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Space Telescope Science Institute
2011 May Symposium: Poster Abstracts

Testing Cosmology and Large Scale Structure With Strong Lensing Selected Galaxy Clusters

Matthew Bayliss (University of Chicago)

We present results from extensive follow-up spectroscopy targeting a sample of galaxy clusters which were systematically selected by virtue of the fact that they strongly lens background galaxies into bright giant arcs. Statistical properties of galaxy cluster-scale strong lenses inform us about both the structure formation history of the universe and also the halo density profiles of the most massive structures in the universe, as well as the overall cosmology from which these structures arise. Multi-object spectroscopy with Gemini/GMOS-North and Magellan/IMACS+GISMO enables us to characterize a large, well-defined sample of massive foreground cluster lenses and lensed giant arcs. We make a direct measurement of the redshift distribution of a spectroscopically complete subsample of giant arcs, and argue that this result could explain the previously claimed tension between the number counts of giant arcs we observe on the sky and the number predicted by a Lambda-CDM model cosmology. Dynamical measurements of the masses of cluster lenses on large scales (~R_200) are also combined with strong lensing constraints for the mass in the lensing cluster cores, and we compare this measured Einstein Radius - M_200 relation to semi-analytic predictions for Lambda-CDM.

Halo Finding in Phase Space and Time: the First Seven-Dimensional Halo Finder

Peter Behroozi (Stanford University)

We present a new algorithm for identifying dark matter halos, substructure, and tidal features in phase space. Our approach is based on adaptive hierarchical refinement of friends-of-friends groups in six phase space dimensions, which allows for robust (grid-independent, shape-independent, and noise-resilient) tracking of substructure; a comparison paper (Knebe et al. 2011) has demonstrated the excellent recovery of halo properties for our approach as compared to other halo finders. We also present a new algorithm for ensuring consistency of halo catalogs and merger trees across time, thus forming the first system designed to find halo properties using all seven dimensions of phase space and time. We use this new system to answer fundamental questions about the distribution of satellite halos, giving radial distributions all the way into the innermost 10-100 kpc of the host halo, as well as to demonstrate that satellite halos reach their peak Vmax well outside of the virial radius of their eventual host halo. Comparison of these results to observations will enable precision tests of the effects of baryonic physics on dark matter halos in the context of LCDM.

Luminous Satellites Versus Dark Subhaloes: Clustering in the Milky Way

Brandon Bozek (Johns Hopkins University)

The known satellite galaxies of the Milky Way are anisotropically distributed in a flattened 'Disk of Satellites' and have been suggested to contain anisotropic subsets of 'ghostly streams' and groups. The analyses of subhaloes in Cold Dark Matter simulations predict that such structures form either by subhalo accretion onto the host halo in clustered groups or through relaxation in a triaxial halo, following filamentary accretion in preferred directions. We used a suite of clustering estimators to quantify the amount of substructure in the distribution of the Milky Way satellite galaxies in configuration space, line-of-sight velocity space, and four-dimensional phase space. We found that the Milky Way satellite galaxies are clustered on small scales in both configuration space and four-dimensional phase-space, but are randomly distributed in line-of-sight velocity space. We also applied the same analysis to all the subhaloes in the Via Lactea II Cold Dark Matter simulation first, and then several samples of the subhaloes defined to represent the luminous dwarfs. All of the clustering methods found a greater degree of clustering in the distributions of the "luminous" Via Lactea II subhalo sets than was found in the Milky Way satellite galaxy distribution. Due to the small observed sample size of the Milky Way satellite galaxies, the difference in the clustering results between the satellites and the "luminous" subhalo sets is only of moderate significance. The inconsistency between the two for all clustering methods suggests Cold Dark Matter simulations are unable to produce an accurate representation of the Milky Way satellite galaxy population. While this is a promising technique, a larger sample size is required to claim a real difference.

Beyond the Bullet: Constraints on Dark Matter from Merging Galaxy Clusters

Douglas Clowe (Ohio University)

The Bullet Cluster showed that the majority of mass in a cluster of galaxies must be in some form of dark matter regardless of the form of the gravitational force law. It did not, however, place stringent constraints on the form of the gravitational force law as it was a single system and had few constraints on where the dark matter needed to be located. I will present results for several other merging clusters and discuss the possibility of using multiple systems simultaneously to place constraints on alternative models of gravity.

CLASH, The Cluster Lensing And Supernova survey with Hubble: Obtaining Robust Dark Matter Maps for 25 Galaxy Clusters

Dan Coe (Space Telescope Science Institute)

The Cluster Lensing And Supernova survey with Hubble (CLASH) is a 524-orbit multi-cycle treasury program to analyze the gravitational lensing properties of 25 galaxy clusters and precisely constrain their dark matter and total mass distributions. The survey will definitively establish the degree of concentration of dark matter in the cluster cores, a key prediction of Lambda-CDM simulations that has increasingly come into tension with existing observations. The CLASH cluster sample is larger and less biased than the current sample of clusters with confidently measured dark matter profiles, as we have minimized lensing-based selection that favors systems with overly dense cores. Specifically, twenty CLASH clusters are solely X-ray selected to be massive (kT > 4.8 keV; 5-30e14 Msun). Five additional clusters were selected for their lensing strength (R_Einstein > 35") to further quantify the lensing bias on concentration measurements and to yield some of the highest resolution dark matter maps obtainable today. Each cluster is being imaged to a depth of 20 orbits in 16 WFC3 and ACS filters from the near-UV to the near-IR (~2,000 - 17,000 A). These data are yielding dozens of multiple images per cluster of strongly lensed galaxies with precise distance measurements (photometric redshifts accurate to ~0.02(1+z)), all essential ingredients toward robustly mapping the dark matter distributions in galaxy cluster cores. Joint analyses of strong and weak lensing, using HST and Subaru imaging respectively, and complemented with X-ray and SZ observations, will allow robust mass profiles to be derived for each cluster from ~10 kpc to beyond ~2 Mpc.

Gravitational Model of the Three Elements Theory

Lassialle Frederic (Polytech'Sophia University of Nice Sophia-Antipolis)

The gravitational model of the three elements theory gives explanations for the dark matter mysteries, and the Pioneer anomaly. Concerning the earth flyby anomalies, the theoretical order of magnitude is the same as the experimental one. A very small change of the perihelion advance of the planet orbits is calculated by this model. The disparity of the gravitational constant measurements might also be explained. Meanwhile, this gravitational model is perfectly compatible with restricted and general relativity, and is part of the three element theory, a unifying theory.

Two New Samples of Strong Lenses

Mike Gladders (U of Chicago / KICP)

A detailed and statistically robust visual inspection of the SDSS DR7 imaging footprint, and similarly the entire RCS-2 cluster catalog has produced hundreds of candidate group- and cluster-scale strong lensing systems, over a volume of approximately 9 Gpc^3 out to a redshifts beyond z=1. An extensive multi-wavelength multi-telescope follow-up program has confirmed the lensing interpretation of several hundred of these lensing candidates, providing a dramatic increase the total number of known lenses from all previous samples. The resulting samples probe strong lensing from group mass scales all the way to the most massive clusters, covering two orders of magnitude in mass, out to redshift one. These lenses inform our understanding of massive halos - the lensing is primarily by dark matter at these mass scales - through studies both of the statistics of the source images and lenses, and detailed lensing studies of individual systems. This poster will present details of the sample construction and verification, and highlight some of the basic statistics of lensing in the samples.

Sunyaev Zel'dovich Effect Observations of Strong Lensing Galaxy Clusters

Megan Gralla (University of Chicago)

Previous studies based on strong and weak lensing measurements have found that strong lensing galaxy clusters are more concentrated than cosmological simulations predict, as could be explained if dark matter halos assemble earlier than expected. However, the results, mostly based on small samples for which it is difficultto model the sample selection, have been disputed. We have conducted Sunyaev Zel'dovich (SZ) effect observations of a sample of ten strong lensing selected galaxy clusters using the SZA/CARMA. We use strong lensing mass models to constrain the mass at small scales and the SZ observations to trace the mass at large scales. Combining the two, we find that clusters tend to be more concentrated than expected from models based on cosmological simulations even when carefully accounting for the selection of large Einstein radius systems.

Calibrating the Sunyaev-Zel'dovich Cluster Mass-Observable Relation for the South Pole Telescope

Fredrick High (University of Chicago)

The South Pole Telescope (SPT) has detected clusters above a nearly uniform mass threshold over an extremely broad range in redshift by searching for the Sunyaev-Zel'dovich effect. The observed abundance of clusters from such a sample is directly sensitive to the growth function of matter perturbations over the majority of the history of the universe, and thereby provides crucial constraints on dark energy with systematics that are complementary to traditional distance-based measures. The dominant source of uncertainty on dark energy constraints from this technique is our estimate of total cluster mass. We are now obtaining observations of 15 SPT clusters at 0.3 < z < 0.6 with Megacam on the Magellan-Clay 6.5 m telescope, and 7 SPT clusters at 0.6 < z < 1 with ACS in Cycle 18, to estimate total cluster mass for these (0.3-1)x10^15 Msun systems with weak gravitational lensing. Empirically verifying whether massive clusters as distant as z ~ 1 have bulk matter properties similar to the better understood z <~ 0.3 samples is necessary for obtaining the best possible estimates of cosmological parameters. The weak lensing mass estimates will be combined with existing SZ, Chandra and XMM-Newton, Spitzer/IRAC, and multi-object spectroscopy data that have already been obtained or are scheduled, as part of a multi-wavelength, multi-technique effort to measure cluster masses with minimal systematic error.

Magnificent Magnification: Extracting the other half of the weak lensing Signal

Eric Huff (UC, Berkeley/LBNL)

Weak gravitational lensing has attracted a great deal of interest in recent years as a powerful measurement technique capable of probing the invisible components of the concordance cosmology. To date, most of the information available from weak lensing studies has come from analyses of the shearing of galaxy shapes. In this presentation, we describe a method for measuring weak lensing magnification by galaxies. Our method makes use of the existence of tight photometric scaling relations (analogous to the Fundamental Plane) between photometric galaxy properties that are perturbed by gravitational lensing, such as apparent size, and other photometric properties, such as mean surface brightness and light profile concentration, that are not. We present a first detection of magnification by massive galaxy halos at significance approaching that achievable with conventional shear-based measurements. We identify the major sources of systematic error inherent in magnification measurements based on galaxy scaling relations and demonstrate techniques to suppress these systematics below the statistical errors of the measurement. Further work holds the potential to dramatically increase the magnification signal beyond that presented here. Applying this technique to current and future surveys will yield a significant boost in the cosmological information available to lensing experiments and a much-needed device for the suppression of systematic errors.

The RESOLVE Survey: REsolved Spectroscopy Of a Local VolumE

Sheila Kannappan (University of North Carolina, Chapel Hill)

The RESOLVE Survey is a volume-limited census of stellar, gas, and dynamical mass as well as merging and star formation within 53,000 cubic Mpc of the nearby cosmic web in two long equatorial strips. The survey's primary science drivers include relating the galaxy velocity and mass functions to environment, constraining the "missing baryons" problem from a complete accounting perspective, and understanding galaxy disk building in large-scale context. RESOLVE's unique data product is high-resolution multiple-longslit spectroscopy from the SOAR and SALT telescopes, targeting all 1500 galaxies with baryonic (stellar + cold gas) mass > ~10^9 Msun in the volume. Combined with a complete redshift survey, this spectroscopy will enable an unprecedented high dynamic-range view of how kinematically estimated mass is distributed on scales from dwarf galaxies to clusters. Here we present early results bearing on four key challenges for RESOLVE: mitigating cosmic variance, achieving mass-limited completeness, tracing multi-phase gas, and measuring the complex dynamics of S0 galaxies.

Observable Signatures of Cuspy and Cored Dark Matter Halos

Rachel Kuzio de Naray (Royal Military College of Canada)

High-resolution Halpha velocity fields provide important observational constraints on the dark matter distribution in low surface brightness galaxies. These two-dimensional data show that dark matter-dominated galaxies tend to be more consistent with cored halos than cuspy halos, at odds with theoretical expectations. Under a variety of realistic conditions, we "observe" simulated galaxies formed in spherical and triaxial cuspy and spherical cored dark matter halos. We find that the appearance of the velocity field is distinctly different depending on the underlying halo type. Cuspy halos appear cuspy in the data and cored halos appear cored. Our results suggest that the cores observed using high-resolution velocity fields in real dark matter-dominated galaxies are genuine and cannot be ascribed to systematic errors, halo triaxiality, or non-circular motions.

A Dark Matter Cloud in the Milky Way Sky: Exploring New Possibilities for Substructure

Mariangela Lisanti (Princeton Center for Theoretical Science)

We propose that dark matter particles tidally-stripped from subhalos falling into the Milky Way form a cloud-like structure near the galactic center with unique velocity behavior out to distances of ~40 kpc. The Via Lactea-II N-body simulation is used to show that 3% of the local dark matter density exists in this cloud, with velocities strongly peaked at 340 km/s. These results have important implications for dark matter detection experiments, as well as stellar surveys searching for substructure in the inner halo.

Interplay between dark matter and baryons in Abell 2744, one of the most active galaxy cluster mergers known

Julian Merten (ITA University of Heidelberg)

The Bullet Cluster presents us with a simple paradigm regarding collisions between galaxy clusters: gas is self-collisional causing it to be stripped from galaxies and dark matter which are not. However some cluster mergers appear to deviate from this prescription, likely because of more complicated merger physics yet to be fully understood or perhaps, as some suggest, due to dark matter self-collisionality. Abell 2744 is a merger of four galaxy clusters all with ~10^14 solar masses or more, making it one of the most active mergers known, and the only to feature a Mach ~3 shock front aside from the Bullet Cluster. In this strongly shocked portion of Abell 2744, gas trails mass as in the Bullet Cluster. However the other two merging clusters exhibit strange phenomenology as revealed by our recent analysis based in part on newly obtained Hubble images. We find gas apparently leading rather than trailing mass of one cluster, perhaps by far the largest "ram-pressure slingshot" yet observed. Mass appears to be offset from galaxies in our "dark cluster" (if confirmed, perhaps the first of its kind). And gas appears to be stripped more cleanly and to a greater distance (> 250 kpc) from one of our clusters than any other yet known. Additional observations and simulations are required to confirm and understand these strange behaviors unleashed by this merger which we dub "Pandora's Cluster".

Weak Lensing Results of the Merging Cluster A1758

Brett Ragozzine (Ohio University)

We present our weak lensing analysis of merging cluster A1758 that actually has two separate mergers, A1758N and A1758S. There is no evidence of interaction between the northern and southern mergers. The geometry of A1758N is different than previously published mergers that have X-ray peaks between the two weak lensing peaks. One of the X-ray peaks of A1758N is between the weak lensing peaks and one X-ray peak is on top of a weak lensing peak. We estimate the combined mass of the northern clusters to be 2.2 x 10^15 solar masses with r200=2297 kpc.

Constraining the Dark Halo Mass through Proper Motion Measurements of Local Group Galaxies

S. Tony Sohn (Space Telescope Science Institute)

The Local Group and its two dominant spiral galaxies have been the benchmark for testing many aspects of cosmological and galaxy formation theories. This includes, e.g., dark halo profiles and shapes, substructure and the "missing satellite" problem, and the minimum mass for galaxy formation. But despite the extensive work in all of these areas, our knowledge of the mass of the Milky Way and M31, and thus the total mass of the Local Group remains one of the most poorly established astronomical parameters (uncertain by a factor of ~4). One important reason for this problem is the lack of information in tangential motions of galaxies, which can be only obtained through proper motion measurements. In this study, we introduce our projects for measuring absolute proper motions of (1) the dwarf spheroidal galaxy Leo I, (2) M31, and (3) the 4 dwarf galaxies near the edge of the Local Group (Cetus, Leo A, Tucana, and Sag DIG). Results from these three independent measurements will provide important clues to the mass of the Milky Way, M31, and the Local Group as a whole, respectively. We also present our proper motion measurement technique that uses compact background galaxies as astrometric reference sources.

Deep Radio Observations of the Draco dSph: Implications for Particle Dark Matter

Kristine Spekkens (Royal Military College of Canada)

We present deep radio observations of the Draco dwarf spheroidal galaxy, designed to detect synchrotron emission resulting from dark matter self-annihilations in its halo. Recent models predict the existence of a relatively "clean", smoothly distributed, degree-scale synchrotron halo in Drac that stems from electron and positron decay products. We map a 4x4 degree region with the GBT at 1.4 GHz to detect this annihilation signature, beating confusion by excising background sources using the NVSS catalog. We find no evidence for a radio halo in Draco, and place a conservative upper limit on its flux by simulating the impact of uncertainties due to foregrounds and backgrounds. We then explore the implications of our upper limit for the self-annihilation cross-section of weakly interacting dark matter. We find that our radio observations place meaningful constraints on this property for a range of Draco models, and thus that deep radio observations are highly complementary to indirect dark matter searches at gamma-ray energies.

Reproducing the statistics and scaling relations of galaxies in LCDM

Sebastian Trujillo-Gómez (NMSU)

It has long been difficult for the concordance cosmological model to simultaneously account for the observed distributions of galaxies in luminosity, stellar mass and circular velocity as well as their scaling relations and clustering. Using halo abundance matching (HAM) along with simple and robust dynamical corrections, we reproduce the observed galaxy statistics in the local universe. To compare these statistics with observations we have assembled a large sample of data for galaxies ranging from dwarf irregulars to giant ellipticals. Our new “Bolshoi” cosmological dark matter simulation has the very high resolution and large volume necessary to model the most luminous galaxies in the SDSS while probing the structure of the dark matter halos that host dwarf galaxies. We find a remarkable agreement of the model with the observed average relation between luminosity and circular velocity for galaxies of all morphological types. In detail, we report the presence of a bimodality in the observed positions of late- vs. early-type galaxies in the LV diagram. Implementing scatter motivated by studies of the TF relation, we are able to account for this bimodality in our model. The model also yields a good fit to the relation between cold baryonic mass and circular velocity and reproduces the abundance of galaxies as a function of circular velocity for galaxies brighter than M_r = -17.5. The LCDM model using stochastic halo abundance matching yields an outstanding agreement with both the shape and the normalization of the SDSS two-point correlation function of bright galaxies without the need to model the bias. We find that imcluding halo contraction in response to galaxy formation provides a better fit to the LV relation and the galaxy velocity function. We overpredict the abundance of dwarf galaxies with V_circ < 80 km/s compared to the results of the HIPASS survey.