Entity

Time filter

Source Type


Faltenbacher A.,University of the Western Cape | Faltenbacher A.,Max Planck Institute for Astrophysics | Faltenbacher A.,Joint Center for Astrophysical Cosmology at Shanghai Astronomical Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2010

We examine the effects of environment on the dynamical structure of satellite systems based on the Millennium II Simulation. Satellite haloes are defined as subhaloes within the virial radius of a host halo. The satellite sample is restricted to those subhaloes which showed a maximum circular velocity above 30 km s-1 at the time of accretion. Host halo masses range from 1011 to 1014 h-1 M.. We compute the satellites' average accretion redshift, zacc, velocity dispersion, σ, and velocity anisotropy parameter, β, utilizing stacked satellite samples of equal-mass hosts at similar background densities. The main results are as follows. (1) On an average, satellites within hosts in high-density environments are accreted earlier (Δz≈ 0.1) compared to their counterparts at low densities. For host masses above 5 × 1013 h-1 M. this trend weakens, and may reverse for higher host masses. (2) The velocity dispersion of satellites in low-density environments follows that of the host, i.e. no velocity bias is observed for host haloes at low densities independent of host mass. However, for low-mass hosts in high-density environments the velocity dispersion of the satellites can be up to ~30 per cent larger than that of the host halo, i.e. the satellites are dynamically hotter than their host haloes. (3) The anisotropy parameter depends on host mass and environment. Satellites of massive hosts show more radially biased velocity distributions. Moreover in low-density environments, satellites have more radially biased velocities (Δβ≳ 0.1) as compared to their counterparts in high-density environments. We believe that our approach allows us to predict a similar behaviour for observed satellite galaxy systems. © 2010 The Author. Journal compilation © 2010 RAS. Source


Zavala J.,Max Planck Institute for Astrophysics | Zavala J.,Joint Center for Astrophysical Cosmology at Shanghai Astronomical Observatory | Springel V.,Max Planck Institute for Astrophysics | Boylan-Kolchin M.,Max Planck Institute for Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2010

If dark matter is composed of neutralinos, one of the most exciting prospects for its detection lies in observations of the gamma-ray radiation created in pair annihilations between neutralinos, a process that may contribute significantly to the extragalactic gamma-ray background (EGB) radiation. We here use the high-resolution Millennium-II simulation of cosmic structure formation to produce the first full sky maps of the expected radiation coming from extragalactic dark matter structures. Our map-making procedure takes into account the total gamma-ray luminosity from all haloes and their subhaloes, and includes corrections for unresolved components of the emission as well as an extrapolation to the damping scale limit of neutralinos. Our analysis also includes a proper normalization of the signal according to a specific supersymmetric model based on minimal supergravity. The new simulated maps allow a study of the angular power spectrum of the gamma-ray background from dark matter annihilation, which has distinctive features associated with the nature of the annihilation process and may be detectable in forthcoming observations by the recently launched Fermi satellite. Our results are in broad agreement with analytic models for the gamma-ray background, but they also include higher order correlations not readily accessible in analytic calculations and, in addition, provide detailed spectral information for each pixel. In particular, we find that difference maps at different energies can reveal cosmic large-scale structure at low and intermediate redshifts. If the intrinsic emission spectrum is characterized by an emission peak, cosmological tomography with gamma-ray annihilation radiation is in principle possible. © 2010 The Authors. Journal compilation © 2010 RAS. Source


Guo Q.,Max Planck Institute for Astrophysics | White S.,Max Planck Institute for Astrophysics | Li C.,Joint Center for Astrophysical Cosmology at Shanghai Astronomical Observatory | Boylan-Kolchin M.,Max Planck Institute for Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2010

For any assumed standard stellar initial mass function, the Sloan Digital Sky Survey (SDSS) gives a precise determination of the abundance of galaxies as a function of their stellar mass over the full stellar mass range 108 M⊙ < M* < 1012 M⊙. Within the concordance Λ cold dark matter (ΛCDM) cosmology, the Millennium Simulations give precise halo abundances as a function of mass and redshift for all haloes within which galaxies can form. Under the plausible hypothesis that the stellar mass of a galaxy is an increasing function of the maximum mass ever attained by its halo, these results combine to give halo mass as a function of stellar mass. The result agrees quite well with observational estimates of mean halo mass as a function of stellar mass from stacking analyses of the gravitational lensing signal and the satellite dynamics of SDSS galaxies. For M* ∼ 5.5 × 1010 M⊙, the stellar mass usually assumed for the Milky Way (MW), the implied halo mass is ∼2 × 1012 M⊙, consistent with most recent direct estimates and inferences from the MW/M31 timing argument. The fraction of the baryons associated with each halo which are present as stars in its central galaxy reaches a maximum of 20 per cent at masses somewhat below that of the MW and falls rapidly at both higher and lower masses. These conversion efficiencies are lower than in almost all recent high-resolution simulations of galaxy formation, showing that these are not yet viable models for the formation of typical members of the galaxy population. When inserted in the Millennium-II Simulation, our derived relation between stellar mass and halo mass predicts a stellar mass autocorrelation function in excellent agreement with that measured directly in the SDSS. The implied Tully-Fisher relation also appears consistent with observation, suggesting that galaxy luminosity functions and Tully-Fisher relations can be reproduced simultaneously in a ΛCDM cosmology. © 2010 The Authors. Journal compilation © 2010 RAS. Source


Faltenbacher A.,Max Planck Institute for Astrophysics | Faltenbacher A.,Joint Center for Astrophysical Cosmology at Shanghai Astronomical Observatory | Faltenbacher A.,University of the Western Cape | White S.D.M.,Max Planck Institute for Astrophysics
Astrophysical Journal | Year: 2010

Based on the Millennium Simulation we examine assembly bias for the halo properties: shape, triaxiality, concentration, spin, shape of the velocity ellipsoid, and velocity anisotropy. For consistency, we determine all these properties using the same set of particles, namely all gravitationally self-bound particles belonging to the most massive substructure of a given friends-of-friends halo. We confirm that near-spherical and high-spin halos show enhanced clustering. The opposite is true for strongly aspherical and low-spin halos. Further, below the typical collapse mass, M *, more concentrated halos show stronger clustering, whereas less concentrated halos are less clustered which is reversed for masses above M *. Going beyond earlier work we show that: (1) oblate halos are more strongly clustered than prolate ones; (2) the dependence of clustering on the shape of the velocity ellipsoid coincides with that of the real-space shape, although the signal is stronger; (3) halos with weak velocity anisotropy are more clustered, whereas radially anisotropic halos are more weakly clustered; (4) for all highly clustered subsets we find systematically less radially biased velocity anisotropy profiles. These findings indicate that the velocity structure of halos is tightly correlated with environment. © 2010. The American Astronomical Society. Source


Donoso E.,Max Planck Institute for Astrophysics | Li C.,Max Planck Institute for Astrophysics | Li C.,Joint Center for Astrophysical Cosmology at Shanghai Astronomical Observatory | Kauffmann G.,Max Planck Institute for Astrophysics | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

We compute the cross-correlation between a sample of 14 000 radio-loud active galactic nuclei (RLAGN) with redshifts between 0.4 and 0.8 selected from the Sloan Digital Sky Survey and a reference sample of 1.2 million luminous red galaxies in the same redshift range. We quantify how the clustering of RLAGN depends on host galaxy mass and on radio luminosity. RLAGN are clustered more strongly on all scales than control samples of radio-quiet galaxies with the same stellar masses and redshifts, but the differences are largest on scales less than ∼1 Mpc. In addition, the clustering amplitude of the RLAGN varies significantly with radio luminosity on scales less than ∼1 Mpc. This suggests that the gaseous environment of a galaxy on the scale of its dark matter halo, plays a key role in determining not only the probability that a galaxy is RLAGN, but also the total luminosity of the radio jet. Next, we compare the clustering of radio galaxies with that of radio-loud quasars in the same redshift range. Unified models predict that both types of active nuclei should cluster in the same way. Our data show that most RLAGN are clustered more strongly than radio-loud QSOs, even when the AGN and QSO samples are matched in both black hole mass and radio luminosity. Only the most extreme RLAGN and radio-loud QSOs (RLQSOs) in our sample, with radio luminosities in excess of ∼1026 W Hz-1, have similar clustering properties. The majority of the strongly evolving RLAGN population at redshifts ∼0.5 are found in different environments to the quasars, and hence must be triggered by a different physical mechanism. © 2010 The Authors. Journal compilation © 2010 RAS. Source

Discover hidden collaborations