Constraints on the Baryonic Compression and Implications for the Fraction of Dark Halo Lenses

Abstract

We predict the fraction of dark halo lenses, that is, the fraction of lens systems produced by the gravitational potential of dark halos, on the basis of a simple parametric model of baryonic compression. The fraction of dark halo lenses primarily contains information on the effect of baryonic compression and the density profile of dark halos, and is expected to be insensitive to cosmological parameters and source population. The model we adopt comprises the galaxy formation probability p_g(M) which describes the global efficiency of baryonic compression and the ratio of circular velocities of galaxies to virial velocities of dark halos gamma_v=v_c/v_{vir} which means how the inner structure of dark halos is modified due to baryonic compression. The model parameters are constrained from the velocity function of galaxies and the distribution of image separations in gravitational lensing, although the degeneracy between model parameters still remains. We show that the fraction of dark halo lenses depends strongly on gamma_v and the density profile of dark halos such as inner slope alpha. This means that the observation of the fraction of dark halos can break the degeneracy between model parameters if the density profile of dark halo lenses is fully settled. On the other hand, by restricting gamma_v to physically plausible range we can predict the lower limit of the fraction of dark halo lenses on the basis of our model. Our result indicates that steeper inner cusps of dark halos (alpha >~ 1.5) or too centrally concentrated dark halos are inconsistent with the lack of dark halo lenses in observations.