iow_20121105 - Gaia

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Galaxy detection with Gaia

Gaia's main goal is to study our Galaxy and its stellar content and to provide us with their highly accurate astrometric and photometric parameters. However the satellite will also survey many others objects. Especially Gaia will observe a few millions of galaxies and will give us a unique chance to access a whole sky survey of these objects, something that no ground-based survey has ever recorded. The high resolution of Gaia's observations, privilege of space astronomy, will allow the observation of small galaxies that could not be resolved from ground base surveys such as Sloan Digital Sky Survey. The "Extended Objects" DU470 of CU4 has dedicated efforts to try to recover and analyze the morphology of these galaxies.

The Gaia observations are very specific: the information from most windows is binned in the across scan direction of transit and the resulting information transmitted to earth is one-dimensional. Since objects will be observed, on average, ~80 times (depending on their position in the sky) in various transit angles, it is possible to numerically stack theses one-dimensional windows to produce a two dimensional map of their surrounding (radius<3"). A global fit of the individual transits, based on Evolutionary Algorithms and Radon transforms, is adopted to measure the morphological properties of the galaxies.

Nevertheless not all observations will be transmitted to earth. An onboard selection is performed by the Video Processing Algorithm (VPA). The VPA selects "starlike" profiles to be transmitted. Therefore many extended objects, such as some solar system objects and extended galaxies will be filtered out. It is important to understand the characteristics (morphologic types, light repartition, ...) of the objects that will pass this filter.

The GIBIS simulator at the pixel level has since about one year included a version of the Video Processing Algorithm. DU470 has performed GIBIS simulations of the transits over 10 000 synthetic galaxies of extreme types (pure bulge and pure disc). The properties of the simulated objects covered the parameter space of disc radius between 0.2" and 2.0" and integrated magnitudes from G=14 to G=20. We present in this figure the probability of detection by Gaia of each type of objects. The main conclusion of this study is that pure disc galaxies will be very poorly detected while the vast majority of elliptical ones (pure bulge) will mostly be transmitted to earth.

credits: DU470

[Published: 05/11/2012; update: 22/04/2014]