Reinterpreting the puzzling properties of z>6 galaxies within a variable IMF framework

Abstract

Recent results form the James Webb Space Telescope (JWST) report space densities for bright and massive galaxies at z>7 that far exceed expectations of theoretical models of galaxy formation, prompting a revision of our understanding of the physical processes leading to the assembly of the first luminous structures. In this work we present predictions from a realization of the GAlaxy Evolution and Assembly (GAEA) model, which implements a prescription for a variable stellar initial mass function (IMF). This prescription is inspired by high-resolution numerical simulations that account for the role of cosmic rays (CR) as regulators of the star formation rate (SFR) in giant molecular clouds. In our approach, SFR density is assumed to be a proxy for the CR density, providing a link between the IMF shape and the predicted physical conditions of the star forming interstellar medium. Our results show that, in our model framework, assuming such a variable IMF reproduces several properties of the z>6 galaxy population, with no further modification of the feedback model, including their UV luminosity functions up to z~13. In order to compare model predictions with available estimates for the galaxy stellar mass function (GSMF), we reconstruct stellar masses from the model's synthetic photometry assuming a universal IMF, reflecting standard observational practice. Under this approach, we show that the model can reproduce the evolution of the GSMF up to the highest redshifts accessible. Our findings highlight the need to consider a variable IMF shape in the error budget associated with stellar mass estimates. We show that the evolution of both the slope and normalization of the gas-phase mass metallicity relation can be used as powerful discriminant between models of early galaxy formation assuming different IMF evolution.

Figures (8)

• Figure 1: Cumulative distribution function (CDF) for the different IMFs used in this work. Individual IMF shapes correspond to the IMF library described in Fontanot18a, which assumes a dependence of the IMF characteristic stellar mass (i.e. the IMF knee) on the CR density background ($U_{\rm CR}$), following the numerical simulations of Papadopoulos11. At increasing $U_{\rm CR}$, the characteristic mass get larger, which translates into an increasing the number of massive stars (i.e. a top-heavy IMF - see main text for more details). The black dashed line corresponds to a MW-like IMF, with the shape corresponding to a Chabrier IMF.
• Figure 2: Redshift Evolution of the UV Luminosity Function at z>6. Black dashed and red solid lines correspond to the predictions of the gaea realization assuming a universal MW-like IMF and the variable IMF scenario from Sec. \ref{['sec:varimf']}. Observational datapoints from Finkelstein15, Bouwens21, PerezGonzalez23, Castellano23, Donnan23, Harikane23, Leung23, Donnan24, Harikane24, McLeod24, Finkelstein24, Adams24, Whitler25.
• Figure 3: Predicted $U_{\rm CR}/U_{\rm MW}$ distribution for gaea model galaxies in the variable IMF realization. Different redshift are shown in different colours as in the legend.
• Figure 4: Photometric differences in JWST filters between a gaea realization assuming a universal MW-like IMF and the run with the variable IMF. Each panel shows results for a given JWST wide filter (indicated in the label), predictions at different redshifts are marked by a different colour as in the legend.
• Figure 5: Logarithmic Mass difference $\alpha_M$ between the intrinsic stellar mass in the variable IMF run and M$^{\rm phot}_\star$, i.e. the stellar mass reconstructed from synthetic photometry assuming a MW-like IMF. Coloured lines refer to different redshifts as in the legend. Solid and dashed lines refer to the $\alpha_M$ dependence on M$_\star$ and M$^{\rm phot}_\star$, respectively. Note that, on average, stellar masses in the variable IMF scenario are lower than those estimated for a MW-like IMF (see the main text for a discussion)