The Dark Galaxy Problem and the Effects of Substructure on Gravitational Lenses.pdfVIP

The Dark Galaxy Problem and the Effects of Substructure on Gravitational Lenses R. Benton Metcalf Institute of Astronomy Madingley Road Cambridge CB3 0HA England Abstract. I argue that the cold dark matter (CDM) model requires that even within a few 1 kpc of the center of a galactic halo a significant fraction (greater than a few percent) of the 0 surface density is contained in substructures with masses 103 M⊙ . These structures should be 0 ∼ 2 light enough to avoid dynamical friction and dense enough to avoid tidal disruption. I then show using the results of numerical simulations that this substructure will significantly alter the flux p e ratios of multiply imaged quasars (QSOs) without changing the image positions. The degree to S which this occurs will depend on the angular size of the QSO and thus the wavelength of the observations. 0 2 1 v 7 1 Introduction 4 3 9 The ΛCDM model of structure formation has so far been enormously successful in explaining the 0 large scale structure of the universe and the anisotropies in the cosmic microwave background (CMB). 1 However the model appears to face some difficulties explaining observations on galactic and smaller 0 scales. A number of such problems have been discussed in the literature [13, 6, 2, 15, for example]. For / h the present discussion the most relevant of these problems is the observation that CDM simulations p of the local group of galaxies predict an order of magnitude more dwarf galaxy halos with masses o- greater than ∼ 108 M⊙ than are