodal. The northern subclummp, distinct in position and velocity, has no detectable X-ray gas. The eastern subclump, apparent in the optical contour map, is indistinguishable from the clump in velocity space, but is clearly visible in the X-ray image. X-ray spectra derived from the central peak of emission yield a best-fit temperature of 1.6 keV. The emission coincident with the eastern clump of galaxies is cooler, 0.8 keV, and is outside the 90% confidence intervals of the central peak temperature. We suggest that the eastern and central subclusters have recently undergone a merger event. The lack of X-ray emission to the north suggests that those galaxies do not form a physically distinct structure (i.e., they are not located within a distinct gravitational potential), but rather that they are falling into the cluster core along the filament defined by the Hercules Supercluster.
04/1995;
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ABSTRACT: We report 472 new redshifts for 416 galaxies in the regions of the 23 poor clusters of galaxies originally identified by Morgan, Kayser, and White (MKW), and Albert, White, and Morgan (AWM). Eighteen of the poor clusters now have 10 or more available redshifts within 1.5/h Mpc of the central galaxy; 11 clusters have at least 20 available redshifts. Based on the 21 clusters for which we have sufficient velocity information, the median velocity scale is 336 km/s, a factor of 2 smaller than found for rich clusters. Several of the poor clusters exhibit complex velocity distributions due to the presence of nearby clumps of galaxies. We check on the velocity of the dominant galaxy in each poor cluster relative to the remaining cluster members. Significantly high relative velocities of the dominant galaxy are found in only 4 of 21 poor clusters, 3 of which we suspect are due to contamination of the parent velocity distribution. Several statistical tests indicate that the D/cD galaxies are at the kinematic centers of the parent poor cluster velocity distributions. Mass-to-light ratios for 13 of the 15 poor clusters for which we have the required data are in the range 50 less than or = M/L(sub B(0)) less than or = 200 solar mass/solar luminosity. The complex nature of the regions surrounding many of the poor clusters suggests that these groupings may represent an early epoch of cluster formation. For example, the poor clusters MKW7 and MKWS are shown to be gravitationally bound and likely to merge to form a richer cluster within the next several Gyrs. Eight of the nine other poor clusters for which simple two-body dynamical models can be carried out are consistent with being bound to other clumps in their vicinity. Additional complex systems with more than two gravitationally bound clumps are observed among the poor clusters.
04/1995;
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ABSTRACT: X-ray observations of Abell 548 reveal that the hot gas in this cluster is at least as complex as the galaxy distribution. Our ROSAT Position Sensitive Proportional Counter (PSPC) image is used in conjunction with optical data from the Minnesota plate-scanning project and redshift data in the literature to investigate the degree of substructure in the intracluster medium and the galaxy distribution. A548 has several X-ray components: hot gas associated with clumps of galaxies, individual sources, and a newly discovered diffuse component. This diffuse component may be promordial gas falling into the cluster for the first time. The optical data suggest that this cluster consists of four major components, not three (as indicated when velocity data are ignored). Simple models of the orbital dynamics suggest that the cluster may not yet have reached its turnaround radius.
03/1995;
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Christina M. Bird
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ABSTRACT: Although numerous studies of individual galaxy clusters have demonstrated the presence of significant substructure, previous studies of the distribution of masses of galaxy clusters determined from optical observations have failed to explicitly correct for substructure in those systems. In this {\it Letter} I present the distributions of velocity dispersion, mean separation, and dynamical masses of clusters when substructure is eliminated from the cluster datasets. I also discuss the changes in these distributions because of the substructure correction. Comparing the masses of clusters with central galaxies before and after correction for the presence of substructure reveals a significant change. This change is driven by reductions in the mean separation of galaxies, not by a decrease in the velocity dispersions as has generally been assumed. Correction for substructure reduces most significantly the masses of systems with cool X-ray temperatures, suggesting that the use of a constant linear radius (1.5$h _{100} ^{-1}$ Mpc in this study) to determine cluster membership is inappropriate for clusters spanning a range of temperatures and/or morphologies. Comment: LaTeX file; PostScript figures available via anonymous ftp at ftp://kula.phsx.ukans.edu/ . Accepted for publication in the Astrophysical Journal Letters
03/1995;
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ABSTRACT: We discuss statistical techniques for detecting and quantifying bimodality in astronomical datasets. We concentrate on the KMM algorithm, which estimates the statistical significance of bimodality in such datasets and objectively partitions data into sub-populations. By simulating bimodal distributions with a range of properties we investigate the sensitivity of KMM to datasets with varying characteristics. Our results facilitate the planning of optimal observing strategies for systems where bimodality is suspected. Mixture-modeling algorithms similar to the KMM algorithm have been used in previous studies to partition the stellar population of the Milky Way into subsystems. We illustrate the broad applicability of KMM by analysing published data on globular cluster metallicity distributions, velocity distributions of galaxies in clusters, and burst durations of gamma-ray sources. PostScript versions of the tables and figures, as well as FORTRAN code for KMM and instructions for its use, are available by anonymous ftp from kula.phsx.ukans.edu. Comment: 32 pages
08/1994;
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Christina M. Bird
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ABSTRACT: Formation theories for central dominant galaxies in clusters require them to be located at the minimum of the cluster gravitational potential. However, 32\% (8 out of 25) of the clusters with more than 50 measured redshifts have central galaxies with significant velocity offsets (with respect to other cluster members). By studying their velocity distributions and correlations between velocity and position, I show that the presence of a large peculiar velocity is strongly correlated with the presence of substructure in these massive systems. About 85\% (21 of 25) of all well-studied clusters show some evidence for substructure, in contrast to the 30-40\% found when using only galaxy or gas distributions. The correlation between substructure and central galaxy location verifies the hypothesis of Merritt (1985) and Tremaine (1990) that high peculiar velocities are indicative of recent merger events between less-massive systems of galaxies. Dynamical friction should act quickly to pull the central galaxy, the most massive discrete object in a cluster, to the minimum of the potential. The less-massive galaxies retain information about their primordial subclusters for a longer period of time. I use an objective partitioning algorithm to assign cluster galaxies to their host subclumps. When galaxies are allocated in this fashion to their subclusters, 75\% of the significant velocity offsets are eliminated. Only 2 out of the 25 clusters have central galaxies which are not centrally-located when substructure is considered in the analysis. Comment: 40 pages (text, figures and tables)
02/1994;
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ABSTRACT: There is increasing evidence that globular cluster systems around many
galaxies are comprised of two or more distinct populations. This idea is
well-established for the Milky Way, where the globular cluster system
can be separated into disk and halo components. More recent analyses of
the metallicity distributions of globular cluster systems around nearby
galaxies suggest that these systems are also comprised of two or more
components. We suggest that the metal-poor subsystems contain the only
genuine ``first generation" globulars, with secondary events such as
galaxy mergers being responsible for more metal-rich globular clusters.
We show that current data are consistent with a universal mean
metallicity for such first generation globular cluster systems. The
observation that the metallicity of globular cluster systems increases
with galaxy luminosity appears to be a result of bright galaxies having
a higher fraction of metal-rich clusters (possibly because a larger
proportion of their globulars formed in merging events). On the basis of
these results, we argue that first generation globulars form within
protogalaxies from primordial material, with self-enrichment producing
their observed metal content.
11/1993; 25:1408.
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ABSTRACT: We have obtained a 12.5 ksec image of the Hercules Cluster, A2151, with
the ROSAT PSPC. The sensitivity of the PSPC to low surface brightness
features and its large field of view (almost twice the size of the
galaxy distribution) enables us to directly compare the large-scale
morphology of the gas and galaxy distributions in this spiral-rich
irregular cluster. Comparison of the optical and X-ray data suggest the
presence of at least three distinct subclusters in A2151. The brightest
X-ray peak coincides with the highest-density peak in the galaxy
distribution. As also seen in the Einstein image of this cluster, this
peak is bimodal, with no corresponding feature in the galaxy
distribution. The northern subclump, distinct in position and velocity,
has no associated X-ray gas. The western subclump, apparent in the
optical contour map is indistinguishable from the main clump in velocity
space and is only a 2sigma enhancement of the X-ray background, The
three group partition of the optical data is a significant improvement
over the one group fit, at a >99.9% confidence level. X-ray spectra
derived from the central peak of emission yield a best-fit temperature
of 1.6 keV. The emission coincident with the western clump of galaxies
is cooler, 0.8 keV, and is outside the 90% confidence intervals of the
central peak temperature. Within these groups there is little evidence
for morphological segregation. This may imply that the subgroups were
well-mixed before the merger which formed the rich cluster.
11/1993; 25:1297.
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ABSTRACT: We present an analysis of the positions and velocities of M31 globular
clusters, which indicates the presence of significant substructure in
the globular cluster system. We suggest this clustering is the surviving
signature of gaseous clumps out of which the halo of M31 formed. A
mixture-modeling analysis of the metallicity distribution of the M31
globulars indicates that, like the Milky Way, the metallicity
distribution is bimodal. We show that the metallicity of genuine halo
clusters in the Milky Way and M31 is similar to that of globulars around
nearby dwarf galaxies. This suggests that such clsuters formed from
primordial material, whereas the disk globulars in the Galaxy and M31,
as well as metal-rich globulars in elliptical galaxies, formed in some
secondary event. Implications of these findings for formation models of
galaxies and globular clusters are discussed.
The Astronomical Journal 11/1993; 106:2281-2290. · 4.03 Impact Factor
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ABSTRACT: We present alternative measures to the classical coefficients of
skewness and kurtosis, called the asymmetry and tail indices, for
comparing the shapes of one-dimensional velocity distributions. We then
apply these estimators to the Dressler and Shectman catalogs of clusters
of galaxies to compare the dynamical evolution of these systems.
The Astronomical Journal 03/1993; 105:1596-1606. · 4.03 Impact Factor
