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A Multiwavelength Study of ELAN Environments (AMUSE$^2$): Detection of a dusty star-forming galaxy within the enormous Lyman $α$ nebula at $z=2.3$ sheds light on its origin

We present ALMA observations on and around the radio-quiet quasar UM287 at $z=2.28$. Together with a companion quasar, UM287 is believed to play a major role in powering the surrounding enormous Ly$α$ nebula (ELAN), dubbed the Slug ELAN, that has an end-to-end size of 450 physical kpc. In addition to the quasars, we detect a new dusty star-forming galaxy (DSFG), dubbed the Slug-DSFG, in 2 mm continuum with a single emission line consistent with CO(4-3). The Slug-DSFG sits at a projected distance of 100 kpc south-east from UM287, with a systemic velocity difference of $-360\pm30$ km s$^{-1}$ with respect to UM287, suggesting it being a possible contributor to the powering of the Slug ELAN. With careful modeling of SED and dynamical analyses it is found that the Slug-DSFG and UM287 appear low in both gas fraction and gas-to-dust ratio, suggesting environmental effects due to the host massive halo. In addition, our Keck long-slit spectra reveal significant Ly$α$ emissions from the Slug-DSFG, as well as a Ly$α$ tail that starts at the location and velocity of the Slug-DSFG and extends towards the south, with a projected length of about 100 kpc. Supported by various analytical estimates we propose that the Ly$α$ tail is a result of the Slug-DSFG experiencing ram pressure stripping. The gas mass stripped is estimated to be about 10$^9$ M$_\odot$, contributing to the dense warm/cool gas reservoir that is believed to help power the exceptional Ly$α$ luminosity.