Variation of the stellar initial mass function in semi-analytical models

III. Testing the cosmic-ray regulated integrated galaxy-wide initial mass function

¹ INAF – Astronomical Observatory of Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
e-mail: fabio.fontanot@inaf.it
² IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
³ INAF – Astronomical Observatory of Capodimonte, Sal. Moiariello, 16, 80131 Napoli, Italy
⁴ Astronomy Sector, Department of Physics, University of Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁵ Tianjin Astrophysics Center, Tianjin Normal University, Binshuixidao 393, 300384 Tianjin, PR China
⁶ Institute for Physics, Laboratory for Galaxy Evolution, EPFL, Observatoire de Sauverny, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁷ Instituto de Radioastronomía y Astrofísica, UNAM, Campus Morelia, CP 58089 Morelia, Mexico
⁸ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
⁹ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
¹⁰ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain

Received: 21 November 2023
Accepted: 15 April 2024

Abstract

Context. In our previous work, we derived the CR-IGIMF, which is a new scenario for a variable stellar initial mass function (IMF) that combines numerical results on the role played by cosmic rays (CR) in setting the thermal state of star-forming gas with the analytical approach of the integrated galaxy-wide IMF (IGIMF).

Aims. In this work, we study the implications of this scenario for the properties of local early-type galaxies (ETG) as inferred from dynamical, photometric, and spectroscopic studies.

Methods. We implemented a library of CR-IGIMF shapes in the framework of the galaxy evolution and assembly (GAEA) model. GAEA provides predictions for the physical and photometric properties of model galaxies and for their chemical composition. Our realization includes a self-consistent derivation of the synthetic spectral energy distribution for each model galaxy, which allows a direct derivation of the mass fraction in the mean IMF of low-mass stars (i.e., the dwarf-to-giant ratio, f_dg) and a comparison with IMF-sensitive spectral features.

Results. The predictions of the GAEA model implementing the CR-IGIMF confirm our previous findings: It correctly reproduces both the observed excess of z ∼ 0 dynamical mass (mass-to-light ratio) with respect to spectroscopic (photometric) estimates assuming a universal MW-like IMF, and the observed increase in [α/Fe] ratios with stellar mass in spheroidal galaxies. Moreover, this realization reproduces the increasing trends of f_dg and IMF-sensitive line strengths with velocity dispersion, although the predicted relations are significantly shallower than the observed ones.

Conclusions. Our results show that the CR-IGIMF is a promising scenario that reproduces at the same time dynamical, photometric, and spectroscopic indications of a varying IMF in local ETGs. The shallow relations found for spectral indices suggest that either a stronger variability as a function of galaxy properties or additional dependences (e.g., as a function of star forming gas metallicity) might be required to match the strength of the observed trends.