JWST lens model for A370: A very low dark matter fraction for a brightest cluster galaxy and lensing properties for the Dragon arc

TL;DR

This paper presents a new JWST-based lens model for galaxy cluster Abell 370, revealing a very low dark matter fraction in one BCG and analyzing lensing phenomena like microlensing and potential Cepheid detection.

Contribution

The study introduces a refined lens model using JWST data, highlighting a surprisingly low dark matter fraction in one BCG and detailed analysis of microlensing events along the Dragon arc.

Findings

Dark matter fraction in south BCG aligns with ΛCDM

Very low dark matter in north BCG suggests alternative models

Lens model accurately reproduces microlensing event distribution

Abstract

We present a new lens model for the $z=0.375$ galaxy cluster Abell 370 based on previously spectroscopically confirmed lensed galaxies and new lensed systems identified in JWST data, including recent data from the MAGNIF program. Based on the best models able to reproduce two radial arcs near the BCGs, we compare the stellar mass to the total mass from the lens model and find that the fraction of dark matter in the south BCG is consistent with $\Lambda$CDM while in the north BCG we find a very small amount of dark matter, more consistent with alternative models to $\Lambda$CDM. We discuss possible causes for this and conclude that additional data is needed to clarify the situation. We study the lensing properties, magnification, time delay and strength of the critical curve, along the Dragon arc, where previous studies have reported tens of alleged microlensing events from supergiant stars at $z=0.7251$. The new lens model is able to reproduce the distribution of microlensing events with great accuracy. Some of the microlensing events may be reinterpreted as long-period Cepheid in future observations. We consider this possibility and study in more detail the challenges for such detection from intervening microlenses.