Chemoarchaeological downsizing in a hierarchical universe: impact of a top-heavy IGIMF

Abstract

We make use of a semi-analytical model of galaxy formation to investigate the origin of the observed correlation between [α/Fe] abundance ratios and stellar mass in elliptical galaxies. We implement a new galaxy-wide stellar initial mass function (Top Heavy Integrated Galaxy Initial Mass Function, TH-IGIMF) in the semi-analytic model SAG and evaluate its impact on the chemical evolution of galaxies. The SFR-dependence of the slope of the TH-IGIMF is found to be key to reproducing the correct [α/Fe]-stellar mass relation. Massive galaxies reach higher [α/Fe] abundance ratios because they are characterized by more top-heavy IMFs as a result of their higher SFR. As a consequence of our analysis, the value of the minimum embedded star cluster mass and of the slope of the embedded cluster mass function, which are free parameters involved in the TH-IGIMF theory, are found to be as low as 5 M ⊙ and 2, respectively. A mild downsizing trend is present for galaxies generated assuming either a universal IMF or a variable TH-IGIMF. We find that, regardless of galaxy mass, older galaxies (with formation redshifts 2) are formed in shorter time-scales ( 2 Gyr), thus achieving larger [α/Fe] values. Hence, the time-scale of galaxy formation alone cannot explain the slope of the [α/Fe]-galaxy mass relation, but is responsible for the big dispersion of [α/Fe] abundance ratios at fixed stellar mass. We further test the hyphothesis of a TH-IGIMF in elliptical galaxies by looking into mass-to-light ratios, and luminosity functions. Models with a TH-IGIMF are also favoured by these constraints. In particular, mass-to-light ratios agree with observed values for massive galaxies while being overpredicted for less massive ones; this overprediction is present regardless of the IMF considered.

Physical drivers of galaxies’ cold-gas content: exploring environmental and evolutionary effects with Dark Sage

“…This feature remains a shared standard for semi-analytic models in general (see the latest versions of other models, e.g. Gargiulo et al 2015;Henriques et al 2015;Lacey et al 2016). If subhaloes were able to accrete hot gas from the ICM within a semi-analytic framework, this should systematically raise H i fractions and lower quiescent fractions.…”

The metal enrichment of passive galaxies in cosmological simulations of galaxy formation

“…Simultaneously explaining the increasing α-to-Fe ratio with velocity dispersion (or stellar mass) and increasing stellar metallicity with stellar mass is therefore fundamentally difficult within the current framework of galaxy formation. One possible solution would be the introduction of a variable IMF which becomes more top-heavy for a higher SFR as suggested by Calura & Menci (2009) and Gargiulo et al (2015) and previously suggested on independent grounds by Baugh et al (2005) and Lacey et al (2016).…”

“…They found that a combination of a power-law DTD and metal-rich winds, which drive light α elements directly out of galaxies, gives the best agreement with a wide range of observational data including the [α/Fe]-mass and mass-metallicity relations. On the other hand, Gargiulo et al (2015) employed a variable IMF whose slope becomes shallower (more top-heavy) for a higher SFR of a galaxy. They tuned their model to reproduce the observed [α/Fe]-galaxy mass relation, and they claimed that the SFR-dependent IMF is key to reproducing the [α/Fe]-galaxy mass relation and that a short star formation time-scale due to radio-mode AGN feedback alone cannot explain the slope of the relation.…”

Stellar Populations in a semi-analytic model I: Bulges of Milky Way-like galaxies

“…While the treatment of mergers is the same as the one implemented in previous versions of SAG, many aspects of DI have been modified for the purposes of the present study. Both mechanisms trigger starbursts that involve a timescale that regulates the consumption of the cold gas involved in the process as implemented by Gargiulo et al (2015), instead of being instantaneous as in former versions of SAG. We refer the reader to their work for more details about the treatment of extended bursts.…”

“…It follows the production of eleven chemical elements (H, 4 He, 12 C, 14 N, 16 O, 20 Ne, 24 Mg, 28 Si, 32 S, 40 Ca, 56 Fe) generated by stars in different mass ranges. As described in Gargiulo et al (2015), we adopt a new set of stellar yields given by Karakas (2010), (Hirschi et al 2005) and Kobayashi et al (2006) for low and intermediate-mass stars (mass interval 1 − 8M ⊙ ), for the mass loss of pre-supernova stars (He and CNO elements), and for the explosive nucleosynthesis (SNe CC), respectively, all of them corresponding to solar metallicities. These yields are selected to be in accordance with the large number of constraints for the Milky Way (Romano et al 2010).…”