A High Resolution Atlas of Composite SDSS Galaxy Spectra
  In this work we present an atlas of composite spectra of galaxies based on
the data of the Sloan Digital Sky Survey Data Release 7 (SDSS DR7). Galaxies
are classified by colour, nuclear activity and star-formation activity to
calculate average spectra of high signal-to-noise ratio and resolution (S/N =
132 - 4760 at {Dlambda = 1 A), using an algorithm that is robust against
outliers. Besides composite spectra, we also compute the first five principal
components of the distributions in each galaxy class to characterize the nature
of variations of individual spectra around the averages. The continua of the
composite spectra are fitted with BC03 stellar population synthesis models to
extend the wavelength coverage beyond the coverage of the SDSS spectrographs.
Common derived parameters of the composites are also calculated: integrated
colours in the most popular filter systems, line strength measurements, and
continuum absorption indices (including Lick indices). These derived parameters
are compared with the distributions of parameters of individual galaxies and it
is shown on many examples that the composites of the atlas cover much of the
parameter space spanned by SDSS galaxies. By co-adding thousands of spectra, a
total integration time of several months can be reached, which results in
extremely low noise composites. The variations in redshift not only allow for
extending the spectral coverage bluewards to the original wavelength limit of
the SDSS spectrographs, but also make higher spectral resolution achievable.
The composite spectrum atlas is available online at
http://www.vo.elte.hu/compositeatlas.
