Discovery of two Einstein crosses from massive post--blue nugget galaxies at z>1 in KiDS

TL;DR

This paper reports the discovery and confirmation of two Einstein Cross gravitational lens systems involving massive post-blue nugget galaxies at redshifts greater than 1, providing insights into galaxy evolution and dark matter content.

Contribution

It presents the first identification and detailed analysis of Einstein Crosses with post-blue nugget galaxies at high redshift, including lens modeling and characterization of the lensed sources.

Findings

The two Einstein Crosses are at redshifts 0.38 and 0.24 with Einstein radii of 5.2 and 5.4 kpc.

Dark matter fractions inside the half-light radius are approximately 56-60%.

The lensed sources are ultra-compact, blue, absorption-dominated galaxies at redshifts 1.10 and 1.59.

Abstract

We report the discovery of two Einstein Crosses (ECs) in the footprint of the Kilo-Degree Survey (KiDS): KIDS J232940-340922 and KIDS J122456+005048. Using integral field spectroscopy from MUSE@VLT, we confirm their gravitational-lens nature. In both cases, the four spectra of the source clearly show a prominence of absorption features, hence revealing an evolved stellar population with little star formation. The lensing model of the two systems, assuming a singular isothermal ellipsoid (SIE) with external shear, shows that: 1) the two crosses, located at redshift $z=0.38$ and 0.24, have Einstein radius $R_{\rm E}=5.2$ kpc and 5.4 kpc, respectively; 2) their projected dark matter fractions inside the half effective radius are 0.60 and 0.56 (Chabrier IMF); 3) the sources are ultra-compact galaxies, $R_{\rm e}\sim0.9$ kpc (at redshift $z_{\rm s}=1.59$) and $R_{\rm e}\sim0.5$ kpc ($z_{\rm s}=1.10$), respectively. These results are unaffected by the underlying mass density assumption. Due to size, blue color and absorption-dominated spectra, corroborated by low specific star-formation rates derived from optical-NIR spectral energy distribution fitting, we argue that the two lensed sources in these ECs are blue nuggets migrating toward their quenching phase.