Dark matter halos around galaxies

We present evidence that all galaxies, of any Hubble type and luminosity, bear the kinematical signature of a mass component distributed differently from the luminous matter. We review and/or derive the DM halo properties of galaxies of different morphologies: spirals, LSBs, ellipticals, dwarf irregulars and dwarf spheroidals. We show that the halo density profile M-h(x) = M-h(1)(1+a(2))x(3)/x(2)+a(2) (with x = R/R-opt), across both the Hubble and luminosity sequences: matches all the available data that include, for ellipticals: properties of the X-ray emitting gas and the kinematics of planetary nebulae, stars: and HI disks: for spirals, LSBs and dIrr's: stellar and HI rotation curves: and, finally. for dSph's the motions of individual stars. The dark + luminous mass structure is obtained: (a) in spirals, LSBs. and dLrr's by modelling the extraordinary properties of the Universal Rotation Curve (URC), to which all these types conform (i.e. the URC luminosity dependence and the smallness of its rms scatter and cosmic variance); (b) in ellipticals and dSph's, by modelling the coadded mass profiles (or the M/L ratios) in terms of a luminous spheroid and the above-specified dark halo. A main feature of galactic structure is that the dark and visible matter are well mixed already in the luminous region. The transition between the inner, star-dominated regions and the outer, halo-dominated region, moves progressively inwards with decreasing luminosity, to the extent that very-low-L stellar systems (disks or spheroids) are not self-gravitating, while in high-L systems the dark matter becomes a main mass component only beyond the optical edge. A halo core radius, comparable to the optical radius, is detected at all luminosities and for all morphologies. The luminous mass fraction varies with luminosity in a fashion common to all galaxy types: it is comparable with the cosmological baryon fraction at L > L, but it decreases by more than a factor 10(2) at L << L. For each Hubble type, the central halo density increases with decreasing luminosity: sequences of denser stellar systems (dwarfs, ellipticals, HSBs. LSBs in decreasing order) correspond in turn to sequences of denser halos. Then, the dark halo structure of galaxies fits into a well ordered pattern underlying a unified picture for the mass distribution of galaxies across the Hubble sequence.