Downsizing revised: Star formation timescales for elliptical galaxies with an environment-dependent IMF and a number of SNIa

Abstract

Previous studies of the stellar mean metallicity and [Mg/Fe] values of massive elliptical (E) galaxies suggest that their stars were formed over a very short timescale that cannot be reconciled with estimates from stellar population synthesis (SPS) studies and with hierarchical assembly. Applying the previously developed chemical evolution code, GalIMF, which allows an environment-dependent stellar initial mass function (IMF) to be applied to the integrated galaxy initial mass function theory instead of an invariant canonical IMF, the star formation timescales (SFT) of E galaxies are re-evaluated. The code’s uniqueness lies in it allowing the galaxy-wide IMF and associated chemical enrichment to evolve as the physical conditions in the galaxy change. The calculated SFTs become consistent with the independent SPS results if the number of type Ia supernovae (SNIa) per unit stellar mass increases for more massive E galaxies. This is a natural outcome of galaxies with higher star formation rates producing more massive star clusters, spawning a larger number of SNIa progenitors per star. The calculations show E galaxies with a stellar mass ≈109.5 M⊙ to have had the longest mean SFTs of ≈2 Gyr. The bulk of more massive E galaxies were formed faster (SFT ≈ 1 Gyr) leading to domination by M dwarf stars and larger dynamical mass-to-light ratios as observed, while lower mass galaxies tend to lose their gas supply more easily due to their shallower potential and therefore also have similarly-short mean SFTs. This work achieves, for the first time, consistency of the SFTs for early-type galaxies between chemical-enrichment and SPS modelling. Equally, it leads to an improved understanding of how the star formation environment may affect the total number of SNIa per unit stellar mass formed.

The formation of early-type galaxies through monolithic collapse of gas clouds in Milgromian gravity

“…The models with 𝑅 initial = 50 kpc and 𝑅 initial = 100 kpc do not agree with any of the observed relations. We note that our models do not agree with the new downsizing relation obtained by Yan et al (2021) and this is discussed in Section 5. Fig.…”

“…If, on the other hand, the galaxy-wide IMF varies such that it becomes top-heavy at a high SFR (eg. Jeřábková et al 2018;Yan et al 2021) then the collapse is likely less deep and 𝑟 eff will end up larger. We do not address the chemical evolution of the models here as the chemical enrichment has not yet been incorporated into PoR and would also require a systematically varying galaxy-wide IMF (the current simulationss effectively assume the canonical IMF for the formation of stellar particles).…”

“…It has been shown that the stellar remnant population is comparable to the mass of living stars in the IGIMF theory but using a canonical invariant IMF would decrease the remnant mass to be much lower than the stellar mass (Yan et al 2019). Yan et al (2021) finds that the ratio of dynamical mass and stellar mass, 𝑀 dyn /𝑀 ★ , is higher for more massive galaxies because they have a more top-heavy galaxy-wide IMF and more stellar remnants. Given the high SFRs (> 10 2 𝑀 /yr) the model galaxies reach (Table 1), it is expected that the IGIMF is top-heavy (Yan et al 2017;Jeřábková et al 2018;Yan et al 2021) leading to significant mass loss from the evolving stars.…”

“…Yan et al (2021) finds that the ratio of dynamical mass and stellar mass, 𝑀 dyn /𝑀 ★ , is higher for more massive galaxies because they have a more top-heavy galaxy-wide IMF and more stellar remnants. Given the high SFRs (> 10 2 𝑀 /yr) the model galaxies reach (Table 1), it is expected that the IGIMF is top-heavy (Yan et al 2017;Jeřábková et al 2018;Yan et al 2021) leading to significant mass loss from the evolving stars. This mass lost would be in the form of gas and if it is heated sufficiently enough it would expand and lead to the expansion of the galaxy (Suzuki et al 2022).…”

From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity

“…The small fraction of outliers is especially evident in their Figure 3, which shows that the much larger galaxy sample of [186] is quite consistent with a tight BTFR. Even the two outliers at the high-mass end might naturally be accounted for using the integrated galactic initial mass function (IGIMF [187][188][189][190]), a framework in which more massive galaxies would be expected to have a higher proportion of stellar remnants and thus a higher M /L (A. H. Zonoozi et al, in preparation). More massive spirals also tend to have a larger fraction of their baryons in a central bulge, which generally has a higher M /L.…”