Lensed stars in galaxy–galaxy strong lensing – a <i>JWST</i> prediction for the Cosmic Horseshoe

ABSTRACT We explore for the first time the possibility of detecting lensed star transients in galaxy–galaxy strong lensing systems upon repeated, deep imaging using the James Webb Space Telescope (JWST). Our calculation predicts that the extremely high recent star formation rate of ${\sim} 140\ {\rm M}_{\odot } \, \textrm {yr}^{-1}$ over the last 50 Myr (not accounting for image multiplicity) in the ‘Cosmic Horseshoe’ lensed system ($z = 2.381$) generates many young, bright stars, of which their large abundance is expected to lead to a detection rate of ${\sim} 60$ transients per pointing in JWST observations with a $5\sigma$ limiting magnitude of ${\sim} 29\, m_{\rm AB}$. With the high expected detection rate and little room for uncertainty for the lens model compared with cluster lenses, our result suggests that the Cosmic Horseshoe could be an excellent tool to test the nature of dark matter based on the spatial distribution of transients, and can be used to constrain axion mass if dark matter is constituted of ultra-light axions. We also argue that the large distance modulus of ${\sim} 46.5\,$ mag at $z \approx 2.4$ can act as a filter to screen out less massive stars as transients and allow one to better constrain the high-mass end of the stellar initial mass function based on the transient detection rate. Follow-up JWST observations of the Cosmic Horseshoe would allow one to better probe the nature of dark matter and the star formation properties, such as the initial mass function at the cosmic noon, via lensed star transients.