A Salpeter IMF and an NFW halo: disentangling the dark and stellar mass of an elliptical galaxy through precise lens modelling of a double-source-plane system

ABSTRACT We present a strong lensing analysis of the double source plane lens J0946+1006 (colloquially ‘Jackpot’ lens) to measure the inner dark matter density profile, the stellar-to-halo mass ratio, and the stellar initial mass function normalization using a two-component stellar plus dark matter mass model. The stellar mass follows a multi-Gaussian expansion light model with a free global mass-to-light ratio and an allowed radial $M/L$ gradient, while the dark matter is described by an elliptical generalized NFW halo. The double-source-plane geometry provides additional leverage against the mass-sheet transformation and helps constrain the radial mass profile. Despite allowing both a radial stellar $M/L$ gradient and a generalized NFW halo, the data prefer an approximately constant stellar mass-to-light ratio with a Salpeter-like IMF normalization, and a dark matter halo consistent with NFW. We infer $M_{\star } = 4.4^{+0.25}_{-0.39}\times 10^{11}\, {\rm M}_{\odot }$ and an inner halo slope $\gamma _{\rm in}^{\rm halo} = 1.04^{+0.10}_{-0.14}$. The halo mass is $M_{200}^{\rm halo} = 1.11^{+0.37}_{-0.32}\times 10^{13}\, {\rm M}_{\odot }$, implying $\log _{10}(M_{200}/M_{\star })=1.41^{+0.13}_{-0.14}$. At fixed halo mass, the inferred stellar mass lies $\sim 0.1$ dex above typical literature stellar halo mass relations at similar redshift, which is comparable to the intrinsic scatter of these relations. We expect this approach to provide a practical template for future dark matter studies with the large double-source-plane lens samples from Euclid.