Probing the Mass Distribution with IRAS Galaxies

We present the results of three independent analyses in which we show that IRAS galaxies faithfully trace the underlying distribution the mass in the local universe. In the first analysis we compare the mass and the galaxy density fields and show that they are consistent within 60 $h^{-1}$Mpc. In the second one we show that the tidal velocity field of the Mark III catalog is consistent with the tidal velocity field predicted from the distribution of IRAS galaxies, hence indicating that IRAS galaxies trace the mass distribution well beyond 60 $h^{-1}$Mpc. Finally, a third analysis aimed at determining the mean biasing relation of IRAS galaxies showed no appreciable deviations from the linear biasing prescription.Comment: 5 pages including 4 Figures + 1 LaTex macro. Contribution to ``Where's the Matter? Tracing Dark and Bright Matter with the New Generation of Large Scale Surveys'', June 2001, Treyer & Tresse Eds, Frontier Grou

On the detectability of gamma-rays from Dark Matter annihilation in the Local Group with ground-based experiments

Recent studies have suggested the possibility that the lightest supersymmetric particle is a suitable dark matter candidate. In this theoretical framework, annihilations in high density environments like the center of dark matter haloes may produce an intense flux of gamma-rays. In this paper we discuss the possibility of detecting the signatures of neutralino annihilation in nearby galaxies with next generation ground-based detectors.Comment: to appear in Proceedings of ICRC 200

γ\gamma-ray flux from Dark Matter Annihilation in Galactic Caustics

In the frame 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 caustics 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 $\check {\rm C}$erenkov 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 $\sim 15^{\circ}$ circle around the Galactic center if the mass density profile 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-halos populating our Galaxy rather than in caustics.Comment: 17 pages, 8 figures. Accepted for publication in JCA

Testing the Least Action Principle in an Omega_0=1 Universe

The least action principle (LAP) is a dynamically rigorous method for deriving the history of galaxy orbits. In particular it is an Omega_0 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.Comment: accepted by APJ, uuencoded-compressed-tarred PostScript file including figures. SISSA Ref. 56/94/

On the recovery of Local Group motion from galaxy redshift surveys

There is a $\sim 150 km s^{-1}$ discrepancy between the measured motion of the Local Group of galaxies (LG) with respect to the CMB and the linear theory prediction based on the gravitational force field of the large scale structure in full-sky redshift surveys. We perform a variety of tests which show that the LG motion cannot be recovered to better than $150-200 km s^{-1}$ in amplitude and within a $\approx10^\circ$ in direction. The tests rely on catalogs of mock galaxies identified in the Millennium simulation using semi-analytic galaxy formation models. We compare these results to the $K_s=11.75$ Two-Mass Galaxy Redshift Survey, which provides the deepest, widest and most complete spatial distribution of galaxies available so far. In our analysis we use a new, concise relation for deriving the LG motion and bulk flow from the true distribution of galaxies in redshift space. Our results show that the main source of uncertainty is the small effective depth of surveys like the 2MRS that prevents a proper sampling of the large scale structure beyond $\sim100 h^{-1} Mpc$. Deeper redshift surveys are needed to reach the "convergence scale" of $\approx 250 h^{-1}Mpc$ in a $\Lambda$CDM universe. Deeper survey would also mitigate the impact of the "Kaiser rocket" which, in a survey like 2MRS, remains a significant source of uncertainty. Thanks to the quiet and moderate density environment of the LG, purely dynamical uncertainties of the linear predictions are subdominant at the level of $\sim 90 km s^{-1}$. Finally, we show that deviations from linear galaxy biasing and shot noise errors provide a minor contribution to the total error budget.Comment: 14 pages, 7 figure

Growth rate of cosmological perturbations at z ~ 0.1 from a new observational test

Spatial variations in the distribution of galaxy luminosities, estimated from redshifts as distance proxies, are correlated with the peculiar velocity field. Comparing these variations with the peculiar velocities inferred from galaxy redshift surveys is a powerful test of gravity and dark energy theories on cosmological scales. Using ~ 2 $\times$ 10$^{5}$ galaxies from the SDSS Data Release 7, we perform this test in the framework of gravitational instability to estimate the normalized growth rate of density perturbations f$\sigma_{8}$ = 0.37 +/- 0.13 at z ~ 0.1, which is in agreement with the $\Lambda$CDM scenario. This unique measurement is complementary to those obtained with more traditional methods, including clustering analysis. The estimated accuracy at z ~ 0.1 is competitive with other methods when applied to similar datasets.Comment: 4 pages, 2 figures, matches version accepted for publication in PR

Anisotropy probe of galactic and extra-galactic Dark Matter annihilations

We study the flux and the angular power spectrum of gamma-rays produced by Dark Matter (DM) annihilations in the Milky Way (MW) and in extra-galactic halos. The annihilation signal receives contributions from: a) the smooth MW halo, b) resolved and unresolved substructures in the MW, c) external DM halos at all redshifts, including d) their substructures. Adopting a self-consistent description of local and extra-galactic substructures, we show that the annihilation flux from substructures in the MW dominates over all the other components for angles larger than O(1) degrees from the Galactic Center, unless an extreme prescription is adopted for the substructures concentration. We also compute the angular power spectrum of gamma-ray anisotropies and find that, for an optimistic choice of the particle physics parameters, an interesting signature of DM annihilations could soon be discovered by the Fermi LAT satellite at low multipoles, l<100, where the dominant contribution comes from MW substructures with mass M>10^4 solar masses. For the substructures models we have adopted, we find that the contribution of extra-galactic annihilations is instead negligible at all scales.Comment: 14 pages, 7 figure

Cluster versus POTENT Density and Velocity Fields: Cluster Biasing and Omega

The density and velocity fields as extracted from the Abell/ACO clusters are compared to the corresponding fields recovered by the POTENT method from the Mark~III peculiar velocities of galaxies. In order to minimize non-linear effects and to deal with ill-sampled regions we smooth both fields using a Gaussian window with radii ranging between 12 - 20\hmpc. The density and velocity fields within 70\hmpc exhibit similarities, qualitatively consistent with gravitational instability theory and a linear biasing relation between clusters and mass. The random and systematic errors are evaluated with the help of mock catalogs. Quantitative comparisons within a volume containing $\sim\!12$ independent samples yield \betac\equiv\Omega^{0.6}/b_c=0.22\pm0.08, where $b_c$ is the cluster biasing parameter at 15\hmpc. If $b_c \sim 4.5$, as indicated by the cluster correlation function, our result is consistent with $\Omega \sim 1$.Comment: 18 pages, latex, 2 ps figures 6 gif figures. Accepted for pubblications in MNRA