A multi-wavelength study of the gravitational lens COSMOS J095930+023427

Abstract

We present a multi-wavelength study of the gravitational lens COSMOS J095930+023427 (z1 = 0.892), together with the associated galaxy group along the line of sight located at z ∼ 0.7, and the lensed background galaxy. The source redshift is currently unknown, but estimated to be at zs ∼ 2. This analysis is based on publicly available HST, Subaru and Chandra imaging data, as well as VLT spectroscopy. The lensing system is an early-type galaxy showing a strong [OII] emission line, and produces four bright images of the distant background source. It has an Einstein radius of 0.79″, about four times larger than the effective radius. We perform a lensing analysis using both a singular isothermal ellipsoid and a peudo-isothermal elliptical mass distribution for the lensing galaxy, and find that the final results on the total mass, the dark matter (DM) fraction within the Einstein radius and the external shear due to a foreground galaxy group are robust with respect to the choice of the parametric model and the source redshift (yet unknown). We measure the luminous mass from the photometric data, and find the DM fraction within the Einstein radius fDM to be between 0.71 ± 0.13 and 0.79 ± 0.15, depending on the unknown source redshift. Meanwhile, the non-null external shear found in our lensing models supports the presence and structure of a galaxy group at z ∼ 0.7, and an independent measurement of the 0.5−2 keV X-ray luminosity within 20″ around the X-ray centroid provides a group mass of M = (3 − 10) × 1013 M⊙, in good agreement with the previous estimate derived through weak lensing analysis. Finally, by inverting the HST/ACS I814 image with the lensing equation, we obtain the reconstructed image of the magnified source galaxy, which has a scale of about 3.3 kpc at zs = 2 (2.7 kpc at zs = 4) and the typical disturbed disk-like appearance observed in low-mass star-forming galaxies at z ∼ 3. However, deep, spatially resolved spectroscopic data for similar lensed sources are still required to detect the first stage of galaxy evolution, since the available spectrum shows no clear features due to the background source.