We investigate the radial distribution of galaxies within their host dark matter halos as measured in the Sloan Digital Sky Survey by modeling their small-scale clustering .
Specifically , we model the Jiang et al .
( 2011 ) measurements of the galaxy two-point correlation function down to very small projected separations ( 10 \leq r \leq 400 h ^ { -1 } \mathrm { kpc } ) , in a wide range of luminosity threshold samples ( absolute r -band magnitudes of -18 up to -23 ) .
We use a halo occupation distribution ( HOD ) framework with free parameters that specify both the number and spatial distribution of galaxies within their host dark matter halos .
We assume one galaxy resides in the halo center and additional galaxies are considered satellites that follow a radial density profile similar to the dark matter Navarro-Frenk-White ( NFW ) profile , except that the concentration and inner slope are allowed to vary .
We find that in low luminosity samples ( M _ { r } < -19.5 and lower ) , satellite galaxies have radial profiles that are consistent with NFW . M _ { r } < -20 and brighter satellite galaxies have radial profiles with significantly steeper inner slopes than NFW ( we find inner logarithmic slopes ranging from -1.6 to -2.1 , as opposed to -1 for NFW ) .
We define a useful metric of concentration , M _ { 1 / 10 } , which is the fraction of satellite galaxies ( or mass ) that are enclosed within one tenth of the virial radius of a halo .
We find that M _ { 1 / 10 } for low luminosity satellite galaxies agrees with NFW , whereas for luminous galaxies it is 2.5 - 4 times higher , demonstrating that these galaxies are substantially more centrally concentrated within their dark matter halos than the dark matter itself .
Our results therefore suggest that the processes that govern the spatial distribution of galaxies , once they have merged into larger halos , must be luminosity dependent , such that luminous galaxies become poor tracers of the underlying dark matter .