SWAN: galaxy morphology and evolution from adaptive optics-assisted imaging

Abstract

Morphology is among the most appropriate ways to characterize the properties of galaxies. Indeed, we will reach a complete understanding of galaxies only by deriving the mechanisms responsible for their structures. However, disentangling these processes in nearby systems is already extremely difficult, and the challenge is even greater at higher redshift, where sources are compact (∼ 0.1− 0.3”) and larger galaxies are rare. In this context, the study of galaxy size, and of the evolution of other galaxy properties according to morphological type (disk or spheroid galaxy), have made use mainly of the classification derived from deep optical Hubble Space Telescope (HST) imaging. This is because of the higher angular resolution achievable at optical wavelengths with space-based observations using HST. However, near-infrared (NIR) surveys provide one of the best opportunities to investigate the cosmic evolution of galaxies and their mass assembly. In particular, at these wavelengths we gain direct sensitivity to the galaxy stellar mass rather than to ongoing or recent star formation, and we see smaller dust extinction effects. The use of adaptive optics (AO) systems allows groundbased telescopes to operate at or near the diffraction limit in the NIR (∼0.07” in the K band for an 8m telescope, a resolution comparable with optical HST observations), correcting for the blurring introduced by the atmosphere. The advantages of NIR AO observations for studying how galaxies form and evolve in the early universe are clear. However, until now there have been only a few attempts using natural guide star sensing.1–3 This is because of the very small number of known extragalactic sources lying at distances ∆θ ≤ 30” from the bright (V ≤ 13) stars needed to correct the wavefront for AO guiding, and also because of problems arising from the space variance of the shape of the point spread function (PSF) in Figure 1. The upper panel shows a comparison of the effective radius (Re) distribution as a function of magnitude for late-type (left) and early-type (right) galaxies with the pure luminosity evolution model (solid line) and hierarchical model (dashed line) predictions. The error bars are the standard error on the mean; no error bar is drawn for a bin with one galaxy. The lower panel shows a comparison of the SWAN (Survey of a Wide Area with NACO) completeness corrected counts for late-type (left) and early-type (right) galaxies with the predictions of the two galaxy evolution models.