A Dense Dark Matter Core of the Subhalo in the Strong Lensing System JVAS B1938+666

Abstract The nature of dark matter remains unknown, motivating the study of fuzzy/wave dark matter (FDM/ ψ DM) and self-interacting dark matter (SIDM) as alternative frameworks to address small-scale discrepancies in halo profiles inferred from observations. This study presents a nonparametric reconstruction of the mass distribution of the previously found dark subhalo in the strong lensing system JVAS B1938+666. Compared with the standard Navarro–Frenk–White profile, both SIDM and ψ DM ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ψ</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1.3</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.31</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.22</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>22</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">eV</mml:mi> </mml:math> ) provide significantly better fits to the resulting density profile. Moreover, the SIDM model is favored over ψ DM with a Bayes factor of 14.44. The reconstructed density profile features a characteristic kiloparsec-scale core ( r c ≈ 0.5 kpc) with central density <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mi>c</mml:mi> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>2.5</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mn>7</mml:mn> </mml:msup> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mspace width="0.25em"/> <mml:mi>kpc</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:math> , exhibiting remarkable consistency with the core–halo mass scaling relations observed in Local Group dwarf spheroidals. These findings offer insights that may help address the core–cusp discrepancy in ΛCDM substructure predictions.