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In this work we present an atlas of composite spectra of galaxies based onthe data of the Sloan Digital Sky Survey Data Release 7 (SDSS DR7). Galaxiesare classified by colour, nuclear activity and star-formation activity tocalculate average spectra of high signal-to-noise ratio and resolution (S/N =132 - 4760 at {Dlambda = 1 A), using an algorithm that is robust againstoutliers. Besides composite spectra, we also compute the first five principalcomponents of the distributions in each galaxy class to characterize the natureof variations of individual spectra around the averages. The continua of thecomposite spectra are fitted with BC03 stellar population synthesis models toextend the wavelength coverage beyond the coverage of the SDSS spectrographs.Common derived parameters of the composites are also calculated: integratedcolours in the most popular filter systems, line strength measurements, andcontinuum absorption indices (including Lick indices). These derived parametersare compared with the distributions of parameters of individual galaxies and itis shown on many examples that the composites of the atlas cover much of theparameter space spanned by SDSS galaxies. By co-adding thousands of spectra, atotal integration time of several months can be reached, which results inextremely low noise composites. The variations in redshift not only allow forextending the spectral coverage bluewards to the original wavelength limit ofthe SDSS spectrographs, but also make higher spectral resolution achievable.The composite spectrum atlas is available online athttp://www.vo.elte.hu/compositeatlas.

A high‐resolution atlas of composite Sloan Digital Sky Survey galaxy spectra