Light Traces Mass Research Articles

Brightest Cluster Galaxy Offsets in Cold Dark Matter

The distribution of offsets between the brightest cluster galaxies of galaxy clusters and the centroid of their dark matter distributions is a promising probe of the underlying dark matter physics. In particular, since this distribution is sensitive to the shape of the potential in galaxy cluster cores, it constitutes a test of dark matter self-interaction on the largest mass scales in the universe. We examine these offsets in three suites of modern cosmological simulations; IllustrisTNG, MillenniumTNG and BAHAMAS. For clusters above , we examine the dependence of the offset distribution on gravitational softening length, the method used to identify centroids, redshift, mass, baryonic physics, and establish the stability of our results with respect to various nuisance parameter choices. We find that offsets are overwhelmingly measured to be smaller than the minimum converged length scale in each simulation, with a median offset of in the highest resolution simulation considered, TNG300-1, which uses a gravitational softening length of . We also find that centroids identified via source extraction on smoothed dark matter and stellar particle data are consistent with the potential minimum, but that observationally relevant methods sensitive to cluster strong gravitational lensing scales, or those using the the “light traces mass” approach, in this context meaning gas is used as a tracer for the potential, can overestimate offsets by factors of ∼10 and ∼30, respectively. This has the potential to reduce tensions with existing offset measurements which have served as evidence for a nonzero dark matter self-interaction cross section.

MARS: A New Maximum-entropy-regularized Strong Lensing Mass Reconstruction Method

Free-form strong-lensing (SL) mass reconstructions typically suffer from overfitting, which manifests itself as false-positive small-scale fluctuations. We present a new free-form MAximum-entropy ReconStruction (MARS) method without the assumption that light traces mass (LTM). The MARS algorithm enables us to achieve excellent convergence in source positions (∼0.″001), minimize spurious small-scale fluctuations, and provide a quasi-unique solution independently of initial conditions. Our method is tested with the publicly available synthetic SL data FF-SIMS. The comparison with the truth shows that the mass reconstruction quality is on par with those of the best-performing LTM methods published in the literature, which have been demonstrated to outperform existing free-form methods. In terms of the radial mass profile reconstruction, we achieve <1% agreement with the truth for the regions constrained by multiple images. Finally, we apply MARS to A1689 and find that the cluster mass in the SL regime is dominated by the primary halo centered on the brightest cluster galaxy and the weaker secondary halo is also coincident with the bright cluster member ∼160 kpc northeast. Within the SL field, the A1689 radial profile is well described by a Navarro–Frenk–White profile with c 200 = 5.53 ± 0.77 and kpc, and we find no evidence that A1689 is overconcentrated.

Lessons from the First Multiply Imaged Supernova: Revised Strong-lensing Models for the Galaxy Cluster MACS J1149.5+2223

Abstract We present two grid-based lens models for the galaxy cluster MACS J1149.5+2223, concentrating on the properties of the first multiply imaged supernova Refsdal, which first appeared in this cluster as an Einstein cross (images S1–S4) around a cluster galaxy in late 2014, and about a year later as an additional image (SX) in a different image of the supernova (SN) host galaxy. One model is our updated light-traces-mass (LTM) strong-lensing model for this cluster, and the second is a parametric model (dPIEeNFW), generated using the same pipeline but with a different parameterization. Together these two models probe different possible solutions in a relatively self-consistent manner and can be used to examine systematic uncertainties and relevant differences between the two parameterizations. We obtain reasonably similar (agreeing to within ≃1σ–3σ, in most cases) time delays and magnification ratios, with respect to S1, from the two different methods, although the LTM predictions seem to be systematically shorter/smaller for some of the images. Most notably, the time delay [and 95% CI] between the Einstein cross (in particular, image S1), and SX, differs substantially between the parametric method (326 [300–359] days) and the LTM method (224 [198–306] days), which seems to underestimate the true reappearance time. The cause for this systematic difference is unclear at present. We speculate on its possible origin and note that a refined measurement of SN Refsdal’s properties should help to more strongly discriminate between the two solutions, and thus between the two descriptions for the intrinsic shape of the underlying matter distribution. We also discuss the implications of our results for the Hubble constant.

AStroLens: automatic strong-lens modelling of X-ray selected galaxy clusters

ABSTRACT We use AStroLens, a newly developed gravitational lens-modelling code that relies only on geometric and photometric information of cluster galaxies as input, to map the strong-lensing regions and estimate the lensing strength of 96 galaxy clusters at z = 0.5–0.9. All clusters were identified during the extended Massive Cluster Survey (eMACS) based on their X-ray flux and optical appearance. Building on the well-tested assumption that the distribution of both luminous and dark matter in galaxy clusters is approximately traced by the distribution of light, i.e. that light traces mass, AStroLens uses three global parameters to automatically model the deflection from strong-gravitational lensing for all galaxy clusters in this diverse sample. We test the robustness of our code by comparing AStroLens estimates derived solely from shallow optical images in two passbands with the results of in-depth lens-modelling efforts for two well-studied eMACS clusters and find good agreement, both with respect to the size and the shape of the strong-lensing regime delineated by the respective critical lines. Our study finds 31 eMACS clusters with effective Einstein radii (θE) in excess of 20″ and eight with θE &amp;gt; 30″, thereby underlining the value of X-ray selection for the discovery of powerful cluster lenses that complement giants like MACSJ0717 at ever-increasing redshift. As a first installment towards the public release of the eMACS sample, we list physical properties of the 10 calibration clusters as well as of the 10 most powerful eMACS cluster lenses, according to AStroLens.

EasyCritics – I. Efficient detection of strongly lensing galaxy groups and clusters in wide-field surveys

We present EasyCritics, an algorithm to detect strongly-lensing groups and clusters in wide-field surveys without relying on a direct recognition of arcs. EasyCritics assumes that light traces mass in order to predict the most likely locations of critical curves from the observed fluxes of luminous red early-type galaxies in the line of sight. The positions, redshifts and fluxes of these galaxies constrain the idealized gravitational lensing potential as a function of source redshift up to five free parameters, which are calibrated on few known lenses. From the lensing potential, EasyCritics derives the critical curves for a given, representative source redshift. The code is highly parallelized, uses fast Fourier methods and, optionally, GPU acceleration in order to process large datasets efficiently. The search of a $\smash{1 \, \mathrm{deg}^2}$ field of view requires less than 1 minute on a modern quad-core CPU, when using a pixel resolution of $0.25''/\mathrm{px}$. In this first part of a paper series on EasyCritics, we describe the main underlying concepts and present a first demonstration on data from the Canada-France-Hawaii-Telescope Lensing Survey. We show that EasyCritics is able to identify known group- and cluster-scale lenses, including a cluster with two giant arc candidates that were previously missed by automated arc detectors.

Combining spectroscopic and photometric surveys using angular cross-correlations – III. Galaxy bias and stochasticity

In the first paper of this series, we studied the effect of baryon acoustic oscillations (BAO), redshift space distortions (RSD) and weak lensing (WL) on measurements of angular cross-correlations in narrow redshift bins. Paper-II presented a multitracer forecast as Figures of Merit (FoM), combining a photometric and spectroscopic stage-IV survey. The uncertainties from galaxy bias, the way light traces mass, is an important ingredient in the forecast. Fixing the bias would increase our FoM equivalent to 3.3 times larger area for the combined constraints. This paper focus on how the modelling of bias affect these results. In the combined forecast, lensing both help and benefit from the improved bias measurements in overlapping surveys after marginalizing over the cosmological parameters. Adding a second lens population in counts-shear does not have a large impact on bias error, but removing all counts-shear information increases the bias error in a significant way. We also discuss the relative impact of WL, magnification, RSD and BAO, and how results change as a function of bias amplitude, photo-z error and sample density. By default we use one bias parameter per bin (with 72 narrow bins), but we show that the results do not change much when we use other parameterizations, with at least 3 parameters in total. Bias stochasticity, even when added as one new free parameter per bin, only produce moderate decrease in the FoM. In general, we find that the degradation in the figure of merit caused by the uncertainties in the knowledge of bias is significantly smaller for overlapping surveys.

STRONG-LENSING ANALYSIS OF THE POWERFUL LENSING CLUSTER MACS J2135.2-0102 (z = 0.33)

ABSTRACT We present a light-traces-mass (LTM) strong-lensing model of the massive lensing cluster MACS J2135.2-0102 (z = 0.33; hereafter MACS2135), known in part for hosting the cosmic eye galaxy lens. MACS2135 is also known to multiply lens a z = 2.3 sub-millimeter galaxy near the brightest cluster galaxy (BCG), as well as a prominent, triply imaged system at a large radius of ∼37″ south of the BCG. We use the latest available Hubble imaging to construct an accurate lensing model for this cluster, identifying six new multiply imaged systems with the guidance of our LTM method, so that we have roughly quadrupled the number of lensing constraints. We determine that MACS2135 is among the top lensing clusters known, comparable in size to the Hubble Frontier Fields. For a source at , we find an effective Einstein radius of , enclosing . We make our lens model, including mass and magnification maps, publicly available, in anticipation of searches for high-z galaxies with the James Webb Space Telescope, for which this cluster is a compelling target.

Quantifying substructures inHubbleFrontier Field clusters: comparison with ΛCDM simulations

The Hubble Frontier Fields (HFF) are six clusters of galaxies, all showing indications of recent mergers, which have recently been observed for lensed images. As such they are the natural laboratories to study the merging history of galaxy clusters. In this work, we explore the 2D power spectrum of the mass distribution $P_{\rm M}(k)$ as a measure of substructure. We compare $P_{\rm M}(k)$ of these clusters (obtained using strong gravitational lensing) to that of $\Lambda$CDM simulated clusters of similar mass. To compute lensing $P_{\rm M}(k)$, we produced free-form lensing mass reconstructions of HFF clusters, without any light traces mass (LTM) assumption. The inferred power at small scales tends to be larger if (i)~the cluster is at lower redshift, and/or (ii)~there are deeper observations and hence more lensed images. In contrast, lens reconstructions assuming LTM show higher power at small scales even with fewer lensed images; it appears the small scale power in the LTM reconstructions is dominated by light information, rather than the lensing data. The average lensing derived $P_{\rm M}(k)$ shows lower power at small scales as compared to that of simulated clusters at redshift zero, both dark-matter only and hydrodynamical. The possible reasons are: (i)~the available strong lensing data are limited in their effective spatial resolution on the mass distribution, (ii)~HFF clusters have yet to build the small scale power they would have at $z\sim 0$, or (iii)~simulations are somehow overestimating the small scale power.

Systematic or signal? How dark matter misalignments can bias strong lensing models of galaxy clusters

We explore how assuming that mass traces light in strong gravitational lensing models can lead to systematic errors in the predicted position of multiple images. Using a model based on the galaxy cluster MACSJ0416 (z = 0.397) from the Hubble Frontier Fields, we split each galactic halo into a baryonic and dark matter component. We then shift the dark matter halo such that it no longer aligns with the baryonic halo and investigate how this affects the resulting position of multiple images. We find for physically motivated misalignments in dark halo position, ellipticity, position angle and density profile, that multiple images can move on average by more than 0.2" with individual images moving greater than 1". We finally estimate the full error induced by assuming that light traces mass and find that this assumption leads to an expected RMS error of 0.5", almost the entire error budget observed in the Frontier Fields. Given the large potential contribution from the assumption that light traces mass to the error budget in mass reconstructions, we predict that it should be possible to make a first significant detection and characterisation of dark halo misalignments in the Hubble Frontier Fields with strong lensing. Finally, we find that it may be possible to detect ~1kpc offsets between dark matter and baryons, the smoking gun for self-interacting dark matter, should the correct alignment of multiple images be observed.

THE KINEMATICAL CENTER AND MASS PROFILE OF THE LOCAL GROUP

Abandoning the assumption that light traces mass, I seek the location of the center of the Local Group of galaxies based solely on kinematic data and the plausible assumption of infall. The available set of positions and radial velocities is shown to be a misleading indicator of Local Group motions, giving a direction to the center offset from the true one; statistical techniques of moderate sophistication do not catch the offset. Corrected calculations show the center to lie in the direction to M31 within the uncertainty of the method, a few degrees. The distance to the center is not well determined, lying about 0.5 Mpc from the Milky Way. The pattern of observed (galactocentric) radial velocities excludes both dynamically important `orphan haloes' and any extended dark matter halo for the Group as a whole, and shows the Group to have formed from a much more extended volume than it presently occupies. Kinematics alone indicates that the mass of the Group is concentrated effectively in M31 and the Milky Way.

Comparison of strong gravitational lens model software I. Redshift and model dependence of time delay and mass calculations

Analysis of strong gravitational lensing depends on software analysis of observational data. The purpose of this study is to evaluate the behavior of strong gravitational lens modeling software with changes in redshift. Four different strong gravitational lens software modeling codes are directly compared (Lenstool/glafic, two light traces mass codes, and GRALE/PixeLens, two non-light traces mass codes) in the analysis of a mock model as well as analysis of SDSSJ1004 + 4112. The calculated time delay is proportional to DdDs/Dds. The percent change in time delays calculated at each redshift tested is compared with percent change in DdDs/Dds. A mock model with a singular isothermal ellipsoid and four images is tested with each code. Five models are used with a constant zlens and a varying zsource, and five models with a constant zsource and a varying zlens. The effects of changing geometry are similarly investigated for SDSSJ1004 + 4112. In general, the changes in time delay are of a similar magnitude and direction, although some calculated time delays did not follow changes in DdDs/Dds. This variation is explained by changes in image position calculated by glafic and GRALE, which varied according to Dds/Ds. Changes in enclosed mass for the mock model with a constant zsource are similar to changes in DdDs/Dds for three of the four codes tested. These data demonstrate the effect of changes in redshift on parameters calculated by each of the codes as compared to changes in DdDs/Dds. The paucity of comparative studies in strong gravitational lensing suggests the need for further studies. These results show that small changes in redshift affect the calculated time delay and mass, and that the effect on the calculations is dependent on the particular software used.

Miscentring in galaxy clusters: dark matter to brightest cluster galaxy offsets in 10 000 Sloan Digital Sky Survey clusters

We characterise the typical offset between the Dark Matter (DM) projected centre and the Brightest Cluster Galaxy (BCG) in 10,000 SDSS clusters. To place constraints on the centre of DM, we use an automated strong-lensing analysis, mass-modelling technique which is based on the well-tested assumption that light traces mass. The cluster galaxies are modelled with a steep power-law, and the DM component is obtained by smoothing the galaxy distribution fitting a low-order 2D polynomial (via spline interpolation), while probing a whole range of polynomial degrees and galaxy power laws. We find that the offsets between the BCG and the peak of the smoothed light map representing the DM, \Delta, are distributed equally around zero with no preferred direction, and are well described by a log-normal distribution with <log_{10}(\Delta [h^{-1} Mpc])>=-1.895^{+0.003}_{-0.004}, and \sigma=0.501\pm0.004 (95% confidence levels), or <log_{10}(\Delta [\arcsec])>=0.564\pm0.005, and \sigma=0.475\pm0.007. Some of the offsets originate in prior misidentifications of the BCG or other bright cluster members by the cluster finding algorithm, whose level we make an additional effort to assess, finding that ~10% of the clusters in the probed catalogue are likely to be misidentified, contributing to higher-end offsets in general agreement with previous studies. Our results constitute the first statistically-significant high-resolution distributions of DM-to-BCG offsets obtained in an observational analysis, and importantly show that there exists such a typical non-zero offset in the probed catalogue. The offsets show a weak positive correlation with redshift, so that higher separations are generally found for higher-z clusters in agreement with the hierarchical growth of structure, which in turn could help characterise the merger, relaxation and evolution history of clusters, in future studies. [ABRIDGED]

The universal Einstein radius distribution from 10 000 SDSS clusters

We present results from strong-lens modelling of 10,000 SDSS clusters, to establish the universal distribution of Einstein radii. Detailed lensing analyses have shown that the inner mass distribution of clusters can be accurately modelled by assuming light traces mass, successfully uncovering large numbers of multiple-images. Approximate critical curves and the effective Einstein radius of each cluster can therefore be readily calculated, from the distribution of member galaxies and scaled by their luminosities. We use a subsample of 10 well-studied clusters covered by both SDSS and HST to calibrate and test this method, and show that an accurate determination of the Einstein radius and mass can be achieved by this approach "blindly", in an automated way, and without requiring multiple images as input. We present the results of the first 10,000 clusters analysed in the range $0.1<z<0.55$, and compare them to theoretical expectations. We find that for this all-sky representative sample the Einstein radius distribution is log-normal in shape, with $< Log(\theta_{e}\arcsec)>=0.73^{+0.02}_{-0.03}$, $\sigma=0.316^{+0.004}_{-0.002}$, and with higher abundance of large $\theta_{e}$ clusters than predicted by $\Lambda$CDM. We visually inspect each of the clusters with $\theta_{e}>40 \arcsec$ ($z_{s}=2$) and find that $\sim20%$ are boosted by various projection effects detailed here, remaining with $\sim40$ real giant-lens candidates, with a maximum of $\theta_{e}=69\pm12 \arcsec$ ($z_{s}=2$) for the most massive candidate, in agreement with semi-analytic calculations. The results of this work should be verified further when an extended calibration sample is available.

LUMINOUS SATELLITES OF EARLY-TYPE GALAXIES. I. SPATIAL DISTRIBUTION

We study the spatial distribution of faint satellites of intermediate redshift (0.1 < z < 0.8), early-type galaxies, selected from the GOODS fields. We combine high-resolution Hubble Space Telescope images and state-of-the-art host subtraction techniques to detect satellites of unprecedented faintness and proximity to intermediate redshift host galaxies (up to 5.5 mag fainter and as close as 05/2.5 kpc to the host centers). We model the spatial distribution of objects near the hosts as a combination of an isotropic, homogeneous background/foreground population and a satellite population with a power-law radial profile and an elliptical angular distribution. We detect a significant population of satellites (Ns = 1.7+0.9−0.8) that is comparable to the number of Milky Way satellites with similar host–satellite contrast. The average projected radial profile of the satellite distribution is isothermal (γp = −1.0+0.3−0.4), which is consistent with the observed central mass density profile of massive early-type galaxies. Furthermore, the satellite distribution is highly anisotropic (isotropy is ruled out at a >99.99% confidence level). Defining ϕ to be the offset between the major axis of the satellite spatial distribution and the major axis of the host light profile, we find a maximum posterior probability of ϕ = 0 and |ϕ| less than 42° at the 68% confidence level. The alignment of the satellite distribution with the light of the host is consistent with simulations, assuming that light traces mass for the host galaxy as observed for lens galaxies. The anisotropy of the satellite population enhances its ability to produce the flux ratio anomalies observed in gravitationally lensed quasars.

Differentiating dark energy and modified gravity with galaxy redshift surveys

The observed cosmic acceleration today could be due to an unknown energy component (darkenergy), or a modification to general relativity (modified gravity). If dark energy modelsand modified gravity models are required to predict the same cosmic expansion historyH(z), they will predict different growth rates for cosmic large scale structure,fg(z). If gravity is not modified, the measuredH(z) leads to a uniqueprediction for fg(z), fgH(z), if dark energy and dark matter are separate. ComparingfgH(z) with themeasured fg(z) provides a transparent and straightforward test of gravity. We show that a simpleχ2 test provides a general figure of merit for our ability to distinguishbetween dark energy and modified gravity given the measuredH(z) and fg(z). We find that a magnitude-limited NIR galaxy redshift survey covering>10 000 (deg)2 and a redshiftrange of 0.5<z<2 can beused to measure H(z) to 1–2% accuracy via baryon acoustic oscillation measurements, andfg(z) to the accuracy of a few per cent via the measurement of redshift-space distortionsand the bias factor which describes how light traces mass. We show that if theH(z) dataare fitted by both a DGP gravity model and an equivalent dark energy model that predict the sameH(z), a surveyarea of 11 931 (deg)2 is required to rule out the DGP gravity model at the 99.99% confidence level. It is feasiblefor such a galaxy redshift survey to be carried out by the next generation space missionsfrom NASA and ESA, and it will revolutionize our understanding of the universe bydifferentiating between dark energy and modified gravity.

Mass and light in the supercluster of galaxies MS0302+17

We investigate the supercluster MS0302+17 (z ≈ 0.42) using weak lensing analysis and deep wide field BVR photometry with the CFH12K camera. Using (B-V) vs. (V-R) evolution tracks we identify early-type members of the supercluster, and foreground ellipticals. We derive a R band catalogue of background galaxies for weak lensing analysis. We compute the correlation functions of light and mass and their cross-correlation and test if light traces mass on supercluster, cluster and galaxy scales. We show that the data are consistent with this assertion. The ζ-statistic applied in regions close to cluster centers and global correlation analyses over the whole field converge toward the simple relation M/L=300±30 h70 (M/L)⊙ in the B band. This independently confirms the earlier results obtained by \citet{Kaiser98}. If we model dark matter halos around each early-type galaxy by a truncated isothermal sphere, we find that a linear relation M∝ L still holds. In this case, the average halo truncation radius is s* ⪉ 200 h-170kpc close to clusters cores whereas it reaches a lower limit of ˜300 h-170kpc at the periphery. This change of s* as a function of radial distance may be interpreted as a result of tidal stripping of early type galaxies. Nevertheless the lack of information on the spatial distribution of late-type galaxies affects such conclusions concerning variations of s*. Though all the data at hand are clearly consistent with the assumption that mass is faithfully traced by light from early-type galaxies, we are not able to describe in detail the contribution of late type galaxies. We however found it to be small. Forthcoming wide surveys in UV, visible, and near infrared wavelengths will provide large enough samples to extend this analysis to late-type galaxies using photometric redshifts. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut des Sciences de l'Univers of the Centre National de la Recherche Scientifique and the University of Hawaii. Catalogue is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/407

Entropy scaling in galaxy clusters: Insights from anXMM-Newtonobservation of the poor cluster A1983

An XMM-Newton observation of the cool (kT=2.1 keV) cluster A1983, at z=0.044, is presented. Gas density and temperature profiles are calculated for the inner 500 h_{50}^{-1} kpc (~0.35 r_200). The outer regions of the surface brightness profile are well described with a beta model with beta=0.74, but the central regions require the introduction of a second component. The temperature profile is flat at the exterior with a slight dip towards the centre. The total mass profile, calculated assuming hydrostatic equilibrium, is consistent with an NFW profile, but with a low concentration parameter c=3.75 +/- 0.74. The M/L_B ratio profile shows that, at large scale, light traces mass to a reasonable extent, and the M/L_B ratio at 0.35 r_200 is consistent with the trends with mass observed in the optical. The M_Fe/L_B ratio is about two times less than that observed for a cluster at 5 keV. The gas mass fraction rises rapidly to level off at ~200 kpc; the value at 0.35 r_200 is ~8%. The scaling properties of the emission measure profile are consistent with the empirical relation \mgas \propto \Tx^{1.94}, and not with the self-similar relation \mgas \propto \Tx^{1.5}. Comparison of the entropy profile of A1983 with that of the hot cluster A1413 shows that the profiles are well scaled using the empirically determined relation S \propto \Tx^{0.65}, suggesting that the slope of the S-T relation is shallower than in the self-similar model. The form of the entropy profiles is remarkably similar, and there is no sign of a larger isentropic core in the cooler cluster. These data provide powerful agruments against preheating models. In turn, there is now increasing observational support for a trend of f_gas with system mass, which may go some way towards explaining the observed scaling behaviour. (Abridged.)

Energy, entropy and mass scaling relations for elliptical galaxies. Towards a physical understanding of their photometric properties

In the present paper, we show that elliptical galaxies (Es) obey a scaling relation between potential energy and mass. Since they are relaxed systems in a post violent-relaxation stage, they are quasi-equilibrium gravitational systems and therefore they also have a quasi-constant specific entropy. Assuming that light traces mass, these two laws imply that in the space defined by the three S\\ersic law parameters (intensity Sigma_0, scale a and shape nu), elliptical galaxies are distributed on two intersecting 2-manifolds: the Entropic Surface and the Energy-Mass Surface. Using a sample of 132 galaxies belonging to three nearby clusters, we have verified that ellipticals indeed follow these laws. This also implies that they are distributed along the intersection line (the Energy-Entropy line), thus they constitute a one-parameter family. These two physical laws (separately or combined), allow to find the theoretical origin of several observed photometrical relations, such as the correlation between absolute magnitude and effective surface brightness, and the fact that ellipticals are located on a surface in the [log R_eff, -2.5 log Sigma_0, log nu] space. The fact that elliptical galaxies are a one-parameter family has important implications for cosmology and galaxy formation and evolution models. Moreover, the Energy-Entropy line could be used as a distance indicator.

Measurement of the Bias Parameter from Weak Lensing

We have measured the correlation between the lensing signal induced by (dark) matter and number counts of galaxies on scales ranging from 0.15-3.0 h_{50}^{-1} Mpc (which correspond to aperture radii of 1-15 arcminutes). This provides a direct probe of the scale dependence of the ratio of the classical bias parameter b and the galaxy-mass correlation coefficient r. The results presented here are based on 16 deg^2 of R_C band data taken with the CFHT as part of the Red-Sequence Cluster Survey. We used a sample of lens galaxies with 19.5<R_C<21, and a sample of source galaxies with 21.5<R_C<24. The results are consistent with a scale independent value of b/r, which provides valuable constraints on models of galaxy formation on scales that can only be probed through weak lensing. For the currently favored cosmology (\Omega_m=0.3, \Omega_\Lambda=0.7) we find b/r=1.05^{+0.12}_{-0.10}, similar to what is found on larger scales (~10 h_{50}^{-1} Mpc) from local dynamical estimates. These results support the hypothesis that light traces mass on scales ranging from 0.15 out to ~10 h_{50}^{-1} Mpc. The accuracy of the measurement will improve significantly in the coming years, enabling us to measure both b and r separately as a function of scale.

Large-Scale Structure Tests of Warm Dark Matter

view Abstract Citations (197) References (94) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Large-Scale Structure Tests of Warm Dark Matter Colombi, Stephane ; Dodelson, Scott ; Widrow, Lawrence M. Abstract The nature of the dark matter critically affects the large-scale structure of the universe. Under the assumptions that the universe is spatially flat with zero cosmological constant and that primordial perturbations were adiabatic with a Harrison-Zeldovich spectrum, neither hot dark matter (HDM) nor cold dark matter (CDM) appears consistent with the observed large-scale structure. Warm dark matter (WDM) is an intriguing alternative from the point of view of both cosmology and particle physics. We consider a one-parameter family of WDM models. The linear power spectra for these models is calculated and compared with the corresponding spectra for CDM, HDM, and mixed dark matter (MDM), as well as the power spectrum derived from observations. Our linear analyses suggest that a model universe dominated by a particle whose mass-to-temperature ratio mx/Tx is increased by a factor of 2 as compared with the standard HDM neutrino gives a reasonable fit to the data on large (>8 h-1 Mpc) scales. N-body simulations for this particular WDM model show features of both HDM and CDM. As in HDM, the first objects to collapse are large pancake-like structures. The final matter distribution is rather smooth, and structures as small as galaxy halos are excluded. However, there appear to be virialized rich clusters evident in the CDM but not in the HDM simulations. Unfortunately, a simple comparison of the matter distribution and its statistical properties with observations indicates that WDM, like CDM, has too much power at small scales. This is particularly evident in the small-scale pairwise velocity dispersion. The cluster multiplicity function has the wrong shape, with too many rich clusters being produced, although this conclusion is based on the simple assumption that light traces mass in groups of galaxies. Publication: The Astrophysical Journal Pub Date: February 1996 DOI: 10.1086/176788 arXiv: arXiv:astro-ph/9505029 Bibcode: 1996ApJ...458....1C Keywords: COSMOLOGY: THEORY; COSMOLOGY: DARK MATTER; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; METHODS: NUMERICAL; Astrophysics E-Print: uuencoded Postscript with most figures included [300 kBytes]. Figures 4-6 [21 MBytes] available directly from authors or at http://fnas08.fnal.gov/pub/Publications/Pub-95-093-A full text sources arXiv | ADS |