2025/03/13 by Li, Rui, Napolitano, Nicola R., Ago, Giuseppe D +16
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2503.10180
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 (SIE) 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+0.06-0.07, 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 8-band (griZYJHKs) photometries from KiDS and VIKING shows that the total stellar mass of the lens is M*=(3.95± 0.35)× 1011 M_\odot (Salpeter Initial Mass Function, IMF), implying a dark matter fraction inside the effective radius to be f\rm 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 IMF heavier than Salpeter is assumed.