Resolved Stellar Population of Distant Galaxies in the ELT Era

Abstract

ABSTRACTThe expected imaging capabilities of future Extremely Large Telescopes (ELTs) will offer the unique possibility to investigate the stellar populations of distant galaxies from the photometry of the stars in very crowded fields. Using simulated images and photometric analysis we explore here two representative science cases aimed at recovering the characteristics of the stellar populations in the inner regions of distant galaxies. Specifically, case (1) at the center of the disk of a giant spiral in the Centaurus Group (μB ∼ 21, distance of 4.6 Mpc) and case (2) at half of the effective radius of a giant elliptical in the Virgo Cluster (μI ∼ 19.5, distance of 18 Mpc). We generate synthetic frames by distributing model stellar populations and adopting a representative instrumental setup, i.e., a 42 m telescope operating close to the diffraction limit. The effect of crowding is discussed in detail, showing how stars are measured preferentially brighter than they really are as the confusion limit is approached. We find that (1) accurate photometry (σ ∼ 0.1, completeness ≳90%) can be obtained for case 2 down to I ∼ 28.5, J ∼ 27.5, allowing us to recover the stellar metallicity distribution in the inner regions of ellipticals in Virgo to within ∼0.1 dex; (2) the same photometric accuracy holds for the science case 1 down to J ∼ 28.0, K ∼ 27.0, enabling reconstruction of the star formation history up to the Hubble time via simple star counts in diagnostic boxes. For the latter case, we discuss the possibility of deriving more detailed information on the star formation history from the analysis of their horizontal-branch stars. We show that the combined features of high sensitivity and angular resolution of ELTs may open a new era for our knowledge of the stellar content of galaxies of different morphological types up to the distance of the Virgo cluster.