The evolution of dark matter substructure

This thesis investigates the dynamical evolution of systems orbiting within deeper potentials. Initially we use a simple satellite-halo interaction to study the dynamical processes that act on orbiting systems and we compare these results to analytical theory. Deep images of the Centaurus cluster reveal a spectacular arc of diffuse light that stretches for over 100 kpc and yet is just a few kpc wide. We use numerical simulations to show that this feature can be produced by the tidal debris of a spiral galaxy that has been disrupted by the potential of one of the central cD galaxies of the cluster. The evolution of sub-halos is then studied in a cosmological context using high resolution N-body simulations of galactic mass halos that form in a cold dark matter (CDM) simulation. CDM halos form via a complex series of mergers, accretion events and violent relaxation. Halos are non-spherical, have steep singular density profiles and contain many thousands of surviving dark matter substructure clumps. This will lead to several unique signatures for experiments that aim to detect dark matter either indirectly, through particle annihilation, or directly in a laboratory. For the first time it is possible to construct maps of the gamma-ray sky that result from the annihilation of dark matter particles within simulated dark matter halo distributions

Gamma-ray and synchrotron emission from neutralino annihilation in the Large Magellanic Cloud

We calculate the expected flux of gamma-ray and radio emission from the LMC due to neutralino annihilation. Using rotation curve data to probe the density profile and assuming a minimum disk, we describe the dark matter halo of the LMC using models predicted by N-body simulations. We consider a range of density profiles including the NFW profile, a modified NFW profile proposed by Hayashi et al.(2003) to account for the effects of tidal stripping, and an isothermal sphere with a core. We find that the gamma-ray flux expected from these models may be detectable by GLAST for a significant part of the neutralino parameter space. The prospects for existing and upcoming Atmospheric Cherenkov Telescopes are less optimistic, as unrealistically long exposures are required for detection. However, the effects of adiabatic compression due to the baryonic component may improve the chances for detection by ACTs. The maximum flux we predict is well below EGRET's measurements and thus EGRET does not constrain the parameter space. The expected synchrotron emission generally lies below the observed radio emission from the LMC in the frequency range of 19.7 to 8550 MHz. As long as <2x 10^-26 cm^3 s^-1 for a neutralino mass of 50 GeV, the observed radio emission is not primarily due to neutralinos and is consistent with the assumption that the main source is cosmic rays. We find that the predicted fluxes, obtained by integrating over the entire LMC, are not very strongly dependent on the inner slope of the halo profile, varying by less than an order of magnitude for the range of profiles we considered.Comment: 24 pages, 5 figures; detailed discussion of how the neutralino induced signals compare with the cosmic-ray induced ones was added. Main conclusions unchanged. Matches accepted version, to appear in Astroparticle Physic

A large HI cloud near the centre of the Virgo cluster

We report the discovery of a large HI cloud in the central regions of the Virgo cluster. It is 110 x 25 kpc in size and contains 3.4 x 10^8 Msol of HI. The morphology and kinematics of this cloud strongly suggest that it consists of HI removed from the galaxy NGC 4388 by ram-pressure stripping. It is more likely the result of an interaction of the ISM of NGC 4388 with the hot halo of the M86 group and not with the ICM centred on M87. The large extent of the plume suggests that gas stripped from cluster galaxies can remain neutral for at least 10^8 yr. Locally, the column density is well above 10^20 cm^-2, suggesting that the intra-cluster HII regions known to exist in Virgo may have formed from gas stripped from cluster galaxies. The existence of the HI plume suggests that stripping of infalling spirals contributes to the enrichment of the ICM. The HI object in the Virgo cluster recently reported by Minchin et al. may have a similar origin and may therefore not be a ``dark galaxy''.Comment: Accepted for publication in A&A. 5 pages, 2 figures. Version with high-resolution figures at http://www.astron.nl/~oosterlo/n438

Diffuse light and building history of the galaxy cluster Abell 2667

We have searched for diffuse intracluster light in the galaxy cluster Abell 2667 (z=0.233) from HST images in three filters. We have applied to these images an iterative multi-scale wavelet analysis and reconstruction technique, which allows to subtract stars and galaxies from the original images. We detect a zone of diffuse emission south west of the cluster center (DS1), and a second faint object (ComDif), within DS1. Another diffuse source (DS2) may be detected, at lower confidence level, north east of the center. These sources of diffuse light contribute to 10-15% of the total visible light in the cluster. Whether they are independent entities or are part of the very elliptical external envelope of the central galaxy remains unclear. VLT VIMOS integral field spectroscopy reveals a faint continuum at the positions of DS1 and ComDif but do not allow to compute a redshift. A hierarchical substructure detection method reveals the presence of several galaxy pairs and groups defining a similar direction as the one drawn by the DS1-central galaxy-DS2 axis. The analysis of archive XMM-Newton and Chandra observations shows X-ray emission elongated in the same direction. The X-ray temperature map shows the presence of a cool core, a broad cool zone stretching from north to south and hotter regions towards the north east, south west and north west. This possibly suggests shock fronts along these directions produced by infalling material. These various data are consistent with a picture in which diffuse sources are concentrations of tidal debris and harassed matter expelled from infalling galaxies by tidal stripping and undergoing an accretion process onto the central cluster galaxy; as such, they are expected to be found along the main infall directions.Comment: Accepted for publication in Astronomy and Astrophysic

Neutralino Annihilation at the Galactic Center Revisited

The annihilation of neutralino dark matter in the Galactic Center (GC) may result in radio signals that can be used to detect or constrain the dark matter halo density profile or dark matter particle properties. At the Galactic Center, the accretion flow onto the central Black Hole (BH) sustains strong magnetic fields that can induce synchrotron emission by electrons and positrons generated in neutralino annihilations during advection onto the BH. Here we reanalyze the radiative processes relevant for the neutralino annihilation signal at the GC, with realistic assumptions about the accretion flow and its magnetic properties. We find that neglecting these effects, as done in previous papers, leads to the incorrent electron and photon spectra. We find that the magnetic fields associated with the flow are significantly stronger than previously estimated. We derive the appropriate equilibrium distribution of electrons and positron and the resulting radiation, considering adiabatic compression in the accretion flow, inverse Compton scattering off synchrotron photons (synchrotron self-Compton scattering), and synchrotron self-absorption of the emitted radiation. We derive the signal for a Navarro-Frenk-White (NFW) dark matter halo profile and a NFW profile with a dark matter spike due to the central BH. We find that the observed radio emission from the GC is inconsistent with the scenario in which a spiky distribution of neutralinos is present. We discuss several important differences between our calculations and those previously presented in the literature.Comment: 19 pages, 11 eps figures, version accepted for publication in JCA

Cusps in CDM halos

We resolve the inner region of a massive cluster forming in a cosmological LCDM simulation with a mass resolution of 2*10^6 Msun and before z=4.4 even 3*10^5 Msun. This is a billion times less than the clusters final virial mass and a substantial increase over current LCDM simulations. We achieve this resolution using a new multi-mass refinement procedure and are now able to probe a dark matter halo density profile down to 0.1 percent of the virial radius. The inner density profile of this cluster halo is well fitted by a power-law rho ~ r^-gamma down to the smallest resolved scale. An inner region with roughly constant logarithmic slope is now resolved, which suggests that cuspy profiles describe the inner profile better than recently proposed profiles with a core. The cluster studied here is one out of a sample of six high resolution cluster simulations of Diemand et al. (2004) and it's inner slope of gamma = 1.2 lies close to the sample average.Comment: 10 pages,10 figures. Matches version in press. Added Figure 7 and section 3.3. about upper limits of inner slopes, text (incl, title and abstract) revised, conclusions unchange

Large scale diffuse light in the Coma cluster: a multi-scale approach

We have obtained wide field images of the Coma cluster in the B, V, R and I bands with the CFH12K camera at CFHT. In order to search for large scale diffuse emission, we have applied to these images an iterative multi scale wavelet analysis and reconstruction technique which allowed to model all the sources (stars and galaxies) and subtract them from the original images. We found various concentrations of diffuse emission present in the central zone around the central galaxies NGC4874 and NGC4889. We characterize the positions, sizes and colors of these concentrations. Some sources do not seem to have strong star formation, while another one probably exhibits spiral-like color. One possible origin for the star forming diffuse emission sources is that in the region of the two main galaxies NGC4874 and NGC4889 spiral galaxies have recently been disrupted and star formation is still active in the dispersed material. We also use the characteristics of the sources of diffuse emission to trace the cluster dynamics. A scenario in which the group around NGC 4874 is moving north is consistent with our data.Comment: 11 pages, accepted in A&A, jpg figure

The effect of dwarf galaxies disruption in semi-analytic models

We present results for a galaxy formation model that includes a simple treatment for the disruption of dwarf galaxies by gravitational forces and galaxy encounters within galaxy clusters. This is implemented a posteriori in a semi-analytic model by considering the stability of cluster dark matter sub-haloes at z=0. We assume that a galaxy whose dark matter substructure has been disrupted will itself disperse, while its stars become part of the population of intracluster stars responsible for the observed intracluster light. Despite the simplicity of this assumption, our results show a substantial improvement over previous models and indicate that the inclusion of galaxy disruption is indeed a necessary ingredient of galaxy formation models. We find that galaxy disruption suppresses the number density of dwarf galaxies by about a factor of two. This makes the slope of the faint end of the galaxy luminosity function shallower, in agreement with observations. In particular, the abundance of faint, red galaxies is strongly suppressed. As a result, the luminosity function of red galaxies and the distinction between the red and the blue galaxy populations in colour-magnitude relationships are correctly predicted. Finally, we estimate a fraction of intracluster light comparable to that found in clusters of galaxies.Comment: 7 pages, 6 figures, accepted for publication in MNRAS, 2 figures changed and references adde

Convergence and scatter of cluster density profiles

We present new results from a series of LCDM simulations of cluster mass halos resolved with high force and mass resolution. These results are compared with recently published simulations from groups using various codes including PKDGRAV, ART, TPM, GRAPE and GADGET. Careful resolution tests show that with 25 million particles within the high resolution region we can resolve to about 0.3% of the virial radius and that convergence in radius is proportional to the mean interparticle separation. The density profiles of 26 high resolution clusters obtained with the different codes and from different initial conditions agree very well. The average logarithmic slope at one percent of the virial radius is $\gamma = 1.26$ with a scatter of $\pm 0.17$. Over the entire resolved regions the density profiles are well fitted by a smooth function that asymptotes to a central cusp $\rho \propto r^{-\gamma}$, where we find $\gamma=1.16\pm 0.14$ from the mean of the fits to our six highest resolution clusters.Comment: MNRAS accepted version. Due to an error in the initial conditions these simulations have a lower sigma_8 than the published value, 0.7 instead of 0.9. We thank Mike Kuhlen who helped us finding this mistake. See the erratum at http://www-theorie.physik.unizh.ch/~diemand/proerr.pdf . Images and movies available at http://www-theorie.physik.unizh.ch/~diemand/clusters

Bayesian Strong Gravitational-Lens Modeling on Adaptive Grids: Objective Detection of Mass Substructure in Galaxies

We introduce a new adaptive and fully Bayesian grid-based method to model strong gravitational lenses with extended images. The primary goal of this method is to quantify the level of luminous and dark-mass substructure in massive galaxies, through their effect on highly-magnified arcs and Einstein rings. The method is adaptive on the source plane, where a Delaunay tessellation is defined according to the lens mapping of a regular grid onto the source plane. The Bayesian penalty function allows us to recover the best non-linear potential-model parameters and/or a grid-based potential correction and to objectively quantify the level of regularization for both the source and the potential. In addition, we implement a Nested-Sampling technique to quantify the errors on all non-linear mass model parameters -- ... -- and allow an objective ranking of different potential models in terms of the marginalized evidence. In particular, we are interested in comparing very smooth lens mass models with ones that contain mass-substructures. The algorithm has been tested on a range of simulated data sets, created from a model of a realistic lens system. One of the lens systems is characterized by a smooth potential with a power-law density profile, twelve include a NFW dark-matter substructure of different masses and at different positions and one contains two NFW dark substructures with the same mass but with different positions. Reconstruction of the source and of the lens potential for all of these systems shows the method is able, in a realistic scenario, to identify perturbations with masses >=10^7 solar mass when located on the Einstein ring. For positions both inside and outside of the ring, masses of at least 10^9 solar mass are required (...).Comment: 21 pages, 15 figures, 4 tables; accepted for publication in MNRA