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
An independent result confirming the likely counterimage. From left to right, the first and second panels show the (negative) original images of the arc in F090W and F200W; the third panel shows the convolved image from F090W to F200W using WebbPSF models; the fourth image shows F200W - 1.87 × F090W to subtract off the local background in the neighbor of LS1. In all panels, red and blue circles show the LS1 and LS1' positions, respectively.
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
