SOM ensemble for unsupervised outlier analysis. Application to outlier identification in the Gaia astronomical survey

► Gaia ESA mission will survey more than one billion objects in our Galaxy and beyond. ► Automated analysis tools are being developed to classifying the observed objects. ► Our work is devoted to the analysis of classification outliers. ► We present a novel technique for segmentation of outliers based on ensemble SOM. ► It allows for data exploration and knowledge discovery in huge astronomical databases. Gaia is an ESA cornerstone astronomical mission that will observe with unprecedented precision positions, distances, space motions, and many physical properties of more than one billion objects in our Galaxy and beyond. It will observe all objects in the sky in the visible magnitude range from 6 to 20, up to approximately 109 sources. An international scientific consortium, the Gaia Data Processing and Analysis Consortium (Gaia DPAC), has organized itself in several coordination units, with the aim, among others, of addressing the work of classifying the observed astronomical sources, using both supervised and unsupervised classification algorithms. This work focuses on the analysis of classification outliers by means of unsupervised classification. We present a novel method to combine SOMs trained with independent features that are calculated from spectrophotometry. The method as described here can help to improve the models used for the supervised classification of astronomical sources. Furthermore, it allows for data exploration and knowledge discovery in huge astronomical databases such as the upcoming Gaia mission.