### Parallel Session P11: 1545-1730, Tuesday 13th April 2010

#### The Dark Art of Dark Matter - Session 1

**Location:** Bute

**Abstract:**

The goal of this session is to understand the mysterious dark matter that comprises 95% of the matter content of the Universe. The UK is at the forefront of research in the fields of Cosmology and Particle Physics with strengths in both theory and observations. We welcome any researcher in theory or observation, from all fields, working towards the goal of understanding dark matter. Together in this session we will discuss key questions such as: “how can we better exploit synergies between direct and indirect detection?” To aid discussion, in addition to the two scheduled parallel sessions there will be a 20-30 person break-out session, to be held on the morning of 14th April. Our aim is to foster a collaborative environment at the meeting that will lead to strong UK-led research in this rapidly developing field.

**Organisers:**

- Catherine Heymans (IfA, University of Edinburgh)
- Richard Massey (IfA, University of Edinburgh)
- Tom Kitching (IfA, University of Edinburgh)

#### Talks

13 April, 15:45 |
Direct Detection of Dark Matter Pawel Majewski (Rutherford Appleton Laboratory) |

13 April, 16:10 | Dark Matter in the Milky Way Justin Read (University of Leicester) |

13 April, 16:35 | The impact of dark matter cusps and cores on the satellite galaxy populatio Jorge Penarrubia (University of Cambridge, IoA) |

13 April, 16:50 | Applications of a New and Rapid Simulations Method for Weak Lensing Analysis Alina Kiessling (University of Edinburgh) |

13 April, 17:00 | Wave-mechanics of Large Scale Structure Edward Thomson (University of Glasgow) |

13 April, 17:10 | Weighing galaxies using gravitationally lensed SNLS supernovae Jakob Jonsson (University of Oxford) |

13 April, 17:25 | Poster adverts |

#### Posters

Sudden Future Singularity models as an alternative to Dark Energy?

Hoda Ghodsi (University of Glasgow)

LoCuSS: Weak Lensing Analysis of 21 Galaxy Clusters at z=0.15-0.3

Victoria Hamilton-Morris (University of Birmingham)

A New Pixon Weak Lensing Cluster Mass Reconstruction Method

Daniel John (Durham University)

Extreme value statistics: predicting the frequency of the densest clusters and sparsest voids

Olaf Davis (Oxford)

The impact of delensing gravitational wave standard sirens on determining cosmological parameters

Craig Lawrie (University of Glasgow)

Do dark matter halos have cusps?

Chris Brook (Jeremiah Horrocks Institute, UCLan)

Probing the dark matter halos of early-type galaxies via lensing

Ignacio Ferreras (MSSL/UCL)

Probing the Dark Universe with Weak Lensing Tomography and the CFHTLS

Catherine Heymans (IfA, University of Edinburgh)

Bright Ideas and Dark Thoughts: "Universal Baryonic Scale" at "Maximum Halo Gravity"

Hongsheng Zhao (U. of St Andrews (SUPA))

The new path to time delays?

Gülay Gürkan (The Universtiy of Manchester)

TeVeS and the straight arc of A2390

Martin Feix (University of St Andrews)

#### Talk Abstracts

**Direct Detection of Dark Matter**

Majewski, Pawel

Rutherford Appleton Laboratory*13 April, 15:45*

Dark Matter is one of the greatest mysteries in science. Although it makes up five sixths of the matter content of the Universe, it has never been directly detected. For several decades, the hunt for detection of dark mater particle has accelerated and motivated many ingenious experiments around the world. I will review existing and planned dark matter direct detection experiments, focussing on the variety of implemented experimental techniques.

**Dark Matter in the Milky Way**

Read, Justin

University of Leicester*13 April, 16:10*

Experiments designed to detect a dark matter particle in the laboratory need to know the very local phase space density of dark matter, both to motivate detector design and to interpret any future signal. I discuss recent progress on estimating this and its implications.

**The impact of dark matter cusps and cores on the satellite galaxy populatio**

Penarrubia, Jorge, A. Benson, M. Walker, G. Gilmore, A. McConnachie, L. Mayer

University of Cambridge, IoA*13 April, 16:35*

In this talk I will show the results from N-body simulations that study the effects that a divergent (i.e. "cuspy") dark matter (DM) profile introduces on the tidal evolution of dwarf spheroidal galaxies (dSphs). I will show that the resilience of dSphs to tidal stripping is extremely sensitive to the slope of the inner halo profile. I will also outline the results from calculations that simulate the hierarchical build-up of spiral galaxies assuming different halo profiles and disc masses, which show that the size-mass relation established from Milky Way (MW) dwarfs strongly supports the presence of cusps in the majority of these systems, as cored models systematically underestimate the masses of the known Ultra-Faint dSphs. These models also indicate that a massive M31 disc may explain why many of its dSphs fall below the size-mass relationship derived from MW dSphs. We also use our models to constrain the mass threshold below which star formation is suppressed in DM haloes, finding that luminous satellites must be accreted with masses above 10^8--10^9 M_sol in order to explain the size-mass relation observed in MW dwarfs.

**Applications of a New and Rapid Simulations Method for Weak Lensing Analysis**

Kiessling, Alina, Andy Taylor, Alan Heavens

University of Edinburgh*13 April, 16:50*

Gravitational lensing is sensitive to all gravitating mass - both Baryonic and Dark Matter - making it the ideal tool to study Cosmology independently of any assumptions about the dynamical or thermal state of objects. The Next Generation of Survey Telescope will observe more of the sky than ever before and the volume of data they will produce is unprecedented. To realise the potential of these surveys, experiments require full large end-to-end simulations of the Surveys to test analysis methods and provide realistic errors. We have developed a new line-of-sight integration approach to simulating 3-D Weak Gravitational Lens Shear and Convergence fields. These light cones are faster to generate than traditional ray-tracing, so we can run an ensemble of simulations allowing us to generate covariance matricies for cosmological parameter estimation and statistical analysis. This presentation will introduce our new analysis method and discuss some of its many applications in weak lensing experiments.

**Wave-mechanics of Large Scale Structure**

Thomson, Edward, Martin Hendry, Luis Teodoro

University of Glasgow*13 April, 17:00*

Simulations of Large Scale Structure using N-Body codes have helped define the Lambda-CDM paradigm. While N-Body codes remain the most popular approach, a lesser known method was developed in the early 90's that formulates the equations describing large scale structure (LSS) formation within a wave-mechanical framework. This method couples the Schroedinger equation with the Poisson equation of gravity. The wavefunction encapsulates information about the density and velocity fields as a single continuous field with complex values.

In this presentation I will review some of the key features of the wave-mechanical approach to LSS. The method avoids the addition of an artificial smoothing parameter, as seen in N-body codes, and is able to follow 'hot streams' - something that is difficult to do with phase space methods. The method is competitive with N-body codes in terms of processing time. The wave-mechanical approach can be interpreted in two ways: (1) as a purely classical system that includes more physics than just gravity, or (2) as the representation of a dark matter field, perhaps an Axion field, where the de Broglie wavelength of the particles is large.

**Weighing galaxies using gravitationally lensed SNLS supernovae**

Jonsson, Jakob, M. Sullivan, I. Hook, S. Basa, R. Carlberg, A. Conley, D. Fouchez, D.A. Howell, K. Perrett, C. Pritchet

University of Oxford*13 April, 17:10*

Gravitational lensing by foreground matter can magnify or de-magnify background sources. Standard candles, like type Ia supernovae (SNe Ia), can therefore be used to weigh the foreground galaxies via gravitational lensing. We present constraints on dark matter halo properties obtained using 175 SNe Ia from the first 3-years of the Supernova Legacy Survey (SNLS). The dark matter halo of each galaxy in the foreground is modelled as a truncated singular isothermal sphere with velocity dispersion and truncation radius obeying luminosity dependent scaling laws. We cannot constrain the truncation radius, but the best-fitting velocity dispersion scaling law agrees well with results from galaxy-galaxy lensing measurements. The normalisation of the velocity dispersion scaling laws are furthermore consistent with empircal Faber-Jackson and Tully-Fisher relations. We have also measured the brightness scatter of SNe Ia due to gravitational lensing. This scatter contributes only little to the SNLS sample (z < 1), but would contribute significantly at z > 1.6.

#### Poster Abstracts

**Do dark matter halos have cusps?**

Brook, Chris

Jeremiah Horrocks Institute, UCLan

Pure N-body simulations have shown that cold dark matter halos have steep inner density profiles, or "cusps". Yet observations of rotation curves of disk galaxies infer a flatter, cored inner density profile. Using self consistent cosmological galaxy formation simulations, we show that the inclusion of baryons, which are dynamically significant in the inner regions of halos, can dramatically alter the profile of the dark matter. Our simulations result in "bulgeless" disk galaxies with dark matter cores.

**Extreme value statistics: predicting the frequency of the densest clusters and sparsest voids**

Davis, Olaf, Stephane Colombi, Julien Devriendt, Joe Silk

Oxford

One interesting property of random fields - such as the observed density field of the universe - is the distribution of their highest maxima and lowest minima. In particular, the maxima of the dark matter field translate to the the locations of the most massive clusters, which exist in the highly non-linear regime of gravitational clustering and probe the evolution of the power spectrum under gravity.

To relate the theoretical maxima of the DM density with observed maxima in a small region of the universe requires an understanding of the behaviour of sample maxima: this is the domain of extreme value or Gumbel statistics.

We present analytical calculations which can predict the distribution of such maxima and minima from the underlying power spectrum, and demonstrate a good agreement with simulated Gaussian fields. We also compare our predictions to the Horizon 4Pi simualtion, a cosmological scale dark matter simulation containing 70 billion particles.

Potential applications will be discussed, including likelihood constraints on void cosmologies, and application to observed CMB anomalies such as the cold spot and 'Axis of Evil'.

**TeVeS and the straight arc of A2390**

Feix, Martin, HongSheng Zhao, Cosimo Fedeli, José Luis Garrido Pestaña, Henk Hoekstra

University of St Andrews

We suggest to test the combined framework of tensor-vector-scalar theory (TeVeS) and massive neutrinos in galaxy clusters via gravitational lensing, choosing the system A2390 with its notorious straight arc as an example. Adopting quasi-equilibrium models for the matter content of A2390, we show that such configurations cannot produce the observed image. Generally, nonlinear effects induced by the TeVeS scalar field are very small, meaning that curl effects are basically negligible. Based on this result, we outline a systematic approach on how to model strong lenses in TeVeS, which is demonstrated for A2390. Compared to general relativity, we conclude that discrepancies between the independent mass estimates from lensing and X-ray observations are amplified. Finally, we address the question of the model’s feasibility and possible implications/problems for TeVeS.

**Probing the dark matter halos of early-type galaxies via lensing**

Ferreras, Ignacio

MSSL/UCL

The combination of gravitational lensing on galaxy scales and stellar population synthesis enables us to constrain the baryon fraction in galaxies, probing the interplay between the dark matter halo and the baryon physics transforming gas into stars. I will present recent work based on a sample of strong (early-type) lenses from the Castles survey. The combination of a non-parametric approach to the lensing data and the analysis of the HST/NICMOS images of the lens give a remarkably good agreement between baryon and lensing mass in the inner regions. The radial trend of the baryon fraction out to 4-5 Re is shown, along with its connection with the Fundamental Plane. I will put this result in context with recent estimates of the global baryon fraction in galaxies.

**Sudden Future Singularity models as an alternative to Dark Energy?**

Ghodsi, Hoda, Dr Martin A. Hendry

University of Glasgow

One of the key challenges facing cosmologists today is the nature of the mysterious dark energy introduced in the standard model of cosmology to account for the current accelerating expansion of the universe. In this regard, many other non-standard cosmologies have been proposed which would eliminate the need to explicitly include any form of dark energy. One such model is the Sudden Future Singularity (SFS) model, in which no equation of state linking the energy density and the pressure in the universe is assumed to hold. In this model it is possible to have a blow up of the pressure occurring in the near future while the energy density would remain unaffected. The particular evolution of the scale factor of the Universe in this model that results in a singular behaviour of the pressure also admits acceleration in the current era as required. In this contribution I will present the results of the tests of an example SFS model against the current data from high redshift supernovae, baryon acoustic oscillations (BAO) and the cosmic microwave background (CMBR). We explore the limits placed on the SFS model parameters by the current data through employing grid-based and MCMC search methods. This lets us discuss the viability of the SFS model in question as an alternative to the standard concordance cosmology.

**The new path to time delays?**

Gürkan, Gülay, Neal Jackson

The Universtiy of Manchester

To better understand the universe and its dynamics, the Hubble constant is a crucial parameter which provides valuable information about the expansion rate of the universe. So far, the Hubble constant has been determined by various methods such as Cepheid variables by utilizing HST Key Project data and WMAP. The accuracy of the Hubble constant value is not better than 10% due to intrinsic constraints/assumptions of each method.

Gravitational lens systems provide another probe of the Hubble constant using time delay measurements. Current investigations of time delay lenses have resulted in different values of Ho ranging from 50-80 km/s/Mpc. The main problem in gravitational lens systems is that requires a mass model for the lens which is difficult to measure independently unless observational constraints are available. Moreover, in order to see time delays clearly, fluxes of sources have to be variable. On the other hand, using a typical value of the Hubble constant and measured time delays enable us to determine a better/more accurate mass model for the lens galaxy.

Here we attempt to develop a new and more efficient method for measuring time delays, which does not require regular monitoring with a high-resolution interferometer array or with optical telescopes. Instead, the WSRT is used for flux monitoring of double image lens systems in which the brighter image is expected to vary first. Triggered VLA observations can then be used to catch the subsequent variability of the fainter image. We present preliminary results from such a program.

**LoCuSS: Weak Lensing Analysis of 21 Galaxy Clusters at z=0.15-0.3**

Hamilton-Morris, Victoria, G.P. Smith, E. Egami, T. Targett, C. Haines, A. Sanderson...

University of Birmingham

The Local Cluster Substructure Survey (LoCuSS) is a multi-wavelength survey of 100 X-ray luminous galaxy clusters at 0.15

**Probing the Dark Universe with Weak Lensing Tomography and the CFHTLS**

Heymans, Catherine, Emma Grocutt, Alan Heavens, Tom Kitching, CFHTLenS team

IfA, University of Edinburgh

Weak gravitational lensing is a powerful technique for measuring the properties of dark matter and dark energy from their gravitational effects alone. The Canada-France-Hawaii Telescope Legacy Survey is currently the largest deep optical data set for weak lensing analysis covering 172 square degrees over 5 optical bands. We present an investigation into the optimal tomographic three-dimensional analysis of the CFHTLS weak lensing signal that minimises the impact of systematics arising from intrinsic galaxy alignments. With systematics under control, since the influence of dark energy on structure growth is redshift-dependent, tomographic analysis of redshift bins will allow us to constrain the properties of the Dark Universe.

**A New Pixon Weak Lensing Cluster Mass Reconstruction Method**

John, Daniel, V. R. Eke, L. F. A. Teodoro

Durham University

We present a new pixon-based method for cluster mass reconstructions using weak gravitational lensing. Pixons are an adaptive smoothing scheme for image reconstruction, where the local smoothing scale is determined by the data. We also introduce a new goodness-of-fit statistic based on the autocorrelation of the residuals of the shear field. We test our algorithm on simulated lensing datasets using NFW halos with and without substructure. We compare our results to previous methods such as Kaiser-Squires(KS), Maximum Entropy(ME) and the Intrinsic Correlation Function(ICF) and show an increased accuracy in the mass reconstructions. We finally discuss future applications to data.

**The impact of delensing gravitational wave standard sirens on determining cosmological parameters**

Lawrie, Craig, Martin Hendry, Fiona Speirits, Joshua Logue

University of Glasgow

Recently there has been much attention in the cosmology literature on the potential future use of compact binary inspirals, so-called gravitational wave standard sirens, as high precision probes of the luminosity distance redshift relation.

It has been recognised, however, that weak lensing due to intervening large scale structure will significantly degrade the precision of standard sirens. Shapiro et al (2010) present a method for "de-lensing" sirens, by combining gravitational wave observations with maps of cosmic shear and flexion along each siren's line of sight.

In this presentation we explore the impact of this de-lensing procedure for constraining cosmological parameters. Using Monte Carlo simulations we investigate the accuracy with which the dimensionless density parameters may be determined, before and after de-lensing, with future data from the proposed LISA satellite and Einstein Telescope.

**Bright Ideas and Dark Thoughts: "Universal Baryonic Scale" at "Maximum Halo Gravity"**

Zhao, Hongsheng, Gianfranco Gentile, Benoit Famaey, Paolo Salucci, Andrea Maccio, Baojiu Li, Henk Hoekstra, Martin Feix

U. of St Andrews (SUPA)

I will interpret a very curious conspiracy of dark-bright matter in galaxies (Gentile et al 2009 Nature), insensitive to the sizes and formation histories of the observed galaxies: the baryons are concentrated to approximately the same surface density at the very position where the halo offers locally maximum gravity. While normal gravitational and gas feedback processes must always occur, it is difficult to forge a feedback history-independent universal scale unless there is some help from possibly new physics in the Dark. A partial confirmation is seen in simulations of N-body where the matter is coupled to a cosmological scalar field (Zhao et al. 2009, ApJ Letters).