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Register a new userDetection of CDM Substructure
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C.S. Kochanek n Dept. of Physics
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The properties of multiple image gravitational lenses require a fractional surface mass density in satellites of f=0.02 (0.006 < f < 0.07 at 90% confidence) that is consistent with the expectations for CDM. The characteristic satellite mass scale, 10^6-10^9 Msun, is also consistent with the expectations for CDM. The agreement between the observed and expected density of CDM substructure shows that most low mass galactic satellites fail to form stars, and this absence of star formation explains the discrepancy between the number of observed Galactic satellites and CDM predictions rather than any modification to the CDM theory such as self-interacting dark matter or a warm dark matter component.
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We devise a method to measure the abundance of satellite halos in gravitational lens galaxies, and apply our method to a sample of 7 lens systems. After using Monte Carlo simulations to verify the method, we find that substructure comprises fraction f=0.02 (median, 0.006<f<0.07 at 90% confidence) of the mass of typical lens galaxies, in excellent agreement with predictions of CDM simulations. We estimate a characteristic critical radius for the satellites of 0.0001<b/arcsec<0.006 (90% confidence). For a satellite mass function of dn/dM M^x with x=-1.8 and M_l<M<M_h, the critical radius provides an estimate that the upper mass limit is 10^6Msun < M_h < 10^9Msun. Our measurement confirms a generic prediction of CDM models, and may obviate the need to invoke alternatives to CDM like warm dark matter or self-interacting dark matter.
We use a simple statistical test to show that the anomalous flux ratios observed in gravitational lenses are created by gravitational perturbations from substructure rather than propagation effects in the interstellar medium or incomplete models for the gravitational potential of the lens galaxy. We review current estimates that the substructure represents between 0.6% and 7% (90% confidence) of the lens galaxy mass, and outline future observational programs which can improve the results.
We briefly review some recent Cold Dark Matter (CDM) models. Our main focus are charge symmetric models of WIMPs which are not the standard SUSY LSPs (Lightest Supersymmetric Partners). We indicate which experiments are most sensitive to certain aspects of the models. In particular we discuss the manifestations of the new models in neutrino telescopes and other set-ups. We also discuss some direct detection experiments and comment on measuring the direction of recoil ions--which is correlated with the direction of the incoming WIMP. This could yield daily variations providing along with the annual modulation signatures for CDM.
We report the detection of a dark substructure through direct gravitational imaging - undetected in the HST-ACS F814W image - in the gravitational lens galaxy of SLACS SDSSJ0946+1006 (the Double Einstein Ring). The detection is based on a Bayesian grid reconstruction of the two-dimensional surface density of the galaxy inside an annulus around its Einstein radius (few kpc). [...] We confirm this detection by modeling the system including a parametric mass model with a tidally truncated pseudo-Jaffe density profile; in that case the substructure mass is M_sub=(3.51+-0.15)x10^9 Msun, located at (-0.651+-0.038,1.040+-0.034), precisely where also the surface density map shows a strong convergence peak. [...] We set a lower limit of (M/L)_V}>=120 (Msun/L}_V,sun (3-sigma) inside a sphere of 0.3 kpc centred on the substructure (r_tidal=1.1kpc). The result is robust under substantial changes in the model and the data-set (e.g. PSF, pixel number and scale, source and potential regularization, rotations and galaxy subtraction). Despite being at the limits of detectability, it can therefore not be attributed to obvious systematic effects. Our detection implies a dark matter mass fraction at the radius of the inner Einstein ring of f_CDM=2.15^{+2.05}_{-1.25} percent (68 percent C.L) in the mass range 4x10^6 Msun to 4x10^9 Msun assuming alpha=1.9+-0.1 (with dN/dm ~ m^-alpha). Assuming a flat prior on alpha, between 1.0 and 3.0, increases this to f_CDM=2.56^{+3.26}_{-1.50} percent (68 percent C.L). The likelihood ratio is 0.51 between our best value (f_CDM=0.0215) and that from simulations (f_sim=0.003). Hence the inferred mass fraction, admittedly based on a single lens system, is large but still consistent with predictions. [...]
We study the effects of substructure in the Galactic halo on direct detection of dark matter, on searches for energetic neutrinos from WIMP annihilation in the Sun and Earth, and on the enhancement in the WIMP annihilation rate in the halo. Our central result is a probability distribution function (PDF) P(rho) for the local dark-matter density. This distribution must be taken into account when using null dark-matter searches to constrain the properties of dark-matter candidates. We take two approaches to calculating the PDF. The first is an analytic model that capitalizes on the scale-invariant nature of the structure--formation hierarchy in order to address early stages in the hierarchy (very small scales; high densities). Our second approach uses simulation-inspired results to describe the PDF that arises from lower-density larger-scale substructures which formed in more recent stages in the merger hierarchy. The distributions are skew positive, and they peak at densities lower than the mean density. The local dark-matter density may be as small as 1/10th the canonical value of ~ 0.4 GeV/cm^3, but it is probably no less than 0.2 GeV/cm^3.
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