Strong Lensing Clusters Research Articles

The observed concentration-mass relation for galaxy clusters

The properties of clusters of galaxies offer key insights into the assembly process of structure in the universe. Numerical simulations of cosmic structure formation in a hierarchical, dark matter dominated universe suggest that galaxy cluster concentrations, which are a measure of a halo's central density, decrease gradually with virial mass. However, cluster observations have yet to confirm this correlation. The slopes of the run of measured concentrations with virial mass are often either steeper or flatter than predicted by simulations. In this work, we present the most complete sample of observed cluster concentrations and masses yet assembled, including new measurements for 10 strong lensing clusters, thereby more than doubling the existing number of strong lensing concentration estimates. We fit a power law to the observed concentrations as a function of virial mass, and find that the slope is consistent with the slopes found in simulations, though our normalization factor is higher. Observed lensing concentrations appear to be systematically larger than X-ray concentrations, a more pronounced effect than found in simulations. We also find that at fixed mass, the bulk of observed cluster concentrations are distributed log-normally, with the exception of a few anomalously high concentration clusters. We examine the physical processes likely responsible for the discrepancy between lensing and X-ray concentrations, and for the anomalously high concentrations in particular. The forthcoming Millennium simulation results will offer the most comprehensive comparison set to our findings of an observed concentration-mass power law relation.

Probing the star formation history using the redshift evolution of luminosity functions

We present a self-consistent, semi-analyticalCDM model of star formation and reioniza- tion. For the cosmological parameters favored by the WMAP data, our models consistently reproduce the electron scattering optical depth to reioniz ation, redshift of reionization and the observed luminosity functions (LF) and hence the star formation rate (SFR) density at 3 ≤ z ≤ 6 for a reasonable range of model parameters. While simple photoionization feed- back produces the correct shape of LF at z = 6, for z = 3 we need additional feedback that suppresses star formation activities in halos with 10 10 . (M/M⊙) . 10 11 . Models with prolonged continuous star formation activities are prefer red over those with short bursts as they are consistent with the existence of a Balmer break in considerable fraction of observed galaxies even at z ∼ 6. The halo number density evolution from the standardCDM structure formation model that fits LF up to z = 6 is consistent with the upper limits on z ≃ 7 LF and source counts at 8 ≤ z ≤ 12 obtained from the Hubble Ultra Deep Field (HUDF) observations without requiring any dramatic change in the nature of star formation. However, to reproduce the observed LF at 6 ≤ z ≤ 10, obtained from the near-IR observations around strong lensing clusters, we need a strong evolution in the initial mass func tion, reddening correction and the mode of star formation at z & 8. We show that low mass molecular cooled halos, which may be important for reionizing the universe, are not detectabl e in the present deep field obser- vations even if a considerable fraction of its baryonic mass goes through a star burst phase. However, their presence and contribution to reionization can be inferred indirectly from the redshift evolution of the luminosity function in the redshi ft range 6 ≤ z ≤ 12. In our model calculations, the contribution of low mass halos to global SFR density prior to reionization reveals itself in the form of second peak at z ≥ 6. However this peak will not be visible in the observed SFR density as a function of z as most of these galaxies have luminosity below the detection threshold of various ongoing deep field survey s. Accurately measuring the LF at high redshifts can be used to understand the nature of star formation in the dark ages and probe the history of reionization.

Gemini andChandraObservations of Abell 586, A Relaxed Strong‐lensing Cluster

We analyze the mass content of the massive strong-lensing cluster Abell 586 (z = 0.17). We use optical data (imaging and spectroscopy) obtained with the Gemini Multi-Object Spectrograph (GMOS) mounted on the 8 m Gemini North telescope, together with publicly available X-ray data taken with the Chandra space telescope. Employing different techniques—velocity distribution of galaxies, weak gravitational lensing, and spatially resolved X-ray spectroscopy—we derive mass and velocity dispersion estimates from each of them. All estimates agree well with each other, within a 68% confidence level, indicating a velocity dispersion of 1000-1250 km s-1. The projected mass distributions obtained through weak lensing and X-ray emission are strikingly similar, having nearly circular geometry. We suggest that Abell 586 is probably a truly relaxed cluster whose last major merger occurred more than ~4 Gyr ago.

Noise in Strong Lensing Cosmography

Giant arcs in strong lensing galaxy clusters can provide a purely geometric determination of cosmological parameters, such as the dark energy density and equation of state. We investigate sources of noise in cosmography with giant arcs, focusing in particular on errors induced by density fluctuations along the line of sight and errors caused by modeling uncertainties. We estimate parameter errors in two independent ways, first by developing a Fisher matrix formalism for strong lensing parameters and next by directly ray-tracing through N-body simulations using a multiplane lensing code. We show that for reasonable power spectra, density fluctuations from large-scale structures produce >100% errors in cosmological parameters derived from any single sight line, precluding the use of individual clusters or golden lenses to derive accurate cosmological constraints. Modeling uncertainties can similarly lead to large errors, and we show that the use of parameterized mass models in fitting strong lensing clusters can significantly bias the inferred cosmological parameters. We lastly speculate on the means by which these errors may be corrected.

The Incidence of Strong‐Lensing Clusters in the Red‐Sequence Cluster Survey

The incidence of giant arcs due to strong-lensing clusters of galaxies is known to be discrepant with current theoretical expectations. This result derives from a comparison of several cluster samples with predictions in the framework of the currently favored ΛCDM cosmology, and one possible explanation for the discrepancy is that this cosmological model is not correct. In this paper we discuss the incidence of giant arcs in the Red-Sequence Cluster Survey (RCS), which again shows significant disagreement with theoretical predictions. We briefly describe a total of eight strong-lens systems, seven of which are discussed here for the first time. Based on the details of these systems, in particular on the ratio of single- to multiple-arc systems, we argue that it may be possible to explain this discrepancy in the currently favored cosmology by modifying the details of the lenses themselves. Specifically, the high incidence of multiple-arc systems and their overall high redshift suggests that a subpopulation of the global cluster population is responsible for much of the observed lensing. The lack of lensing clusters at z < 0.64 in the RCS indicates that a property associated with clusters at early times results in the boosted lensing cross sections; likely, a combination of ellipticity and elongation along the line of sight, substructure, and changes in the cluster-mass profiles is responsible. Cluster mass, which should evolve to globally higher values toward lower redshifts, is clearly not the most significant consideration for the formation of giant arcs.

Constraining the cosmological parameters using strong lensing

We investigate the potentiality of using strong lensing clusters to constrain the cosmological parameters m and �. The existence of a multiple image system with known redshift allows, for a given (m, �) cosmol- ogy, absolute calibration of the total mass deduced from lens modelling. Recent Hubble Space Telescope (HST) observations of galaxy clusters reveal a large number of multiple images, which are predicted to be at different redshifts. If it is possible to measure spectroscopically the redshifts of many multiple images then one can in prin- ciple constrain (m, �) through ratios of angular diameter distances, independently of any external assumptions. For a regular/relaxed cluster observed by HST with 3 multiple image systems, each with different spectroscopic redshifts, we show by analytic calculation that the following uncertainties can be expected: m = 0.30 ± 0.11, � = 0.70 ±0.23 or m = 1.00 ±0.17, � = 0.00 ±0.48 for the two most popular world models. Numerical tests on simulated data confirm these good constraints, even in the case of more realistic cluster potentials, such as bimodal clusters, or when including perturbations by galaxies. To investigate the sensitivity of the method to dif- ferent mass profiles, we also use an analytic pseudo-elliptical Navarro, Frenk & White profile in the simulations. These constraints can be improved if more than 3 multiple images with spectroscopic redshifts are observed, or by combining the results from different clusters. Some prospects on the determination of the cosmological parameters with gravitational lensing are given.

Discovery of a [CLC]z[/CLC] = 0.77 Galaxy Cluster with Multiple, Bright, Strong-Lensing Arcs

We report the discovery of a remarkable strong-lensing cluster from the ongoing Red-Sequence Cluster Survey (RCS). RCS 0224-0002, at a spectroscopic redshift of 0.773, lenses two to four separate background sources. These arcs are of relatively high surface brightness and hence amenable to detailed follow-up study. We provide preliminary photometry and spectroscopy of RCS 0224-0002 and discuss in detail the one lensed source for which a redshift has so far been determined. This galaxy, at a redshift of 4.8786, appears to be composed of a star-forming core enveloped in a Lyα emitting cloud. Moreover, it shows no measurable velocity structure to a limit of ~50 km s-1 over a region likely to be ~1 kpc in size. The available data are also used to develop an initial lensing model, which shows RCS 0224-0002 to have a mass corresponding to a central velocity dispersion of ~1000 km s-1, consistent with the measured optical richness of the cluster. This preliminary model is also used to estimate redshifts for all lensed components visible in our ground-based imaging. RCS 0224-0002 is the highest-redshift cluster to show lensing with such a large number of bright arcs (comparable to the best lensing clusters at much lower redshifts), and the highest-redshift strong-lensing cluster for which both the cluster redshift and at least one source redshift have been established spectroscopically, and as such it provides a powerful opportunity to directly measure the geometry of the universe and galaxy properties at high redshift.

Constraints on the asymptotic baryon fractions of galaxy clusters at large radii

While X-ray measurements have so far revealed an increase in the volume-averaged baryon fractions $f_b(r)$ of galaxy clusters with cluster radii $r$, $f_b(r)$ should asymptotically reach a universal value $f_b(\infty)=f_b$, provided that clusters are representative of the Universe. In the framework of hydrostatic equilibrium for intracluster gas, we have derived the necessary conditions for $f_b(\infty)=f_b$: The X-ray surface brightness profile described by the $\beta$ model and the temperature profile approximated by the polytropic model should satisfy $\gamma\approx2(1-1/3\beta)$ and $\gamma\approx1+1/3\beta$ for $\beta 1$, respectively, which sets a stringent limit to the polytropic index: $\gamma 2/3$ breaks down at rather large radii ($\ga30r_c$) where hydrostatic equilibrium has probably become inapplicable. We further explore how to obtain the asymptotic value $f_b(\infty)$ of baryon fraction from the X-ray measurement made primarily over the finite central region of a cluster. We demonstrate our method using a sample of 19 strong lensing clusters, which enables us to place a useful constraint on $f_b(\infty)$: $0.094\pm0.035 \leq f_b(\infty) \leq 0.41\pm0.18$. An optimal estimate of $f_b(\infty)$ based on three cooling flow clusters with $\beta = 0.142\pm0.007$ or $\Omega_M = 0.35\pm0.09$.