Using Strong Lensing to Detect Subhalos with Steep Inner Density Profiles

Abstract: The inner region of a subhalo's density distribution is particularly sensitive to dark matter microphysics, with alternative dark matter models leading to both cored and steeply-rising inner density profiles. This work investigates how the lensing signature and detectability of dark matter subhalos in mock HST-, Euclid-, and JWST-like strong lensing observations depends on the subhalo's radial density profile, especially with regards to the inner power-law slope, $\beta$. We demonstrate that the minimum-mass subhalo detectable along the Einstein ring of a system is strongly dependent on $\beta$. In particular, we show that subhalos with $\beta \sim 2.2$ can be detected down to masses over an order-of-magnitude lower than their Navarro-Frenk-White (NFW) counterparts with $\beta \sim 1$. Importantly, we find that the detectability of subhalos with steep inner profiles is minimally affected by increasing the complexity of the main lens galaxy's mass model. This is a unique characteristic of these subhalos, as those with NFW or shallower profiles become essentially undetectable when multipole perturbations are added to the lens model. The results of this work highlight how the underlying dark matter physics can significantly impact the expected number of subhalo detections from strong gravitational lensing observations. This is important for testing Cold Dark Matter against alternatives, such as Self-Interacting Dark Matter, which predict the existence of subhalos with diverse inner density profiles.