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We report the discovery of a low-mass $z=5.200\pm 0.002$ galaxy that is inthe process of ceasing its star formation. The galaxy, MACS0417-z5PSB, ismultiply imaged with magnification factors $\sim40$ by the galaxy cluster MACSJ0417.5-1154, observed as part of the CAnadian NIRISS Unbiased Cluster Survey(CANUCS). Using observations of MACS0417-z5PSB with a JWST/NIRSpec Prismspectrum and NIRCam imaging, we investigate the mechanism responsible for thecessation of star formation of the galaxy, and speculate about possibilitiesfor its future. Using spectrophotometric fitting, we find a remarkably lowstellar mass of $\rm{M_*}=4.3\pm^{0.9}_{0.8} \times 10^{7} \rm{M_{\odot}}$,less than 1% of the characteristic stellar mass at $z\sim5$. We measure ade-lensed rest-UV half-light radius in the source plane of $30\pm^{7}_{5}$ pc,and measure a star formation rate from H$\alpha$ of $0.14\pm^{0.17}_{0.12}$$\rm{M_{\odot}/yr}$. We find that under the assumption of a double power lawstar formation history, MACS0417-z5PSB has seen a recent rise in starformation, peaking $\sim10-30$ Myr ago and declining precipitously since then.Together, these measurements reveal a low-mass, extremely compact galaxy whichis in the process of ceasing star formation. We investigate the possibilitiesof mechanisms that have led to the cessation of star formation inMACS0417-z5PSB, considering stellar and AGN feedback, and environmentalprocesses. We can likely rule out an AGN and most environmental processes, butleave open the possibility that MACS0417-z5PSB could be a star forming galaxyin the lull of a bursty star formation history.

An extremely compact, low-mass post-starburst galaxy at $z=5.2$