Optical+Near-IR Analysis of a Newly Confirmed Einstein Ring at <i>z </i>∼ 1 from the Kilo-Degree Survey: Dark Matter Fraction, Total and Dark Matter Density Slope, and Initial Mass Function*

Abstract 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 z l = 0.9906, while the background source is a Ly α emitter at z s = 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 R Ein = 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>γ</mml:mi> <mml:mi>tot</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2.1</mml:mn> <mml:msubsup> <mml:mn>4</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.07</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.06</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , 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 ( gri ZYJHK s ) photometries from KiDS and VIKING shows that the total stellar mass of the lens is M * = (3.95 ± 0.35) × 10 11 M ⊙ (Salpeter initial mass function (IMF)), implying a dark matter fraction inside the effective radius of f DM = 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.