<i>ULySS</i>: a full spectrum fitting package

<i>Aims. <i/> We provide an easy-to-use full-spectrum fitting package and explore its applications to (i) the determination of the stellar atmospheric parameters and (ii) the study of the history of stellar populations. <i>Methods. <i/>We developed <i>ULySS<i/>, a package to fit spectroscopic observations against a linear combination of non-linear model components convolved with a parametric line-of-sight velocity distribution. The minimization can be either local or global, and determines all the parameters in a single fit. We use maps, convergence maps and Monte-Carlo simulations to study the degeneracies, local minima and to estimate the errors. <i>Results. <i/> We show the importance of determining the shape of the continuum simultaneously to the other parameters by including a multiplicative polynomial in the model (without prior pseudo-continuum determination, or rectification of the spectrum). We also stress the usefulness of using an accurate line-spread function, depending on the wavelength, so that the line-shape of the models properly matches the observation. For simple models, i.e., to measure the atmospheric parameters or the age/metallicity of a single-age stellar population, there is often a unique minimum, or when local minima exist they can be recognized unambiguously. For more complex models, Monte-Carlo simulations are required to assess the validity of the solution. <i>Conclusions. <i/>The <i>ULySS<i/> package is public, simple to use and flexible. The full spectrum fitting makes optimal use of the signal.

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