The nature of the SDSS galaxies in various classes based on morphology, colour and spectral features – II. Multi-wavelength properties

We present a multi-wavelength study of the nature of the SDSS galaxies divided into fine classes based on their morphology, colour and spectral features. The SDSS galaxies are classified into early-type and late-type; red and blue; passive, HII, Seyfert and LINER, which returns a total of 16 fine classes of galaxies. The properties of galaxies in each fine class are investigated from radio to X-ray, using 2MASS, IRAS, FIRST, NVSS, GALEX and ROSAT data. The UV – optical – NIR colours of blue earlytype galaxies (BEGs) seem to result from the combination of old stellar population and recent star formation (SF), if there is no significant difference in their formation epoch between different spectral classes. Non-passive red early-type galaxies (REGs) have larger metallicity and younger age than passive REGs, considering their UV – optical – NIR colours, which implies that non-passive REGs have suffered recent SF adding young and metal-rich stars to them. The radio detection fraction of REGs strongly depends on their optical absolute magnitudes, while that of most late-type galaxies does not, implying the difference in their radio sources: AGN and SF. The optical – NIR colours of red late-type galaxies (RLGs) reveal that they may have considerable old stars as well as young stars. The UV – optical colours and the radio detection fraction of passive RLGs show that they have properties similar to REGs rather than non-passive RLGs. Dust extinction may not be a dominant factor making RLGs red, because RLGs are detected in the midand far-infrared bands less efficiently than blue late-type galaxies (BLGs). The passive BLGs have very blue UV – optical – NIR colours, implying either recent SF quenching or current SF in their outskirts. Including star formation rate, other multi-wavelength properties in each fine class are investigated, and their implication on the identity of each fine class is discussed.