A semi-empirical library of galaxy spectra for Gaia classification based
  on SDSS data and PEGASE models
  Aims:This paper is the third in a series implementing a classification system
for Gaia observations of unresolved galaxies. The system makes use of template
galaxy spectra in order to determine spectral classes and estimate intrinsic
astrophysical parameters. In previous work we used synthetic galaxy spectra
produced by PEGASE.2 code to simulate Gaia observations and to test the
performance of Support Vector Machine (SVM) classifiers and parametrizers. Here
we produce a semi-empirical library of galaxy spectra by fitting SDSS spectra
with the previously produced synthetic libraries. We present (1) the
semi-empirical library of galaxy spectra, (2) a comparison between the observed
and synthetic spectra, and (3) first results of claassification and
parametrization experiments with simulated Gaia spectrophotometry of this
library. Methods: We use chi2-fitting to fit SDSS galaxy spectra with the
synthetic library in order to construct a semi-empirical library of galaxy
spectra in which (1) the real spectra are extended by the synthetic ones in
order to cover the full wavelength range of Gaia, and (2) astrophysical
parameters are assigned to the SDSS spectra by the best fitting synthetic
spectrum. The SVM models were trained with and applied to semi-empirical
spectra. Tests were performed for the classification of spectral types and the
estimation of the most significant galaxy parameters (in particular redshift,
mass to light ratio and star formation history). Results: We produce a
semi-empirical library of 33670 galaxy spectra covering the wavelength range
250 to 1050 nm at a sampling of 1 nm or less. Using the results of the fitting
of the SDSS spectra with our synthetic library, we investigate the range of the
input model parameters that produces spectra which are in good agreement with
observations. (abridged)
