Optical+Near-IR Analysis of a Newly Confirmed Einstein Ring at z ∼ 1 from the Kilo-Degree Survey: Dark Matter Fraction, Total and Dark Matter Density Slope, and Initial Mass Function * * Based on observations with OmegaCam@VST, VIRCAM@VISTA, HAWK-I, and XSHOOTER@VLT (Prog. ID: 107.22S8)

Journal article

We report the spectroscopic confirmation of a bright blue Einstein ring in the Kilo-Degree Survey (KiDS) footprint: the Einstein “blue eye.” Spectroscopic data from X-Shooter at the Very Large Telescope (VLT) show that the lens is a typical early-type galaxy (ETG) at zl = 0.9906, while the background source is a Lyα emitter at zs = 2.823. The reference lens modeling was performed on a high-resolution Y-band adaptive-optics image from HAWK-I at VLT. Assuming a singular isothermal ellipsoid total mass density profile, we inferred an Einstein radius REin = 10.47 ± 0.06 kpc. The average slope of the total mass density inside the Einstein radius, as determined by a joint analysis of lensing and isotropic Jeans equations, is γtot=2.14−0.07+0.06 , showing no systematic deviation from the slopes of lower-redshift galaxies. This can be the evidence of ETGs developing through dry mergers plus moderate dissipationless accretion. Stellar population analysis with eight-band (griZYJHKs) photometries from KiDS and VIKING shows that the total stellar mass of the lens is M* = (3.95 ± 0.35) × 1011 M⊙ (Salpeter initial mass function (IMF)), implying a dark matter fraction inside the effective radius of fDM = 0.307 ± 0.151. We finally explored the dark matter halo slope and found a strong degeneracy with the dynamic stellar mass. Dark matter adiabatic contraction is needed to explain the posterior distribution of the slope, unless an IMF heavier than Salpeter is assumed.

Authors: Li, R; Napolitano, NR; D’Ago, G; Shalyapin, VN; Zhu, K

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal Letters
• Volume: 987
• Issue: 2
• Article number: L31
• Publication date: 2025-07-04
• Acceptance date: 2025-06-17
• DOI: 10.3847/2041-8213/ade680
• EISSN: 2041-8213
• ISSN: 2041-8205
• Language: English
• Keywords: Dark matter; Galaxy evolution; Galaxy dynamics; Galaxy formation; Gravitational lensing