A Stellar Population Synthesis Model for the Study of Ultraviolet Star Counts of the Galaxy

GALEX, the first all sky imaging UV satellite, has imaged a large part of the sky providing an excellent opportunity for studying UV star counts. The aim of our study is to investigate in detail the observed UV star counts obtained by GALEX vis-a-vis the model simulated catalogs produced by the Besancon model of stellar population synthesis in various Galactic directions, and to explore the potential for studying the structure of our Galaxy from images in multiple NUV and FUV filters of the forthcoming Ultraviolet Imaging Telescope (UVIT) to be flown onboard ASTROSAT. We have upgraded the Besancon model of stellar population synthesis to include the UV bands of GALEX and UVIT. Depending on the availability of contiguous GALEX, SDSS, WISE and 2MASS overlapping regions, we have chosen a set of 19 GALEX fields which spread over a range of Galactic directions. We cross-matched GALEX sources with the WISE+2MASS and SDSS catalogs and UV stars in the GALEX catalog are identified by choosing a suitable IR colour, J - W1 (W1 is a WISE band at 3.4 microns). The IR colour cut method, which is used for the first time for separation of stars, is discussed in comparison with the GALEX+SDSS star counts method. We present the results of the UV star counts analysis carried out using the data from GALEX. We find that the Besancon model simulations represent the observed star counts of both the GALEX AIS and MIS well within the error bars in various Galactic directions. Based on the model analysis, we separated out white dwarfs of the disc and blue horizontal branch stars of the halo from the observed sample by selecting a suitable FUV - NUV colour. The Besancon model is now ready for further comparisons in the UV domain and will be used for prospective studies for the UVIT instrument to be flown onboard ASTROSAT.