O : Recap - University of Maryland Observatory · 2016-12-08 · 1 Class 23 : Dark Matter Halos n...
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Class 23 : Dark Matter Halos
n This class l Dark matter halos; the “skeleton” of a galaxy l Halo over-density factor ΔV l Applications of Virial Theorem
O : Recap
n Last class : Linear theory of structure formation l Gravitational Instability l Evolution of perturbations, from big bang to
present day l Find “preferred scale” corresponding to the
particle horizon at matter-radiation equality l Beautiful agreement between observed and
I : Dark Matter Halos
n On small scales, the dark matter collapses to form well defined bound objects (halos!)
n Dark Matter in the halos obeys the virial theorem (i.e. the system is in virial equilibrium)…
n In order to settle down to this state, the Dark Matter has to dissipate excess energy.
n How does collisionless dark matter dissipate energy???
Groups and clusters of galaxies
Jenkins et al. (2001)
Properties of a DM halo
n Definition… let Δ(r) be average density within radius r of a dark matter halo in units where the overall cosmological density is unity.
n It turns out that the “virial radius” rv of a DM halo corresponds to a pretty universal value of Δ , often assume Δv=200.
n Suppose that a DM halo has a mass Mv with the virial radius. If σ is velocity of an average DM particle, Virial theorem gives
n By definition we have
n Combining with the virial theorem…
n For illustrative purposes, evaluating at z=1…