Classification of extragalactic point sources and flux variability characteristics of blazars

Abstract

Classification of different types of astronomical objects in large surveys usually done through spectroscopy requires enormous amounts of time. Hence, many attempts have been made using broad band photometric magnitudes and spectroscopic observations to classify the sources, particularly extragalactic sources such as active galactic nuclei (AGNs), starburst galaxies and newlinenormal galaxies. However, a method which does not involve spectroscopic data would be ideal. With this in view, in this work we have made an effort to classify a sample of 37,492 point sources into Quasi-Stellar Objects (QSOs), galaxies and stars using template fitting technique and multiwavelength photometric magnitudes from the Sloan Digital Sky Survey (SDSS) and newlinethe Galaxy Evolution Explorer (GALEX) with coverage from the optical (z: 8931 Å) to the far ultraviolet (FUV: 1516 Å). Templates for QSOs, galaxies and stars were used to fit the data of the objects to the seven photometric bands of SDSS and GALEX. The results were compared with SDSS spectroscopic classification. Two UV bands (NUV and FUV) were included to remove the possible degeneracies in the classification based only on optical bands or in color-color method. UV bands play a crucial role in the classification and characterization of astronomical objects that emit over a wide range of wavelengths, especially for those that are bright at UV. Classification using template fitting method is consistent with spectroscopic methods, provided UV information of the objects is available. UV bands are particularly important for separating quasars and stars, as well as spiral and starburst galaxies. We have achieved the efficiency of 89% for QSOs, 63% for galaxies and 84% for stars. Objects for which spectroscopic data is not available can also be classified using this method which does not require spectroscopic information.