Parallel Talk | Cosmology from Home 2020
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Talk title:
Small-Scale Structure from Interlopers
Talk abstract:
We present a new probe of dark matter small scale structure: the convergence power spectrum of line-of-sight (LOS) halos in galaxy-galaxy lensing systems. While much of the literature has focused on the lensing perturbation due to subhalos within the main lensing galaxy, we quantify a similar perturbation due to halos not associated with the main lens, lying along the vast LOS. We develop a formalism to analytically calculate the LOS convergence power spectrum, by assuming the LOS halos are small compared to the main lens, which allows us to project them onto the main-lens plane as effective subhalos. We test our formalism against multi-plane ray-tracing simulations and find excellent agreement. We show how the relative contribution of LOS halos and subhalos depends on the source and lens redshift, as well as the assumed halo and subhalo mass functions. For a fiducial system with fraction of dark matter halo mass in substructure $f_{\rm sub}=0.4\%$ for subhalo masses $[10^5-10^8]\rm{M}_{\odot}$, the LOS contribution to the power spectrum is at least several times greater than that of subhalos for source redshifts $z_s\gtrsim0.5$. Furthermore, it is likely that for lenses typical of the SLACS and BELLS surveys, the LOS contribution dominates: $f_{\rm sub}\gtrsim2\%$ ($4\%$) is needed for subhalos to dominate in SLACS (BELLS), which is higher than current upper bounds on $f_{\rm sub}$ for our mass range. Since the halo mass function is better understood than the subhalo mass function from first principles, the dominance of LOS in galaxy-galaxy lenses with high-quality imaging can be seen as a significant advantage when translating this observable into a constraint on dark matter.
