New development: This system will be observed with JWST in Cycle 5 (Program 12088). The multi-wavelength imaging will help disentangle emission from the lensed source and any putative satellite galaxy, providing a decisive test of the interpretation presented here.
Strong gravitational lensing provides one of the most powerful ways to detect small clumps of dark matter in distant galaxies. By studying how these substructures perturb the shapes of lensed images, astronomers can measure the mass and density of otherwise invisible objects.
Previous analyses of the lens system SDSS J0946+1006 suggested the presence of a dark matter subhalo with an unusually high central density. Such an object would be difficult to reconcile with predictions from the standard cold dark matter model and attracted significant attention.
In this work we revisit the system and explore a different possibility: that the perturber may host a faint satellite galaxy. Once this possibility is included in the lens modelling, the data can be explained without requiring an extreme dark matter density.
While this provides a compelling alternative interpretation, it is currently difficult to determine conclusively whether the perturber contains a faint galaxy or is truly dark. Future observations at multiple wavelengths could help resolve this question: colour differences between the lensed background source and any satellite galaxy would allow the two components to be more cleanly separated, providing a stronger test of the model.
