Steepened inner density profiles of group galaxies via interactions: an N-body analysis

Abstract

We continue to see a range of values for the Hubble constant obtained from gravitationally lensed multiple image time-delays when assuming an isothermal lens despite a robust value from the Hubble Space Telescope (HST) key project (72 ± 8 km s -1 Mpc -1 ). One explanation is that there is a variation in Hubble constant values due to a fundamental heterogeneity in lens galaxies present in groups, that is, central galaxies with a high dark matter surface density, and satellite galaxies with a possibly stripped halo, low dark matter surface density, and a more centrally concentrated matter distribution. Our goal is to see if a variety of group interactions between the most-massive group members can result in significant changes in the galaxy density profiles over the scale probed by strong lensing ($15 kpc). While stripping of the outer parts of the halo can be expected, the impact on inner regions where the luminous component is important is less clear in the context of lensing, though still crucial, as a steepened density profile within this inner region allows these lens systems to be consistent with current HST/Wilkinson Microwave Anisotropy Probe (WMAP) estimates on H 0 . We employ the particle-mesh code SUPERBOX to carry out the group interaction simulations. An important advantage of using such a code is that it implements a fast, low-storage fast Fourier transform (FFT) algorithm allowing simulations with millions of particles on desk-top machines. We simulate interactions between group members, comparing the density profile for the satellite before and after interaction for the mass range of 1011 to 10 13 M ⊙ · Our investigations show a significant steepening of the density profile in the region of ∼5-20 kpc, i.e. that which dominates strong lensing in lens galaxies. This effect is independent of the initial mass-to-light ratio. Additionally, the steepening in the inner region is transient in nature, with consecutive interactions returning the profile to an isothermal state within a time-frame of ∼0.5-2.0 Gyr. This factor may help explain why lens galaxies that produce lower values of H 0 (i.e. those with possibly steeper profiles) are far fewer in number than those which agree with both the HST key project value for H 0 and isothermality, since one would have to observe the lens galaxy during this transient steepened phase.