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MULTIWAVELENGTH IMAGE ANALYSIS OF THE SMALL MAGELLANIC CLOUD USING HIERARCHICAL MARKOVIAN SEGMENTATION

This paper addresses the segmentation of astronomical multiband images with missing data. We present some results obtained on Multiwavelength images of the Small Magellanic Cloud, by using the Marginal Posterior Mode (MPM) estimator on a quadtree structure under Markovian assumption : the estimation of the model parameters is then addressed with Expectation-Maximization (EM)-type algorithms, allowing unsupervised hyperparameter estimation. The main interest of this modeling effort lies in its generality : the algorithm handles multiwavelength data (possibly with missing data) in a single Causal-in-scale Markovian model. It is an interesting tool for astronomical image analysis, which exhibits very large dynamic range of intensities and missing data on the sampling grid in this case. 1. MULTIBAND IMAGES IN ASTRONOMY The study of star formation mechanisms and their relationship with interstellar medium is one of the most dynamic fields of research in astronomy. The interstellar medium is composed of a mixture of gas (mainly hydrogen and helium) in different phases and dust grains (mainly carbon and silicium). Stars are forming from the gas in the heart of molecular Hydrogen complexes. The molecular clouds themselves result from the formation of molecules via chemical reactions in dense atomic gas. The ultra violet emission of massive newborn stars ionizes the surrounding gas, giving birth to the socalled HII regions. Observations at various wavelengths are necessary to study the different states of gas and dust and their mutual relationships as well as their links to star formation. The emission line at21cm in the radio wavelength range gives the column density of atomic hydrogen (HI), which is the number of hydrogen atoms in an unitary section cylinder along the line of sight. Emissions in the far Infrared at100 or 170 microns are directly due to the thermic emission of big dust grains. The H α emission, at656.3nm, is the strongest Hydrogen recombination line in the HII regions and is generally superimposed on the optical continuum of stars[1]. Molecular Hydrogen is difficult to detect, but estimation of their density can be infered from the intensity of the CO lines at millimeter wavelengths, assuming that this CO and molecular hydrogen are formed at quite the same time[2].