2020A&A...639A.120L

Context. Young stellar cluster cores in the local Universe provide the most pristine information available on the stellar initial mass function (IMF), but their stellar densities are too high to be resolved by present-day instrumentation. With a resolving power 100 times better than the Hubble Space Telescope, the Multi-Adaptive Optics Imaging CameraA for Deep Observations (MICADO), which is the near-infrared camera on the Extremely Large Telescope (ELT), will for the first time provide access to a significant number of dense young stellar clusters that are critical for direct studies on the universality and shape of the IMF.
Aims. In this work we aim to estimate the lowest stellar mass that MICADO will be able to reliably detect given a stellar density and distance. We also show that instrumental effects that will play a critical role, and report the number of young clusters that will be accessible for IMF studies in the local Universe with the ELT.
Methods. We used SimCADO^*, the instrument simulator package for the MICADO camera, to generate observations of 56 dense stellar regions with densities similar to the cores of young stellar clusters. We placed the cluster fields at distances between 8 kpc and 5 Mpc from the Earth, implying core densities from 10² to 10⁵stars/arcsec², and determined the lowest reliably observable mass for each stellar field through point-spread function fitting photometry.
Results. Our results show that stellar densities of <10³stars/arcsec² will be easily resolvable by MICADO. The lowest reliably observable mass in the Large Magellanic Cloud will be around 0.1M_☉ for clusters with densities <10³stars/arcsec². MICADO will be able to access the stellar content of the cores of all dense young stellar clusters in the Magellanic Clouds, allowing the peak and shape of the IMF to be studied in great detail outside the Milky Way. At a distance of 2Mpc, all stars with M>2M_☉ will be resolved in fields of <10⁴stars/arcsec², allowing the high-mass end of the IMF to be studied in all galaxies out to and including NGC 300.
Conclusions. We show that MICADO on the ELT will be able to probe the IMF of star clusters that are ten times denser than what the James Webb Space Telescope will be able to access, and over one hundred times denser than the clusters that the Hubble Space Telescope can successfully resolve. While the sensitivity of MICADO will not allow us to study the brown dwarf regime outside the Milky Way, it will enable access to all stellar members of over 1000 young clusters in the Milky Way and the Magellanic Clouds. Furthermore, direct measurements of the Salpeter slope of the IMF will be possible in over 1500 young clusters out to a distance of 5Mpc. MICADO on the ELT will be able to measure resolved IMFs for a large ensemble of young clusters under starkly different environments and test the universality of the IMF in the local Universe.