Primordial star clusters at extreme magnification

Gravitationally lensed galaxies with magnification μ ≈ 10-100 are routinely detected at high redshifts, but magnifications significantly higher than this are hampered by a combination of low probability and large source sizes. Magnifications of μ ~ 1000 may none the less be relevant in the case of intrinsically small, high-redshift objects with very high number densities. Here, we explore the prospects of detecting compact (≲10 pc), high-redshift (z ≳ 7) Population III star clusters at such extreme magnifications in large-area surveys with planned telescopes like Euclid, Wide Field Infrared Survey Telescope and Wide-field Imaging Surveyor for High-redshift (WISH). We find that the planned WISH 100 deg² ultradeep survey may be able to detect a small number of such objects, provided that the total stellar mass of these star clusters is ≳10⁴M_⊙. If candidates for such lensed Population III star clusters are found, follow-up spectroscopy of the surrounding nebula with the James Webb Space Telescope or ground-based Extremely Large Telescopes should be able to confirm the Population III nature of these objects. Multiband photometry of these objects with the James Webb Space Telescope also has the potential to confirm that the stellar initial mass function in these Population III star clusters is top-heavy, as supported by current simulations.