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We present a multi‐wavelength study of the nature of the Sloan Digital Sky Survey (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, H  ii , Seyfert and low‐ionization nuclear emission region, 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 the Two‐Micron All‐Sky Survey,  Infrared Astronomical Satellite , Faint Images of the Radio Sky at Twenty‐centimetres, NRAO VLA Sky Survey,  Galaxy Evolution Explorer  and  Roentgen Satellite  data. The UV–optical–NIR colours of blue early‐type 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 imply 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: active galactic nucleus 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 mid‐ and 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 the 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.

monthly notices of the royal astronomical society

The nature of the Sloan Digital Sky Survey galaxies in various classes based on morphology, colour and spectral features – II. Multi‐wavelength properties