Towards a library of synthetic galaxy spectra and preliminary results of
  classification and parametrization of unresolved galaxies for Gaia - II
  This paper is the second in a series, implementing a classification system
for Gaia observations of unresolved galaxies. Our goals are to determine
spectral classes and estimate intrinsic astrophysical parameters via synthetic
templates. Here we describe (1) a new extended library of synthetic galaxy
spectra, (2) its comparison with various observations, and (3) first results of
classification and parametrization experiments using simulated Gaia
spectrophotometry of this library. Using the PEGASE.2 code, based on galaxy
evolution models that take account of metallicity evolution, extinction
correction, and emission lines (with stellar spectra based on the BaSeL
library), we improved our first library and extended it to cover the domain of
most of the SDSS catalogue. We produce an extended library of 28885 synthetic
galaxy spectra at zero redshift covering four general Hubble types of galaxies,
over the wavelength range between 250 and 1050 nm at a sampling of 1 nm or
less. The library is also produced for 4 random values of redshift in the range
of 0-0.2. It is computed on a random grid of four key astrophysical parameters
(infall timescale and 3 parameters defining the SFR) and, depending on the
galaxy type, on two values of the age of the galaxy. The synthetic library was
compared and found to be in good agreement with various observations. The first
results from the SVM classifiers and parametrizers are promising, indicating
that Hubble types can be reliably predicted and several parameters estimated
with low bias and variance.
