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 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. 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/WMAP estimates on H_0. We employ the particle-mesh code SUPERBOX to carry out the group interaction simulations. We simulate interactions between group members, comparing the density profile for the satellite before and after interaction for the mass range of 10^11 to 10^13 M_sun. 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. Additionally, the steepening in the inner region is transient in nature, with consecutive interactions returning the profile to an isothermal state within a timeframe 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.