FASTAR

II. Semi-resolved evolutionary stellar population models

¹ Instituto de Astrofísica de Canarias, c/ Vía Láctea s/n, E38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
³ Departamento de Inteligencia Artificial, Universidad Nacional de Educación a Distancia (UNED), Calle Juan del Rosal 16, E-28040 Madrid, Spain
⁴ INAF-Osservatorio Astronomico di Capodimonte, Sal. Moiariello 16 Napoli 80131, Italy
⁵ Centro de Astrobiología (CSIC/INTA), 28692 ESAC Campus, Villanueva de la Cañada, Madrid, Spain
⁶ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Wien, Austria
⁷ Centre for Astrophysics and Supercomputing, Swinburne University, John Street, Hawthorn, VIC 3122, Australia
⁸ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotláská 2 Brno 611 37, Czech Republic
⁹ Observatoire de Paris, LERMA, PSL University, 61 Avenue de l’Observatoire, F-75014 Paris, France
¹⁰ Université Paris-Cité, 5 Rue Thomas Mann, 75014 Paris, France
¹¹ Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 106, Ensenada 22800, BC, Mexico

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 January 2026
Accepted: 21 April 2026

Abstract

Standard evolutionary synthesis models rely on the assumption of a fully sampled stellar initial mass function (IMF). Under this assumption, the age, chemical composition, and IMF uniquely define the predicted absorption spectra. However, with current instrumentation pushing observations towards higher spatial resolutions and lower surface brightnesses, the assumption of a fully sampled IMF does not always hold true. Here we present the semi-resolved version of the FASTAR models, a comprehensive set of evolutionary synthesis predictions able to reproduce the stochastic behavior of discretely-sampled IMFs. Semi-resolved FASTAR predictions share the same evolutionary principles, ingredients, and features of the integral (fully sampled IMF) version of the FASTAR models, expanding a range of ages from 20 Myr to 14 Gyr, metallicities between −2.5 ≤ [M/H] ≤ +0.3, and several IMF functional forms. Detailed spectroscopic measurements can be carried out within the 3540 − 7400 Å wavelength range, and low-resolution spectral energy distributions can also be synthesized over a wider 2000-to-12 000 Å coverage. Semi-resolved FASTAR models also depend on the number of stars contributing to the observed spectra, which determines the effective sampling of the different stellar evolutionary phases along the isochrones. This incomplete sampling implies that semi-resolved FASTAR models are inevitably stochastic. On top of the inherent stochasticity of the models, derived quantities such as equivalent widths, colors, or mass-to-light ratios might present strong deviations compared to standard fully sampled simple stellar population models. This stochasticity dilutes the boundary between model predictions and data, promoting new sampling-based inference approaches. FASTAR semi-resolved models allow for the effective exploration of the parameter space thanks to their optimized, JAX-based computation.