Cluster Mass Measurements Research Articles

Kinematic effect in gravitational lensing by clusters of galaxies

Gravitational lensing provides an efficient tool for the investigation of matter structures, independent of the dynamical or the hydrostatic equilibrium properties of the deflecting system. However, it depends on the kinematic status. In fact, either a translational motion or a coherent rotation of the mass distribution can affect the lensing properties. Here, light deflection by galaxy clusters in motion is considered. Even if gravitational lensing mass measurements of galaxy clusters are regarded as very reliable estimates, the kinematic effect should be considered. A typical peculiar motion with respect to the Hubble flow brings about a systematic error ≤0.3 per cent, independent of the mass of the cluster. On the other hand, the effect of the spin increases with the total mass. For cluster masses ∼10 15 M ⊙ , the effect of the gravitomagnetic term is ≤0.04 per cent on strong lensing estimates and ≤0.5 per cent in the weak-lensing analyses. The total kinematic effect on the mass estimate is then ≤1 per cent, which is negligible in current statistical studies. In the weak-lensing regime, the rotation imprints a typical angular modulation in the tangential shear distortion. This would allow, in principle, a detection of the gravitomagnetic field and a direct measurement of the angular velocity of the cluster but the required background source densities are well beyond current technological capabilities.

Dark energy and the mass of galaxy clusters

Up to now, dark energy evidences are based on the dynamics of the universe on very large scales, above 1 Gpc. Assuming it continues to behave like a cosmological constant $\ensuremath{\Lambda}$ on much smaller scales, I discuss its effects on the motion of nonrelativistic test particles in a weak gravitational field, and I propose a way to detect evidences of $\ensuremath{\Lambda}\ensuremath{\ne}0$ at the scale of about 1 Mpc: the main ingredient is the measurement of galaxy cluster masses.

Impact of systematic errors in Sunyaev–Zel’dovich surveys of galaxy clusters

Future high-resolution microwave background measurements hold the promise of detectinggalaxy clusters throughout our Hubble volume through their Sunyaev–Zel’dovich (SZ)signature, down to a given limiting flux. The number density of galaxy clusters is highlysensitive to cluster mass through fluctuations in the matter power spectrum,as well as redshift through the comoving volume and the growth factor. Thissensitivity in principle allows tight constraints on such quantities as the equation ofstate of dark energy and the neutrino mass. We evaluate the ability of futurecluster surveys to measure these quantities when combined with Planck-like CMBdata. Using a simple effective model for uncertainties in the cluster mass–SZ fluxrelation, we evaluate systematic shifts in cosmological constraints from cluster SZsurveys. We find that a systematic bias of 10% in cluster mass measurements cangive rise to shifts in cosmological parameter estimates at levels larger than the1σ statistical errors. Systematic errors are unlikely to be detected from the massand redshift dependence of cluster number counts alone; increasing survey sizehas only a marginal effect. Implications for upcoming experiments are discussed.

Tracing cosmic evolution with clusters of galaxies

The most successful cosmological models to date envision structure formation as a hierarchical process in which gravity is constantly drawing lumps of matter together to form increasingly larger structures. Clusters of galaxies currently sit atop this hierarchy as the largest objects that have had time to collapse under the influence of their own gravity. Thus their appearance on the cosmic scene is also relatively recent. Two features of clusters make them uniquely useful tracers of cosmic evolution. First, clusters are the biggest things whose masses we can reliably measure because they are the largest objects to have undergone gravitational relaxation and entered into virial equilibrium. Mass measurements of nearby clusters can therefore be used to determine the amount of structure in the universe on scales of ${10}^{14}--{10}^{15}{M}_{\astrosun{}}$, and comparisons of the present-day cluster mass distribution with the mass distribution at earlier times can be used to measure the rate of structure formation, placing important constraints on cosmological models. Second, clusters are essentially ``closed boxes'' that retain all their gaseous matter, despite the enormous energy input associated with supernovae and active galactic nuclei, because the gravitational potential wells of clusters are so deep. The baryonic component of clusters therefore contains a wealth of information about the processes associated with galaxy formation, including the efficiency with which baryons are converted into stars and the effects of the resulting feedback processes on galaxy formation. This article reviews our theoretical understanding of both the dark-matter component and the baryonic component of clusters, providing a context for interpreting the flood of new cluster observations that are now arriving from the latest generation of x-ray observatories, large optical surveys, and measurements of cluster-induced distortions in the spectrum of the cosmic microwave background.

Cluster gas temperature maps with ASCA and beyond

During the past several years, ASCA has nearly completed mapping of gas temperatures in bright nearby clusters of galaxies. In several clusters, it discovered strong and irregular spatial temperature variations indicative of ongoing violent subcluster mergers. In other systems, the temperature distribution appears symmetric, indicating a relaxed state which is a necessary condition for reliable hydrostatic total mass measurements. I present a (very incomplete) review of these results, discuss their implications for understanding of physical processes in the intracluster gas and for the cluster mass measurements. I also mention some interesting studies that will become possible with the forthcoming Chandra X-ray observatory.

A New Weak-Lensing Analysis of MS 1224.7+2007

Galaxy cluster mass distributions are useful probes of Ω0 and the nature of the dark matter. Large clusters will distort the observed shapes of background galaxies through gravitational lensing, allowing the measurement of the cluster mass distributions. For most cases, the agreement between weak-lensing and radial velocity mass measurements of clusters is reasonably good. There is, however, one significant exception, the z = 0.32 cluster MS 1224.7+2007, which has a lensing mass substantially greater than the virial mass and, also, a very high mass-to-light ratio. Since this controversial object might be an unusually dark mass, a follow-up study is definitely warranted. In this paper we study the mass and light distributions of MS 1224.7+2007 out to a projected radius of 800 h-1 kpc by measuring the gravitationally induced distortions of background galaxies. We detect a shear signal in the background galaxies in the radial range 275 ≤ r ≤ 275'' at the 5.5 σ level. The resultant mass map exhibits a peak centered on the dominant cluster galaxy and strong evidence for substructure, which is seen even more strongly in the galaxy distribution. Assuming that all the detected shear is due to mass at z = 0.32, we find a cluster mass-to-light ratio of M/LR = 640 ± 150 (M/LR)⊙. The mass profile is quite flat compared with other clusters, disagreeing with a pseudo–singular isothermal sphere at the 95% confidence level. Our mass and M/L estimates are consistent with the previous weak-lensing result. The discrepancy between the lensing and virial mass remains, although it might be partially explained by subclustering and infall perpendicular to the line of sight. This cluster remains a candidate dark object deficient in baryons and, as such, severely tests cosmological models.

Measurement of cluster masses up to 107 amu arising as a result of inelastic sputtering of thin layers by ions

A special mass spectrograph is developed to detect simultaneously ions within a mass range of 102–102 amu. It is shown that a fast heavy ion (fission fragment) can raise Au clusters having masses up to 107 amu (up to 150 Å in size) from the substrate.