SWAN: galaxy morphology and evolution from adaptive optics-assisted imaging
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 ground-based 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 Δθ=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 AO observations. The prospect of AO cosmology will undoubtedly improve with the use of forthcoming large-field multiconjugate systems, and the widespread adoption of laser guide star (LGS) facilities.4 LGS systems trade a gain in sky coverage for a loss in image quality, since LGS samples atmospheric turbulence in a conical (rather than cylindrical) volume. However, to overcome the present shortage of targets for AO-assisted cosmology, it is necessary to identify and characterize extragalactic sources in the vicinity of bright guide stars.
Together with Richard Davies, Andrew Baker, Matthew Lehnert, and Filippo Mannucci, I undertook a campaign of AO imaging of fields in different parts of the sky, selected around stars bright enough for AO guiding (10.3=R=12.4).5 Here we present the results of our Ks-band AO imaging for the first 21 fields in the framework of the Survey of a Wide Area with NACO (SWAN).6 We used the NACO (NAOS-CONICA) instrument at the European Southern Observatory Very Large Telescope, obtaining Strehl ratios of 30–60%. The total coverage of the 21 fields is 15.3arcmin2, within which a total of 383 galaxies are detected down to a magnitude of Ks ~ 23.5.
As the AO PSF is quickly changing in both time and position on the frame, it is crucial to account for these variations in order to extract all the information from our wide-field observations. We therefore developed a new approach to account for the varying PSF in extragalactic AO data, 7 and used it to extract the morphological parameters of the detected galaxies. Because the expected median redshift is z~1,8 our spatial resolution of 0.1" is equivalent to only 500pc for typical cosmologies, hinting at the exciting potential of this work.
We have compared the galaxy counts in SWAN and the size-magnitude relation of the detected galaxies with the predictions of two very different galaxy evolution models. The first is a ‘backwards’ pure luminosity evolution (PLE) model.9 In this model, the local properties of galaxies (e.g., spectral energy distributions, luminosity functions, stellar population) are used to construct star formation history and luminosity evolution and probe backwards into the past properties of the galaxies, neglecting interactions and merging between galaxies. The second model we used is the hierarchical semianalytical Numerical Galaxy Catalogue,10 in which the standard theory of structure formation in a ΛCDM (cold dark matter) cosmology constitutes a starting point. In this case the local properties of the galaxies are an output, to be compared with observations.
Our high-resolution AO morphological classification of the SWAN galaxies allows us to assess the total number counts and the size-magnitude relation. In addition, for the first time we can evaluate the predictions of the models for the counts and sizes of the late-type (disk dominated and irregulars) and early-type (ellipticals and spheroids) galaxies separately in the NIR (see Figure 1).
We find that both models are able to reproduce the observed size distribution, although the PLE model seems to do it better at fainter magnitudes. While the hierarchical model can convincingly reproduce the counts for late-type galaxies, it is not consistent with the observed number counts for the early type. On the other hand, the PLE model can reasonably reproduce both the late-and early-type count distribution of the SWAN galaxies, including the plateau observed for K=20 for the early type. We conclude that a PLE model better replicates the observed properties in our K-selected sample up to K=22, without evidence of relevant number evolution even when distinguishing between early- and late-type galaxies.6
These first results obtained in the framework of SWAN already illustrate the importance of obtaining reliable morphological classifications for better constraining the details of galaxy formation and evolution models. They also demonstrate the unique power of AO observations to extend such work to faint galaxies in the NIR. Despite technological and data-handling problems, AO is one of the most promising techniques for astronomical observations, and will be an essential tool for fully exploiting the capabilities of the new generation of large telescopes.
Giovanni Cresci is currently a postdoctoral research associate in the infrared/submillimeter group at MPE. He received his PhD from the University of Florence in 2006. His research interests include the study of the formation and evolution of galaxies using high spatial resolution imaging and spectroscopic techniques, and the study of supernovae and super star clusters in external galaxies.