Detection of a dark matter subhalo in the strongly lensed system PJ011646

Abstract

We present a strong lensing analysis of the system PJ011646 using high-resolution ($\sim$0.1 arcsec) Atacama Large Millimeter/submillimeter Array (ALMA) dust-continuum observations to test for the presence of dark matter substructures. The lens mass distribution is modelled with an elliptical power law and third- and fourth-order multipoles (PL+MP; $m=3,4$), plus external shear. The multipoles have amplitudes of $\simeq$1.5 per cent of the convergence, consistent with nearby early-type galaxies, and improve the fit by $\hat{I}”\ln Z=52.1$ relative to a pure PL model. Using this best-fitting macromodel, we perform a grid-based subhalo search in the image plane, parametrising the perturber as a spherical NFW. A subhalo in two locations in the image plane improves the fit by $\hat{I}”\ln Z>10$. Both correspond to the same location in the source plane, so they are partially degenerate; follow-up analysis suggests that only one is physically real. This is a subhalo of mass $M_{200}={2.78}_{-0.66}^{+0.43}\times 10^{10}{M}_{\odot }$ and concentration $c_{200}=30_{-7}^{+5}$, detected at $\sim$5.8$\ddot{I}ƒ$ significance (relative to the PL+MP). This concentration is consistent with that expected for a typical tidally stripped Navarro-Frenk-White subhalo. The enclosed projected mass is most tightly constrained within a radius of 2 kpc, where we infer $M_{\mathrm{sub}}={3.57}_{-0.14}^{+0.16}\times 10^9{M}_{\odot }$. From grid cells consistent with no detection ($\hat{I}”\ln Z<10$), we derive limits on the minimum subhalo mass that could have been detected at $3\ddot{I}ƒ$ significance, finding $M_{200}\approx 8\times 10^8{M}_{\odot }$ in the most sensitive regions of the lensed arcs. This demonstrates that ALMA continuum imaging at sub-arcsecond resolution can probe dark matter substructure in a mass regime where cold and warm dark matter models predict different abundances of subhalos.

12 pages, 6 figures