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

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 capable of reproducing two radial arcs near the brightest cluster galaxies (BCGs), we were able to compare the stellar mass to the total mass from the lens model. We find that the fraction of dark matter in the south BCG is consistent with ΛCDM, while in the north BCG, we find a very small amount of dark matter, which is more consistent with alternative models. We discuss possible causes for this and conclude that additional data is needed to clarify the situation. We studied 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 a very high level of accuracy. Some of the microlensing events might be reinterpreted as long-period Cepheid in future observations. We consider this possibility in greater detail and study the challenges in making such detections from intervening microlenses.