Not so dark, not so dense: an alternative explanation for the lensing subhalo in SDSSJ0946+1006

TL;DR

Reevaluating the lensing system SDSSJ0946+1006 by modeling the subhalo as a faint galaxy, which allows for lower density solutions consistent with standard dark matter models and improves the fit to observational data.

Contribution

This study introduces a model that includes the luminosity of the subhalo, challenging previous assumptions of a dark-only subhalo and aligning results with CDM predictions.

Findings

Luminous subhalo models fit the data better.

Lower density subhaloes are consistent with CDM.

Neglecting subhalo light can bias mass inferences.

Abstract

Previous studies of the strong lens system SDSSJ0946+1006 have reported a dark matter subhalo with an unusually high central density, potentially challenging the standard cold dark matter (CDM) paradigm. However, these analyses assumed the subhalo to be completely dark, neglecting the possibility that it may host a faint galaxy. In this work, we revisit the lensing analysis of SDSSJ0946+1006, explicitly modelling the subhalo as a luminous satellite. Incorporating light from the perturber broadens the range of allowed subhalo properties, revealing solutions with significantly lower central densities that are consistent with CDM expectations. The inferred luminosity of the satellite also aligns with predictions from hydrodynamical simulations. While high-concentration subhaloes remain allowed, they are no longer statistically preferred. The luminous subhalo model yields a better fit to the data, while also offering a more plausible explanation that is in line with theoretical expectations. We validate our methodology using mock data, demonstrating that neglecting subhalo light can lead to inferred mass distributions that are artificially compact.