An E-ELT case study: colour–magnitude diagrams of an old galaxy in the Virgo cluster

One of the key science goals for a diffraction limited imager on an extremely large telescope (ELT) is the resolution and accurate photometry of individual stars down to faint magnitudes in a range of environments. The aim of this study is to test the proposed capabilities of a multi-conjugate adaptive optics (MCAO) assisted imager working at the diffraction limit, in IJHKs filters, on a 42 m diameter ELT to carry out accurate stellar photometry in crowded images in an elliptical-like galaxy at the distance of the Virgo cluster. As a basis for realistic simulations we have used the phase A studies of the European-ELT project, including the MICADO imager and the MAORY MCAO module. We convolved a complex resolved stellar population with the telescope and instrument performance expectations to create realistic images. We then tested the ability of the currently available photometric packages STARFINDER and DAOPHOT to handle these simulated images. Our results show that for a one hour exposure deep colour–magnitude diagrams (photometric error, ±0.25 at I ≥ 27.2; H ≥ 25. and Ks ≥ 24.6) of old stellar populations in galaxies, at the distance of Virgo, are feasible at a maximum surface brightness, μV ∼ 17 mag/arcsec 2 (down to MI > − 4a ndMH ∼ MK > −6), and significantly deeper (photometric error, ±0.25 at I ≥ 29.3; H ≥ 26. 6a ndKs ≥ 26.2) for μV ∼ 21 mag/arcsec 2 (down to MI ≥− 2a ndMH ∼ MK ≥− 4.5). The photometric errors, and thus also the depth of the photometry could potentially be improved with photometry packages specifically designed for an ELT MCAO point spread function. We also make a simple comparison between these simulations and what can be expected from a single conjugate adaptive optics feed to MICADO and also the NIRCAM imager on the James Webb space telescope.