496 research outputs found
Using x ray images to detect substructure in a sample of 40 Abell clusters
Using a method for constraining the dynamical state of a galaxy cluster by examining the moments of its x-ray surface brightness distribution, we determine the statistics of cluster substructure for a sample of 40 Abell clusters. Using x-ray observations from the Einstein Observatory Imaging Proportional Counter (IPC), we measure the first moment M1(r), the ellipsoidal orientation angle theta2(r), and the axial ratio eta(r) at several different radii in the cluster. We determine the effects of systematics such as x-ray point source emission, telescope vignetting, Poisson noise, and characteristics of the IPC by measuring the same parameters on an ensemble of simulated cluster images. Due to the small band-pass of the IPC, the ICM emissivity is nearly independent of temperature so the intensity at each point in the IPC images is simply proportional to the emission measure calculated along the line of sight through the cluster (e.g. Fabricant et al. 1980). Therefore, barring a change superposition of two x-ray emitting clusters, a significant variation in the image centroid M1(r) as a function of radius indicates that the center of mass of the intra-cluster medium (ICM) varies with radius. We argue that such a configuration (essentially an m = 1 component in the ICM density distribution) is a non-equilibrium component; it results from an off-center subclump or a recent merger in the ICM
Genetic Map of Bacteriophage [var phi]X174
Bacteriophage [var phi]X174 temperature-sensitive and nonsense mutations in eight cistrons were mapped by using two-, three-, and four-factor genetic crosses. The genetic map is circular with a total length of 24 × 10−4wt recombinants per progeny phage. The cistron order is D-E-F-G-H-A-B-C. High negative interference is seen, consistent with a small closed circular deoxyribonucleic acid molecule as a genome
A Broad Search for Counterrotating Gas and Stars: Evidence for Mergers and Accretion
We measure the frequency of bulk gas-stellar counterrotation in a sample of
67 galaxies drawn from the Nearby Field Galaxy Survey, a broadly representative
survey of the local galaxy population down to M_B-15. We detect 4
counterrotators among 17 E/S0's with extended gas emission (24% +8 -6). In
contrast, we find no clear examples of bulk counterrotation among 38 Sa-Sbc
spirals, although one Sa does show peculiar gas kinematics. This result implies
that, at 95% confidence, no more than 8% of Sa-Sbc spirals are bulk
counterrotators. Among types Sc and later, we identify only one possible
counterrotator, a Magellanic irregular. We use these results together with the
physical properties of the counterrotators to constrain possible origins for
this phenomenon.Comment: 19 pages, 4 figures, AJ, accepte
An Optical/X--ray Study of A576, a Galaxy Cluster with a Cold Core
We analyze the galaxy population and dynamics of the galaxy cluster Abell 576
using 281 redshifts (230 new), R band CCD galaxy photometry over a 2h^{-1} Mpc
by 2h^{-1} Mpc region centered on the cluster, an Einstein IPC image, and an
Einstein MPC spectrum. The cluster galaxies with emission lines in their
spectra have a larger velocity dispersion and are significantly less clustered
on this 2h^{-1} Mpc scale than galaxies without emission lines. We show that
excluding the emission line galaxies decreases the velocity dispersion by 18%
and the virial mass estimate by a factor of two. The central cluster region
contains a non--emission galaxy population and an intracluster medium which are
significantly cooler (sigma_{core}=387^{+250}_{-105}~km/s and
T_X=1.6^{+0.4}_{-0.3}~keV at 90\% confidence) than the global populations
(sigma=977^{+124}_{-96}~km/s for the non--emission population and T_X>4~keV at
90% confidence). Because (1) the low dispersion population is no more luminous
than the global population and (2) the evidence for a cooling flow is weak, we
suggest that the cluster core may contain the remnants of a lower mass
subcluster. We examine the cluster mass, baryon fraction and luminosity
function. The cluster virial mass varies significantly depending on the galaxy
sample used. Consistency between the hydrostatic and virial estimators can be
achieved if (1) the gas temperature at r\sim1h^{-1}~Mpc is T_X\sim8keV (the
best fit value) and (2) several velocity outliers are excluded from the virial
calculation. Although the best fit Schechter function parameters and the ratio
of galaxy to gas mass are typical of other clusters, the baryon fraction is
relatively low. Using a lower limit to the binding mass, we show that the gas
mass fraction is \le3h^{-3/2}% and the baryon fraction is \le6%.Comment: 34 pages postscript, accepted for publication in Ap
Cosmological constraints from cluster x-ray morphologies
We use a representative sample of 65 galaxy clusters observed with the Einstein IPC to constrain the range of cluster X-ray morphologies. We develop and apply quantitative and reproducible measures to constrain the intrinsic distributions of emission weighted centroid variation, axial ratio, orientation, and the radial fall--off. We then use the range of cluster X--ray morphologies to constrain three generic cosmological models (Omega=1, Omega_0=0.2, and Omega_0=0.2 & lambda_0=0.8). For each of these models, we evolve eight sets of Gaussian random initial conditions consistent with an effective power spectrum P(k)\propto k^{-1} on cluster scales. Using this sample of 24 numerical cluster simulations, we compare the X--ray morphologies of the observed and simulated clusters. The comparisons indicate that: (i) cluster centroid variations, axial ratios, and radial fall-offs are sensitive to the underlying cosmological model, and (ii) galaxy clusters with the observed range of X--ray morphologies are very unlikely in low Omega_0 cosmologies. The analysis favors the Omega=1 model, though some discrepancies remain. We discuss the effects of changing the initial conditions and including additional physics in the simulations
- …