1,721,154 research outputs found
Dark matter annihilation in the local group
No full textUnder the hypothesis of a dark matter composed by supersymmetric particles such as neutralinos, we investigate the possibility that their annihilation in the halos of nearby galaxies could produce detectable fluxes of gamma photons. Expected fluxes depend on several, poorly known quantities such as the density profiles of dark matter halos, the existence and prominence of central density cusps and the presence of a population of subhalos. We find that, for all reasonable choices of dark matter halo models, the intensity of the gamma-ray flux from some of the nearest extragalactic objects, such as M31, is comparable to or higher than the diffuse galactic foreground. We show that next generation ground-based experiments could have the sensitivity to reveal such fluxes which could help us to unveil the nature of dark matter particles
TESTING THE LEAST ACTION PRINCIPLE IN AN OMEGA(0)=1 UNIVERSE
No full textThe least action principle (LAP) is a dynamically rigorous method for deriving the history of galaxy orbits. In particular it is an Omega test, predicting current epoch galaxy velocities as a function of position and of the cosmological background. It is most usefully applied to in-falling structures, such as the local group, where its application indicates that the preferred cosmological model is Omega(0) = 0.1 and h = 0.75 (h is the Hubble parameter in units of 100 km s(-1) Mpc(-1)). The method assumes that all the mass acts as if it were distributed as the visible galaxies. We test the reliability of the LAP to Local Group-like systems extracted from Omega(0) = 1 n-body simulations. While the orbits of the galaxies are qualitatively well reconstructed, the LAP systematically underestimates the mass of the system. This failure is attributed to the presence of extended halos weakly clustered around visible galaxies which prevent a large fraction of the group mass from being detected by the LAP technique. We conclude that the LAP method cannot rule out an Omega(0) = 1 value on the Local Group scale. Better constraints on Omega(0) may be obtained by applying this technique to in-falling systems, such as clusters, containing objects with separations large compared to galaxy sizes
Reconstructing positions and peculiar velocities of galaxy clusters within 25,000 kilometers per second: The cluster real space dipole
No full textStarting from the observed distribution of Abell/ACO galaxy clusters in redshift space, we use a two-step procedure to recover their distances and peculiar velocities. After statistically correcting for the unobserved cluster distribution in the zone of avoidance and also for a smooth absorption at higher latitudes, we use a dynamical iterative algorithm, based on that published by Strauss & Davis in 1988, to recover the real-space duster positions by minimizing the redshift-space distortions. The whole procedure assumes that clusters trace the mass, that peculiar velocities are caused by gravity, and that linear perturbation theory applies. The amplitude of the cluster dipole measured in real space turns out to be similar to 23% less than that measured in redshift space. In both cases the dipole direction is aligned with the cosmic microwave background dipole within similar to 10 degrees, taking into account the Virgocentric infall component of the Local Group motion. Using linear theory we obtain beta(c)(=Omega(0)(0.6)/b(c)) approximate to 0.21(+/-0.03), where the uncertainty is due to observational errors and limitations in the reconstruction procedure, while the intrinsic cosmological variance amounts to similar to 0.07. This beta(c) value implies that for a cluster-mass bias parameter of b(c) less than or similar to 5, a flat universe is not excluded, contrary to previous cluster-dipole z-space analysis. A more stringent determination of beta(c) will be obtained from the analysis of the peculiar velocity field in a forthcoming paper
gamma-ray flux from dark matter annihilation in galactic caustics
No full textIn the framework of indirect dark matter searches we investigate the flux of high energy gamma-ray photons produced by annihilation of dark matter in caustics within our Galaxy under the hypothesis that the bulk of dark matter is composed of the lightest supersymmetric particles. Unfortunately, the detection of the caustic annihilation signal with currently available instruments is rather challenging. Indeed, with realistic assumptions concerning particle physics and cosmology, the gamma-ray signal from caustics is below the detection threshold of both. Cerenkov telescopes and satellite-borne experiments. Nevertheless, we find that this signal is more prominent than that expected if annihilation only occurs in the smoothed Galactic halo, with the possible exception of a similar to 15 degrees circle around the Galactic centre if the mass density pro. le of our Galaxy exhibits a sharp cusp there. We show that the angular distribution of this gamma-ray flux changes significantly if DM annihilation preferentially occurs within virialized sub-haloes populating our Galaxy rather than in caustics
Tracing the cosmic velocity field at z similar to 0.1 from galaxy luminosities in the SDSS DR7
No full textSpatial modulations in the distribution of observed luminosities (computed using redshifts) of ~ 5 × 105 galaxies from the SDSS Data Release 7, probe the cosmic peculiar velocity field out to z ~ 0.1. Allowing for luminosity evolution, the r-band luminosity function, determined via a spline-based estimator, is well represented by a Schechter form with M⋆(z) - 5log10h = -20.52 - 1.6(z - 0.1) ± 0.05 and α⋆ = -1.1 ± 0.03. Bulk flows and higher velocity moments in two redshift bins, 0.02 < z < 0.07 and 0.07 < z < 0.22, agree with the predictions of the ΛCDM model, as obtained from mock galaxy catalogs designed to match the observations. Assuming a ΛCDM model, we estimate σ8 ≈ 1.1 ± 0.4 for the amplitude of the linear matter power spectrum, where the low accuracy is due to the limited number of galaxies. While the low-z bin is robust against coherent photometric uncertainties, the bias of results from the second bin is consistent with the ~ 1% magnitude tilt reported by the SDSS collaboration. The systematics are expected to have a significantly lower impact in future datasets with larger sky coverage and better photometric calibration
Peculiar velocity reconstruction with the fast action method: tests on mock redshift surveys
No full textWe present extensive tests of the fast action method (FAM) for recovering the past orbits of mass tracers in an expanding universe from their redshift-space coordinates at the present epoch. The tests focus on the reconstruction of present-day peculiar velocities using mock catalogues extracted from high-resolution N-body simulations. The method allows for a self-consistent treatment of redshift-space distortions by direct minimization of a modified action for a cosmological gravitating system. When applied to ideal, volume-limited catalogues, FAM recovers unbiased peculiar velocities with a one-dimensional, 1sigma error of similar to220 km s(-1), if velocities are smoothed on a scale of 5 h(-1) Mpc. Alternatively, when no smoothing is applied, FAM predicts nearly unbiased velocities for objects residing outside the highest density regions. In this second case the 1sigma error decreases to a level of similar to150 km s(-1). The correlation properties of the peculiar velocity fields are also correctly recovered on scales larger than 5 h(-1) Mpc. Similar results are obtained when FAM is applied to flux-limited catalogues mimicking the IRAS PSCz survey. In this case FAM reconstructs peculiar velocities with similar intrinsic random errors, while velocity-velocity correlation properties are well reproduced beyond scales of similar to8 h(-1) Mpc. We also show that FAM provides better velocity predictions than other, competing methods based on linear theory or the Zel'dovich approximation. These results indicate that FAM can be successfully applied to presently available galaxy redshift surveys such as IRAS PSCz
COLORS, LUMINOSITIES, AND MASSES OF DISK GALAXIES .2. ENVIRONMENTAL DEPENDENCIES
No full textThe B-band and near-infrared (H) luminosity functions of spiral galaxies are derived for the Coma and A1367 clusters and for a reference population of ''field'' galaxies in the Coma supercluster. They are consistent at the bright end, but they differ significantly at the faint end, indicating an overdensity of spirals with blue color (B-H -21.5) in clusters with respect to the field. These objects have disturbed morphology and peculiar velocities significantly larger than the rest of the cluster sample. We discuss these results in the framework of a possible environmental dependence of galaxy evolution, and we conclude that enhanced current star formation in cluster spiral galaxies might occur due to molecular gas collapse stimulated by the ram-pressure mechanism
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