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

2014 
Astronomisches Rechen-Institut, Zentrum fu¨r Astronomie der Universita¨t Heidelberg, Mo¨nchhofstr, 12-14, 69120 Heidelberg,GermanyABSTRACTContext.Galaxy Evolution Explorer (GALEX), the first all sky imaging ultraviolet (UV) satellite, has imaged a large part of the skyproviding an excellent opportunity for studying UV star counts. Combining photometry from the different wavelengths in the infrared(from Wide-field Infrared Survey (WISE) and Two Micron All Sky Survey (2MASS)) to UV allows us to extract a real star catalogfrom the GALEX source catalog.Aims.The aim of our study is to investigate in detail the observed UV star counts obtained by GALEX vis-a-vis the model simulatedcatalogs produced by the Besanc¸on model of stellar population synthesis in various Galactic directions, and to explore the potential forstudying the structure of our Galaxy from images in multiple near-UV (NUV) and far-UV (FUV) filters of the forthcoming UltravioletImaging Telescope (UVIT) to be flown onboard ASTROSAT.Methods. We have upgraded the Besanc¸on model of stellar population synthesis to include the UV bands of GALEX and UVIT.Depending on the availability of contiguous GALEX, Sloan Digital Sky Survey (SDSS), WISE and 2MASS overlapping regions, wehave chosen a set of nineteen GALEX fields which spread over a r ange of Galactic directions. We selected a sample of objects fromthe GALEX database using the CASjobs interface and then cross-matched them with the WISE+2MASS and SDSS catalogs. UVstars in the GALEX catalog are identified by choosing a suitab le infrared (IR) colour, J −W1 (W1 is a WISE band at 3.4 µm), whichcorresponds to a temperature range from 1650 K to 65000 K. The IR colour cut method, which is used for the first time for separ ationof stars, is discussed in comparison with the GALEX+SDSS star counts method.Results.We present the results of the UV star counts analysis carried out using the data from GALEX. We find that the Besanc¸onmodel simulations represent the observed star counts of both the GALEX All-sky Imaging Survey (AIS) and Medium Imaging Survey(MIS) well within the error bars in various Galactic directions. Based on the model analysis, we separated out white dwarfs (WDs) ofthe disc and blue horizontal branch stars (BHBs) of the halo from the observed sample by selecting a suitable FUV − NUV colour.Conclusions.The Besanc¸on model is now ready for further comparisons in the UV domain and will be used for prospective studiesfor the UVIT instrument to be flown onboard ASTROSAT.Key words. Stars: general - Ultraviolet: stars - Galaxy: disc - Galaxy:stellar content - Galaxy: halo
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