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

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: atul.deep@esa.int; fiorentino@astro.rug.nl; etolstoy@astro.rug.nl
² Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
³ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Max – Planck-Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
⁵ ONERA, 92322 Châtillon, France

Received: 28 January 2011
Accepted: 16 May 2011

Abstract

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 IJHK_s 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 K_s ≥ 24.6) of old stellar populations in galaxies, at the distance of Virgo, are feasible at a maximum surface brightness, μ_V ~ 17 mag/arcsec² (down to M_I >  −4 and M_H ~ M_K >  −6), and significantly deeper (photometric error,  ± 0.25 at I ≥ 29.3; H ≥ 26.6 and K_s ≥ 26.2) for μ_V ~  21 mag/arcsec² (down to M_I ≥  −2 and M_H ~ M_K ≥  −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.