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Search Results: 1 - 10 of 44895 matches for " Michael Boylan-Kolchin "
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Extragalactic gamma-ray background radiation from dark matter annihilation
Jesus Zavala,Volker Springel,Michael Boylan-Kolchin
Physics , 2009, DOI: 10.1111/j.1365-2966.2010.16482.x
Abstract: 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.
The Merger Rates and Mass Assembly Histories of Dark Matter Haloes in the Two Millennium Simulations
Onsi Fakhouri,Chung-Pei Ma,Michael Boylan-Kolchin
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.16859.x
Abstract: We construct merger trees of dark matter haloes and quantify their merger rates and mass growth rates using the joint dataset from the Millennium and Millennium-II simulations. The finer resolution of the Millennium-II Simulation has allowed us to extend our earlier analysis of halo merger statistics to an unprecedentedly wide range of descendant halo mass (10^10 < M0 < 10^15 Msun), progenitor mass ratio (10^-5 < xi < 1), and redshift (0 < z < 15). We update our earlier fitting form for the mean merger rate per halo as a function of M_0, xi, and z. The overall behavior of this quantity is unchanged: the rate per unit redshift is nearly independent of z out to z~15; the dependence on halo mass is weak (M0^0.13); and it is nearly a power law in the progenitor mass ratio (xi^-2). We also present a simple and accurate fitting formula for the mean mass growth rate of haloes as a function of mass and redshift. This mean rate is 46 Msun/yr for 10^12 Msun haloes at z=0, and it increases with mass as M^{1.1} and with redshift as (1+z)^2.5 (for z > 1). When the fit for the mean mass growth rate is integrated over a halo's history, we find excellent match to the mean mass assembly histories of the simulated haloes. By combining merger rates and mass assembly histories, we present results for the number of mergers over a halo's history and the statistics of the redshift of the last major merger.
Mapping extragalactic dark matter structures through gamma-rays
Jesus Zavala,Volker Springel,Michael Boylan-Kolchin
Physics , 2010,
Abstract: If dark matter is composed of neutralinos, the gamma-ray radiation produced in their annihilation offers an attractive possibility for dark matter detection. This process may contribute significantly to the extragalactic gamma-ray background (EGB) radiation, which is being measured by the FERMI satellite with unprecedented sensitivity. Using the high-resolution Millennium-II simulation of cosmic structure formation we have produced the first full-sky maps of the expected contribution of dark matter annihilation to the EGB radiation. Our maps include 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. 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 color maps combining different energies can reveal the cosmic large-scale structure at low and intermediate redshifts.
Too big to fail? The puzzling darkness of massive Milky Way subhaloes
Michael Boylan-Kolchin,James S. Bullock,Manoj Kaplinghat
Physics , 2011, DOI: 10.1111/j.1745-3933.2011.01074.x
Abstract: We show that dissipationless LCDM simulations predict that the majority of the most massive subhaloes of the Milky Way are too dense to host any of its bright satellites (L_V > 10^5 L_sun). These dark subhaloes have circular velocities at infall of 30-70 km/s and infall masses of [0.2-4] x 10^10 M_sun. Unless the Milky Way is a statistical anomaly, this implies that galaxy formation becomes effectively stochastic at these masses. This is in marked contrast to the well-established monotonic relation between galaxy luminosity and halo circular velocity (or halo mass) for more massive haloes. We show that at least two (and typically four) of these massive dark subhaloes are expected to produce a larger dark matter annihilation flux than Draco. It may be possible to circumvent these conclusions if baryonic feedback in dwarf satellites or different dark matter physics can reduce the central densities of massive subhaloes by order unity on a scale of 0.3 - 1 kpc.
The Milky Way's bright satellites as an apparent failure of LCDM
Michael Boylan-Kolchin,James S. Bullock,Manoj Kaplinghat
Physics , 2011, DOI: 10.1111/j.1365-2966.2012.20695.x
Abstract: We use the Aquarius simulations to show that the most massive subhalos in galaxy-mass dark matter halos in LCDM are grossly inconsistent with the dynamics of the brightest Milky Way dwarf spheroidal galaxies. While the best-fitting hosts of the dwarf spheroidals all have 12 < Vmax < 25 km/s, LCDM simulations predict at least ten subhalos with Vmax > 25 km/s. These subhalos are also among the most massive at earlier times, and significantly exceed the UV suppression mass back to z ~ 10. No LCDM-based model of the satellite population of the Milky Way explains this result. The problem lies in the satellites' densities: it is straightforward to match the observed Milky Way luminosity function, but doing so requires the dwarf spheroidals to have dark matter halos that are a factor of ~5 more massive than is observed. Independent of the difficulty in explaining the absence of these dense, massive subhalos, there is a basic tension between the derived properties of the bright Milky Way dwarf spheroidals and LCDM expectations. The inferred infall masses of these galaxies are all approximately equal and are much lower than standard LCDM predictions for systems with their luminosities. Consequently, their implied star formation efficiencies span over two orders of magnitude, from 0.2% to 20% of baryons converted into stars, in stark contrast with expectations gleaned from more massive galaxies. We explore possible solutions to these problems within the context of LCDM and find them to be unconvincing. In particular, we use controlled simulations to demonstrate that the small stellar masses of the bright dwarf spheroidals make supernova feedback an unlikely explanation for their low inferred densities.
How do galaxies populate Dark Matter halos?
Qi Guo,Simon White,Cheng Li,Michael Boylan-Kolchin
Physics , 2009, DOI: 10.1111/j.1365-2966.2010.16341.x
Abstract: For any assumed stellar Initial Mass Function, the Sloan Digital Sky Survey (SDSS) gives a precise determination of the stellar mass function of galaxies for 10^8 M_sun < M_* < 10^12 M_sun. Within the concordance LCDM cosmology, the Millennium simulations give a precise halo mass function for all halos 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 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 x 10^10 M_sun, the stellar mass usually assumed for the Milky Way, the implied halo mass is ~ 2 x 10^12 M_sun, 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% at masses somewhat below that of the Milky Way, 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 LCDM cosmology.
M31 Satellite Masses Compared to LCDM Subhaloes
Erik J. Tollerud,Michael Boylan-Kolchin,James S. Bullock
Physics , 2014, DOI: 10.1093/mnras/stu474
Abstract: We compare the kinematics of M31's satellite galaxies to the mass profiles of the subhaloes they are expected to inhabit in LCDM. We consider the most massive subhaloes of an approximately M31-sized halo, following the assumption of a monotonic galaxy luminosity-to-subhalo mass mapping. While this abundance matching relation is consistent with the kinematic data for galaxies down to the luminosity of the bright satellites of the Milky Way and M31, it is not consistent with kinematic data for fainter dwarf galaxies (those with L <~ 10^8 Lsun). Comparing the kinematics of M31's dwarf Spheroidal (dSph) satellites to the subhaloes reveals that M31's dSph satellites are too low density to be consistent with the subhaloes' mass profiles. A similar discrepancy has been reported between Milky Way dSphs and their predicted subhaloes, the "too big to fail" problem. By contrast, total mass profiles of the dwarf Elliptical (and similarly bright) satellites are consistent with the subhaloes. However, they suffer from large systematic uncertainties in their dark matter content because of substantial (and potentially dominant) contributions from baryons within their half-light radii.
Dynamics of the Magellanic Clouds in a LCDM Universe
Michael Boylan-Kolchin,Gurtina Besla,Lars Hernquist
Physics , 2010, DOI: 10.1111/j.1365-2966.2011.18495.x
Abstract: We examine Milky Way-Magellanic Cloud systems selected from the Millennium-II Simulation in order to place the orbits of the Magellanic Clouds in a cosmological context. Our analysis shows that satellites massive enough to be LMC analogs are typically accreted at late times. Moreover, those that are accreted at early times and survive to the present have orbital properties that are discrepant with those observed for the LMC. The high velocity of the LMC, coupled with the dearth of unbound orbits seen in the simulation, argues that the mass of the MW's halo is unlikely to be less than 2 x 10^12 Msun. This conclusion is further supported by statistics of halos hosting satellites with masses, velocities, and separations comparable to those of the LMC. We further show that: (1) LMC and SMC-mass objects are not particularly uncommon in MW-mass halos; (2) the apparently high angular momentum of the LMC is not cosmologically unusual; and (3) it is rare for a MW halo to host a LMC-SMC binary system at z=0, but high speed binary pairs accreted at late times are possible. Based on these results, we conclude that the LMC was accreted within the past four Gyr and is currently making its first pericentric passage about the MW.
On the Hot Gas Content of the Milky Way Halo
Taotao Fang,James S. Bullock,Michael Boylan-Kolchin
Physics , 2012, DOI: 10.1088/0004-637X/762/1/20
Abstract: The Milky Way appears to be missing baryons, as the observed mass in stars and gas is well below the cosmic mean. One possibility is that a substantial fraction of the Galaxy's baryons are embedded within an extended, million-degree hot halo, an idea supported indirectly by observations of warm gas clouds in the halo and gas-free dwarf spheroidal satellites. X-ray observations have established that hot gas does exist in our Galaxy beyond the local hot bubble; however, it may be distributed in a hot disk configuration. Moreover, recent investigations into the X-ray constraints have suggested that any Galactic corona must be insignificant. Here we re-examine the observational data, particularly in the X-ray and radio bands, in order to determine whether it is possible for a substantial fraction of the Galaxy's baryons to exist in ~ 10^6 K gas. In agreement with past studies, we find that a baryonically closed halo is clearly ruled out if one assumes that the hot corona is distributed with a cuspy NFW profile. However, if the hot corona of the galaxy is in an extended, low-density distribution with a large central core, as expected for an adiabatic gas in hydrostatic equilibrium, then it may contain up to 10^11 M_sun of material, possibly accounting for all of the missing Galactic baryons. We briefly discuss some potential avenues for discriminating between a massive, extended hot halo and a local hot disk.
The surprising inefficiency of dwarf satellite quenching
Coral Wheeler,John I. Phillips,Michael C. Cooper,Michael Boylan-Kolchin,James S. Bullock
Physics , 2014, DOI: 10.1093/mnras/stu965
Abstract: We study dwarf satellite galaxy quenching using observations from the Geha et al. (2012) NSA/SDSS catalog together with LCDM cosmological simulations to facilitate selection and interpretation. We show that fewer than 30% of dwarfs (M* ~ 10^8.5-10^9.5 Msun) identified as satellites within massive host halos (Mhost ~ 10^12.5-10^14 Msun) are quenched, in spite of the expectation from simulations that half of them should have been accreted more than 6 Gyr ago. We conclude that whatever the action triggering environmental quenching of dwarf satellites, the process must be highly inefficient. We investigate a series of simple, one-parameter quenching models in order to understand what is required to explain the low quenched fraction and conclude that either the quenching timescale is very long (> 9.5 Gyr, a "slow starvation" scenario) or that the environmental trigger is not well matched to accretion within the virial volume. We discuss these results in light of the fact that most of the low mass dwarf satellites in the Local Group are quenched, a seeming contradiction that could point to a characteristic mass scale for satellite quenching.
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