arXiv subjects

S. Dye

Publications and source records attributed to S. Dye.

156 records · Page 9Linked to original sources

arXiv2004

Mapping the 3-D dark matter with weak lensing in COMBO-17

We present a 3-dimensional lensing analysis of the z=0.16 supercluster A901/2, resulting in a 3-D map of the dark matter distribution within a 3 X 10^{5} [Mpc]^3 volume from the COMBO-17 survey. We perform a chi^2-fit of isothermal spheres to the tangential shear pattern around each cluster as a function of redshift to estimate the 3-D positions and masses of the main clusters in the supercluster from lensing alone. We then present the first 3-D map of the dark matter gravitational potential field, Phi, using the Kaiser-Squires (1993) and Taylor (2001) inversion methods. These maps clearly show the potential wells of the main supercluster components, including a new cluster behind A902, and demonstrates the applicability of 3-D dark matter mapping and projection free-mass-selected cluster finding to current data. Finally, we develop the halo model of dark matter and galaxy clustering and compare this with the auto-and cross-correlation functions of the 3-D gravitational potential, galaxy number densities and galaxy luminosity densities measured in the A901/2 field. We find significant anti-correlations between the gravitational potential field and the galaxy number density and luminosities, as expected due to baryonic infall into dark matter concentrations. We find good agreement with the halo model for the number densities and luminosity correlation functions.

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arXiv2003

Linking star-formation and enivronment in the A901/902 supercluster

We investigate correlations between the location of galaxies in dense environments and their degree of star-formation activity. Using photometric redshifts and spectral classifications from the unique 17-band COMBO-17 survey we are able to precisely isolate galaxies from the Abell 901/902 supercluster within a thin redshift slice around z=0.16. We compare the detailed photometric properties of the supercluster galaxies with the underlying dark matter density field as revealed by weak gravitational lensing. We find strong evidence for segregation by type, with the highest density regions populated almost exclusively by galaxies classified according to their rest-frame U-V colours as quiescent. We also observe a threshold surface mass density from lensing, κ\sim 0.05 (corresponding to a physical density Σ= 2.5x10^(14)h M_\sun Mpc^(-2)), above which star-formation activity is rapidly suppressed. This abrupt transformation affects primarily the faint end of the star-forming galaxy population and occurs at a local surface number density corresponding to roughly 400h^2 Mpc^(-2) to a limit of M*_V+6. When only galaxies brighter than M*+1 are considered the trends with environment remain, but are more gradual and extend beyond 2h^(-1) Mpc radius.

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arXiv2003

The evolution of faint AGN between z~1 and z~5 from the COMBO-17 survey

We present a determination of the optical/UV AGN luminosity function and its evolution, based on a large sample of faint (R<24) QSOs identified in the COMBO-17 survey. Using multi-band photometry in 17 filters within 350nm < lambda_obs < 930nm, we could simultaneously determine photometric redshifts with an accuracy of sigma_z<0.03 and obtain spectral energy distributions. The redshift range covered by the sample is 1.2<z<4.8, which implies that even at z~3, the sample reaches below luminosities corresponding to M_B = -23, conventionally employed to distinguish between Seyfert galaxies and quasars. We clearly detect a broad plateau-like maximum of quasar activity around z~2 and map out the smooth turnover between z~1 and z~4. The shape of the LF is characterised by some mild curvature, but no sharp `break' is present within the range of luminosities covered. Using only the COMBO-17 data, the evolving LF can be adequately described by either a pure density evolution (PDE) or a pure luminosity evolution (PLE) model. However, the absence of a strong L*-like feature in the shape of the LF inhibits a robust distinction between these modes. We present a robust estimate for the integrated UV luminosity generation by AGN as a function of redshift. We find that the LF continues to rise even at the lowest luminosities probed by our survey, but that the slope is sufficiently shallow that the contribution of low-luminosity AGN to the UV luminosity density is negligible. Although our sample reaches much fainter flux levels than previous data sets, our results on space densities and LF slopes are completely consistent with extrapolations from recent major surveys such as SDSS and 2QZ.

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arXiv2003

The shear power spectrum from the COMBO-17 survey

We perform a cosmic shear analysis of the COMBO-17 survey -- a unique dataset with shear quality R-band imaging and accurate photometric redshift estimates (dz=0.05) for ~90% of galaxies to R=24.0. We undertake a full maximum likelihood analysis to measure the weak lensing power spectra, Cl^kk, Cl^bb & Cl^kb from l=400 to l=10^4. We find a strong measurement of the convergence power over five fields. The b-field has a much lower significance, indicating our data is free of major systematics, while the cross-correlation of k & b is consistent with zero. We have also calculated the shear correlation functions and variance over a range of scales between 0.5 and 20 arcmin. In addition, we have used our results to measure cosmological parameters, constraining the normalisation of the matter power spectrum to be sigma_8=(0.72 \pm 0.09)(Omega_m/0.3)^-0.49, where the errors quoted are 1-sigma due to the intrinsic dispersion in galaxy ellipticities, cosmic and sampling variance. We have significantly reduced the usual additional uncertainty in the median redshift (z_m) of the survey by estimating z_m directly from the data. To demonstrate the power of accurate redshift information, we have also measured parameters from a shear analysis of only those galaxies with accurate redshifts. In this case, we have eliminated the uncertainty in the redshift distribution of sources and we show that the uncertainty in the resulting parameter constraints are reduced by more than a factor of 2 compared to the typical uncertainties found in cosmic shear surveys to date. Finally, we combine our parameter measurements with constraints from the 2dFGRS and from the CMB. With these additional constraints, we measure sigma_8=0.73 +0.05/-0.03 and Omega_m=0.27 +/- 0.02.

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arXiv2002

The COMBO-17 Survey: Evolution of the Galaxy Luminosity Function from 25,000 Galaxies with 0.2<z<1.2

We present a detailed empirical assessment of how the galaxy luminosity function and stellar luminosity density evolves over the last half of the universe's age (0.2<z<1.2) for galaxies of different spectral energy distributions (SED). The results are based on ~25,000 galaxies (R<24) with redshift measurements (sigma_z~0.03) and SEDs across 350..930 nm, derived from medium-band photometry in 17 filters, observed as part of the COMBO-17 survey (``Classifying Objects by Medium-Band Observations in 17 Filters'') over three disjoint fields with a total area of 0.78 square degrees. Luminosity functions (LF), binned in redshift and SED-type, are presented in the restframe passbands of the SDSS r-band, the Johnson B-band and a synthetic UV continuum band at 280 nm. We find that the luminosity function depends strongly on SED-type at all redshifts covered. The shape of the LF, i.e. the faint-end power-law slope, does depend on SED type, but not on redshift. However, the redshift evolution of the characteristic luminosity M* and density phi* depends strongly on SED-type: (1) Early-type galaxies, defined as redder than a present-day reference Sa spectrum, become drastically more abundant towards low redshift, by a factor of 10 in the number density phi* from z=1.1 to now, and by a factor of 4 in their contribution to the co-moving r-band luminosity density, j_r. (2) Galaxies resembling present-day Sa- to Sbc-colours show a co-moving number density and contribution to j_r that does not vary much with redshift. (3) Galaxies with blue spectra reflecting strong star formation decrease towards low redshift both in luminosity and density, and by a factor of 4 in their j_r contribution. (abridged)

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arXiv2002

Lens magnification by CL0024+1654 in the U and R band

[ABRIDGED] We estimate the total mass distribution of the galaxy cluster CL0024+1654 from the measured source depletion due to lens magnification in the R band. Within a radius of 0.54Mpc/h, a total projected mass of (8.1+/-3.2)*10^14 M_sol/h (EdS) is measured, which corresponds to a mass- to-light ratio of M/L(B)=470+/-180. We compute the luminosity function of CL0024+1654 in order to estimate contamination of the background source counts from cluster galaxies. Three different magnification-based reconstruction methods are employed using both local and non-local techniques. We have modified the standard single power-law slope number count theory to incorporate a break and applied this to our observations. Fitting analytical magnification profiles of different cluster models to the observed number counts, we find that the cluster is best described either by a NFW model with scale radius r_s=334+/-191 kpc/h and normalisation kappa_s=0.23+/-0.08 or a power-law profile with slope xi=0.61+/-0.11, central surface mass density kappa_0=1.52+/-0.20 and assuming a core radius of r_core=35 kpc/h. The NFW model predicts that the cumulative projected mass contained within a radius R scales as M( 0.15 up to U_AB<=26.4 with 95% confidence.

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arXiv2002

Measurement of intrinsic alignments in galaxy ellipticities

We measure the alignment of galaxy ellipticities in the local universe over a range of scales using digitized photographic data from the SuperCOSMOS Sky Survey. We find for a magnitude cut of b_J < 20.5, corresponding to a median galaxy redshift of z = 0.1, and 2x10^6 galaxies, that the galaxy ellipticities exhibit a non-zero correlation over a range of scales between 1 and 100 arcminutes. In particular, we measure the variance of mean galaxy ellipticities, sg^2(theta), in square angular cells on the sky as a function of cell size and find it lies in the range, 2 x 10^{-4} > sg^2(theta) > 1 x 10^{-5} for cell side lengths between 15 < theta < 100 arcminutes. Considering the low median redshift of the galaxies in the sample and hence the relatively low effective cross-section for lensing of these galaxies by the large-scale structure of the Universe, we propose that we have detected an intrinsic alignment of galaxy ellipticities. We compare our results to recent analytical and numerical predictions made for the intrinsic galaxy alignment and find good agreement. We discuss the importance of these results for measuring cosmic shear from upcoming shallow surveys (e.g. Sloan Digital Sky Survey) and we outline how these measurements could possibly be used to constrain models of galaxy formation and/or measure the mass distribution in the local universe.

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arXiv2001

Probing the distribution of dark matter in the Abell 901/902 supercluster with weak lensing

We present a weak shear analysis of the Abell 901/902 supercluster, composed of three rich clusters at z=0.16. Using a deep R-band image from the 0.5 x 0.5 degree MPG/ESO Wide Field Imager together with supplementary B-band observations, we build up a comprehensive picture of the light and mass distributions in this region. We find that, on average, the light from the early-type galaxies traces the dark matter fairly well, although one cluster is a notable exception to this rule. The clusters themselves exhibit a range of mass-to-light (M/L) ratios, X-ray properties, and galaxy populations. We attempt to model the relation between the total mass and the light from the early-type galaxies with a simple scale-independent linear biasing model. We find M/L_B=130h for the early type galaxies with zero stochasticity, which, if taken at face value, would imply Omega_m < 0.1. However, this linear relation breaks down on small scales and on scales equivalent to the average cluster separation (approximately 1 Mpc), demonstrating that a single M/L ratio is not adequate to fully describe the mass-light relation in the supercluster. Rather, the scatter in M/L ratios observed for the clusters supports a model incorporating non-linear biasing or stochastic processes. Finally, there is a clear detection of filamentary structure connecting two of the clusters, seen in both the galaxy and dark matter distributions, and we discuss the effects of cluster-cluster and cluster-filament interactions as a means to reconcile the disparate descriptions of the supercluster.

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arXiv2001

Deep BVR photometry of the Chandra Deep Field South from the COMBO-17 survey

We report on deep multi-color imaging (R_5-sigma = 26) of the Chandra Deep Field South, obtained with the Wide Field Imager (WFI) at the MPG/ESO 2.2 m telescope on La Silla as part of the COMBO-17 survey. As a result we present a catalog of 63501 objects in a field measuring 31.5' x 30' with astrometry and BVR photometry. A sample of 37 variable objects is selected from two-epoch photometry. We try to give interpretations based on color and variation amplitude.

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arXiv2000

Gravitational lens magnification by Abell 1689: Distortion of the background galaxy luminosity function

Gravitational lensing magnifies the luminosity of galaxies behind the lens. We use this effect to constrain the total mass in the cluster Abell 1689 by comparing the lensed luminosities of background galaxies with the luminosity function of an undistorted field. Since galaxies are assumed to be a random sampling of luminosity space, this method is not limited by clustering noise. We use photometric redshift information to estimate galaxy distance and intrinsic luminosity. Knowing the redshift distribution of the background population allows us to lift the mass/background degeneracy common to lensing analysis. In this paper we use 9 filters observed over 12 hours with the Calar Alto 3.5m telescope to determine the redshifts of 1000 galaxies in the field of Abell 1689. Using a complete sample of 151 background galaxies we measure the cluster mass profile. We find that the total projected mass interior to 0.25h^(-1)Mpc is (0.48 +/- 0.16) * 10^(15)h^(-1) solar masses, where our error budget includes uncertainties from the photometric redshift determination, the uncertainty in the off-set calibration and finite sampling. This result is in good agreement with that found by number count and shear-based methods and provides a new and independent method to determine cluster masses.

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arXiv1998

Self-consistent Gravitational Lens Reconstruction

We present a new method for directly determining accurate, self-consistent cluster lens mass and shear maps in the strong lensing regime from the magnification bias of background galaxies. The method relies upon pixellisation of the surface mass density distribution which allows us to write down a simple, solvable set of equations. We also show how pixellisation can be applied to methods of mass determination from measurements of shear and present a simplified method of application. The method is demonstrated with cluster models and applied to magnification data from the lensing cluster Abell 1689.

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arXiv1998

Gravitational Lens Magnification and the Mass of Abell 1689

We present the first application of lens magnification to measure the absolute mass of a galaxy cluster; Abell 1689. The absolute mass of a galaxy cluster can be measured by the gravitational lens magnification of a background galaxy population by the cluster potential. The lensing signal is complicated by the variation in number counts due to galaxy clustering and shot-noise, and by additional uncertainties in relating magnification to mass in the strong lensing regime. Clustering and shot-noise can be dealt with using maximum likelihood methods. Local approximations can then be used to estimate the mass from magnification. Alternatively if the lens is axially symmetric we show that the amplification equation can be solved nonlocally for the surface mass density and the tangential shear. In this paper we present the first maps of the total mass distribution in Abell 1689, measured from the deficit of lensed red galaxies behind the cluster. Although noisier, these reproduce the main features of mass maps made using the shear distortion of background galaxies but have the correct normalisation, finally breaking the ``sheet-mass'' degeneracy that has plagued lensing methods based on shear. We derive the cluster mass profile in the inner 4' (0.48 Mpc/h). These show a profile with a near isothermal surface mass density κ= (0.5+/-0.1)(θ/1')^{-1} out to a radius of 2.4' (0.28Mpc/h), followed by a sudden drop into noise. We find that the projected mass interior to 0.24 h^{-1}$Mpc is M(<0.24 Mpc/h)=(0.50+/- 0.09) \times 10^{15} Msol/h. We compare our results with masses estimated from X-ray temperatures and line-of-sight velocity dispersions, as well as weak shear and lensing arclets and find all are in fair agreement for Abell 1698.

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