Mass accretion histories & density profiles of LCDM clusters

We analyze the mass accretion histories (MAHs) and density profiles of cluster-size halos in a flat LCDM cosmology.To fit the MAHs of all systems we need to generalize the MAH fit found in previous systematic studies of predominantly galactic halos.We find that the concentration of the density distribution is tightly correlated with the halo MAH and its formation redshift. During the early period of fast mass growth the concentration remains roughly constant and low (~3-4), while during the slow accretion phase it increases with decreasing redshift as 1/(1+z).We consider fits of three widely discussed analytic density profiles.We find that there is no unique best fit for all clusters.At the same time, if a cluster is best fit by a particular analytic profile at z=0, the same is usually true at earlier epochs out to z ~ 1-2.The local logarithmic slope of the density profiles at 3% of the virial radius ranges from -1.2 to -2.0.In addition, the logarithmic slope becomes shallower with decreasing radius without reaching an asymptotic value down to the smallest resolved scale (<1% of the virial radius).During the early MAH period of rapid mass growth the density profiles can be well described by a single power law rho(r) ~ r^-a with a ~ 1.5-2.The relatively shallow power law slopes result in low concentrations at these stages of evolution, as the scale radius where the density profiles reaches the slope of -2 is at large radii.This indicates that the inner power law like density distribution of halos is built up during the periods of rapid mass accretion and active merging, while the outer steeper profile is formed when the mass accretion slows down.Comment: 5 pages, 4 figures, to appear in the proceedings of the XXIst IAP Colloquium 'Mass profiles and shapes of cosmological structures', Paris 4-9 July 2005, France, eds. G. Mamon, F. Combes, C. Deffayet, B. Fort, EAS Publications Serie

Modeling the galaxy/light-mass connection with cosmological simulations

I review some results on the galaxy/light-mass connection obtained by dissipationless simulations in combination with a simple, non-parametric model to connect halo circular velocity to the luminosity of the galaxy they would host. I focus on the galaxy-mass correlation and mass-to-light ratios obtained from galaxy up to cluster scales. The predictions of this simple scheme are shown to be in very good agreement with SDSS observations.Comment: proceedings of the 6th International Workshop on The Identification of Dark Matter, 11 - 16 September 2006, Rhodes Island, Greece; to be published by World Scientifi

Neutralino annihilation in the Large Magellanic Cloud

I present results with respect to the gamma-ray and synchrotron fluxes due to neutralino annihilation in the Large Magellanic Cloud based on Tasitsiomi et al., Astrop. Phys. 21, 637 (2004).Comment: 6 pages, 4 figures; to appear in the 2004 International Symposium of High Energy Gamma Ray Astronomy, Heidelberg, July 2004 (AIP Proceedings Series

Pairwise velocities in the Halo Model: Luminosity and Scale Dependence

We investigate the properties of the pairwise velocity dispersion as a function of galaxy luminosity in the context of a halo model. We derive the distribution of velocities of pairs at a given separation taking into account both one-halo and two-halo contributions. We show that pairwise velocity distribution in real space is a complicated mixture of host-satellite, satellite-satellite and two-halo pairs. The peak value is reached at around 1$h^{-1}$Mpc and does not reflect the velocity dispersion of a typical halo hosting these galaxies, but is instead dominated by the satellite-satellite pairs in high mass clusters. This is true even for cross-correlations between bins separated in luminosity. As a consequence the velocity dispersion at a given separation can decrease with luminosity, even if the underlying typical halo host mass is increasing, in agreement with recent observations. We compare our findings to numerical simulations and find a good agreement. Numerical simulations also suggest a luminosity dependent velocity bias, which depends on the subhalo mass. We develop models of the auto- and cross-correlation function of luminosity subsamples of galaxies in the observable r_\proj - \pi space and calculate the inferred velocity dispersion as a function of wave vector if dispersion model is fit to the redshift space power spectrum. We find that so derived pairwise velocity dispersion also exhibits a bump at $k\sim 1 h/{\rm Mpc}$.Comment: 11 pages, 12 figures; v2: major revision matching version accepted by MNRA

Neutralino annihilation gamma-rays from clumps and the LMC

We discuss the detectability of dark matter clumps in the Milky Way halo due to neutralino annihilation. We then focus on a known ``clump'', the Large Magellanic Cloud.Comment: 4 pages, 4 figures; to appear in New Astronomy Reviews, Proceedings of 2nd VERITAS symposiu

Dark Matter Halo Mergers I: Dependence on Environment & Redshift Evolution

This paper presents a study of the specific merger rate as a function of group membership, local environment, and redshift in a very large, $500h^{-1} Mpc$, cosmological N-body simulation, the \textit{Millennium Simulation}. The goal is to provide environmental diagnostics of major merger populations in order to test simulations against observations and provide further constraints on major merger driven galaxy evolution scenarios. A halo sample is defined using the maximum circular velocity, which is both well defined for subhalos and closely correlated with galaxy luminosity. Subhalos, including the precursors of major mergers, are severely tidally stripped. Major mergers between subhalos are therefore extremely rare. Tidal stripping also suppresses dynamical friction, resulting in long major merger time scales when the more massive halo does not host other subhalos. In contrast, when other subhalos are present major merger time scales are several times shorter. This enhancement is likely due to inelastic unbound collisions between subhalos. Following these results, we predict that major mergers in group environments are dominated by mergers involving the central galaxy, that the specific merger rate is suppressed in groups, and that the frequency of fainter companions is enhanced for mergers and their remnants. We also observe an `assembly bias' in the major merger rate in that mergers of galaxy-like halos are slightly suppressed in overdense environments while mergers of group-like halos are slightly enhanced. A dynamical explanation for this trend is advanced which calls on both tidal effects and interactions between bound halos beyond the virial radii of locally dynamically dominant halos.Comment: 44 pages, 8 figures, Preprint Submitted to Ap

Dark Matter Halo Mergers I: Dependence on Environment & Redshift Evolution

This paper presents a study of the specific merger rate as a function of group membership, local environment, and redshift in a very large, $500h^{-1} Mpc$, cosmological N-body simulation, the \textit{Millennium Simulation}. The goal is to provide environmental diagnostics of major merger populations in order to test simulations against observations and provide further constraints on major merger driven galaxy evolution scenarios. A halo sample is defined using the maximum circular velocity, which is both well defined for subhalos and closely correlated with galaxy luminosity. Subhalos, including the precursors of major mergers, are severely tidally stripped. Major mergers between subhalos are therefore extremely rare. Tidal stripping also suppresses dynamical friction, resulting in long major merger time scales when the more massive halo does not host other subhalos. In contrast, when other subhalos are present major merger time scales are several times shorter. This enhancement is likely due to inelastic unbound collisions between subhalos. Following these results, we predict that major mergers in group environments are dominated by mergers involving the central galaxy, that the specific merger rate is suppressed in groups, and that the frequency of fainter companions is enhanced for mergers and their remnants. We also observe an `assembly bias' in the major merger rate in that mergers of galaxy-like halos are slightly suppressed in overdense environments while mergers of group-like halos are slightly enhanced. A dynamical explanation for this trend is advanced which calls on both tidal effects and interactions between bound halos beyond the virial radii of locally dynamically dominant halos.Comment: 44 pages, 8 figures, Preprint Submitted to Ap

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