James Webb Space Telescope Feed Post
JWST's PEARLS: Mothra, a new kaiju star at z=2.091 extremely magnified by MACS0416, and implications for dark matter models
Illustrations of the VSVPA effect with mock data. The top two panels show the mock data for epochs 1 and 3 and their difference for the band F356W, with and without instrumental noise. The bottom two panels are similar but for the F410M band. Offsets of approximately 0.07" can be appreciated in the difference between epochs. These offsets can be appreciated in some filters, such as F356W, even in the absence of noise. In all cases the images are 1.5" across. The white cross marks the position of the varying point source. The length of the arms of the cross is 0.06" (end-to-end).
Abstract: We report the discovery of Mothra, an extremely magnified monster star, likely a binary system of two supergiant stars, in one of the strongly lensed galaxies behind the galaxy cluster MACS0416. The star is in a galaxy with spectroscopic redshift z=2.091 in a portion of the galaxy that is parsecs away from the cluster caustic. The binary star is observed only on the side of the critical curve with negative parity but has been detectable for at least eight years, implying the presence of a small lensing perturber. Microlenses alone cannot explain the earlier observations of this object made with the Hubble Space Telescope. A larger perturber with a mass of at least 104\,\Msun\ offers a more satisfactory explanation. Based on the lack of perturbation on other nearby sources in the same arc, the maximum mass of the perturber is M<2.5×106\,\Msun, making it the smallest substructure constrained by lensing above redshift 0.3. The existence of this millilens is fully consistent with the expectations from the standard cold dark matter model. On the other hand, the existence of such small substructure in a cluster environment has implications for other dark matter models. In particular, warm dark matter models with particle masses below 8.7\,keV are excluded by our observations. Similarly, axion dark matter models are consistent with the observations only if the axion mass is in the range 0.5×10-22eV
