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Aims:The Gaia astrometric survey mission will, as a consequence of itsscanning law, obtain low resolution optical (330-1000 nm) spectrophotometry ofseveral million unresolved galaxies brighter than V=22. We present the firststeps in a project to design and implement a classification system for thesedata. The goal is both to determine morphological classes and to estimateintrinsic astrophysical parameters via synthetic templates. Here we describe(1) a new library of synthetic galaxy spectra, and (2) first results ofclassification and parametrization experiments using simulated Gaiaspectrophotometry of this library. Methods:We have created a large grid ofsynthetic galaxy spectra using the PEGASE.2 code, which is based on galaxyevolution models that take into account metallicity evolution, extinctioncorrection, emission lines (with stellar spectra based on the BaSeL library).Our classification and regression models are Support Vector Machines (SVMs),which are kernel-based nonlinear estimators. Results:We produce a basic libraryof about 4000 zero redshift galaxy spectra covering the main Hubble types overwavelength range 250 to 1050 nm at a sampling of 1 nm or less. It is computedon a regular grid of four key astrophysical parameters for each type and forintermediate random values of the same parameters. An extended libraryreproduces this at a series of redshifts. Initial results from the SVMclassifiers and parametrizers are promising, indicating that Hubble types canbe reliably predicted and several parameters estimated with low bias andvariance. Comparing the colours of our synthetic library with Sloan Digital SkySurvey (SDSS) spectra we find good agreement over the full range of Hubbletypes and parameters.

Towards a library of synthetic galaxy spectra and preliminary results of classification and parametrization of unresolved galaxies for Gaia