Variation of the stellar initial mass function in semi-analytical models. III. Testing the cosmic-ray regulated integrated galaxy-wide initial mass function

Abstract

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