Panchromatic averaged stellar populations

Abstract

We study how the spectral fitting of galaxies, in terms of light fractions, derived in one spectral region translates into another region, by using results from evolutionary synthesis models. In particular, we examine propagation dependencies on evolutionary population synthesis (EPS) models (grasil, galev, Maraston and galaxev), age, metallicity and stellar evolution tracks over the near-ultraviolet–near-infrared (NUV–NIR, 3500 Å to 2.5 μm) spectral region. Our main results are as follows: as expected, young (t≲ 400 Myr) stellar population fractions derived in the optical cannot be directly compared to those derived in the NIR, and vice versa. In contrast, intermediate to old age (t≳ 500 Myr) fractions are similar over the whole spectral region studied. The metallicity has a negligible effect on the propagation of the stellar population fractions derived from NUV to NIR. The same applies to the different EPS models, but restricted to the range between 3800 and 9000 Å. However, a discrepancy between galev/Maraston and grasil/galaxev models occurs in the NIR. Furthermore, the initial mass function is not important for the synthesis propagation. Compared to starlight synthesis results, our propagation predictions agree at ∼95 per cent confidence level in the optical, and ∼85 per cent in the NIR. In summary, spectral fitting performed in a restricted spectral range should not be directly propagated from the NIR to the UV/optical, or vice versa. We provide equations and an on-line form [panchromatic averaged stellar population (PaASP)] to be used for this purpose.