Chemical evolution of ultra-faint dwarf galaxies in the self-consistently calculated integrated galactic IMF theory

Abstract

The galaxy-wide stellar initial mass function (gwIMF) of a galaxy in dependence on its metallicity and star formation rate can be calculated by the integrated galactic IMF (IGIMF) theory. This theory has been applied in a study of the chemical evolution of the ultra-faint dwarf (UFD) satellite galaxies, but failed to reproduce the data. Here, we find that the IGIMF theory is naturally consistent with the data. We applied the time-evolving gwIMF, which was calculated at each time step. The number of type Ia supernova explosions that forms per unit stellar mass was renormalised according to the gwIMF. The chemical evolution of Boötes I, one of the best-observed UFD, was calculated. Our calculation suggests a mildly bottom-light and top-light gwIMF for Boötes I, and that this UFD has the same gas-consumption timescale as other dwarfs, but was quenched about 0.1 Gyr after formation. This is consistent with independent estimations, and it is similar to Dragonfly 44. The recovered best-fitting input parameters in this work are not covered in previous work, creating a discrepancy between our conclusions. In addition, a detailed discussion of the uncertainties is presented to address the dependence of the chemical evolution model results on the applied assumptions. This study demonstrates the power of the IGIMF theory in understanding star formation in extreme environments and shows that UDFs are a promising pathway to constrain the variation of the low-mass stellar IMF.

A relic from a past merger event in the Large Magellanic Cloud

“…The same stellar nucleosynthesis prescriptions (and zero-point shifts) are then adopted in the models for the LMC and NGC 2005's parent galaxy. Interestingly, it is found that the best agreement between model predictions and relevant data is obtained with a galaxy wide IMF that varies in qualitative agreement with the predictions of the integrated galactic IMF theory (Yan et al 2020).…”

“…Furthermore, in order to reproduce the chemical abundance ratios measured in NGC 2005, we find that it is necessary to require also a reduction of the upper mass limit of the IMF, from 100 to 40 M . These assumptions are justified, at least qualitatively, in the framework of the integrated galactic IMF theory by the low star formation rates and metal-poor environments that characterize dwarf and UFD galaxies (Yan et al 2020).…”

“…If the formation of such massive stars is suppressed, less Zn is formed, this resulting in a lower [Zn/Fe] ratio overall. It has been recently shown (Yan et al 2020) that the very low star formation rates expected in low-luminosity, metal-poor stellar systems lead to a lower upper mass limit for the galaxy-wide initial mass function (IMF). Indeed, if we consider star formation rates lower than ∼ 5 • 10 −4 M yr −1 and an upper mass limit of 40 M for the galaxy-wide IMF, a remarkably good fit of the observed [Zn/Fe] ratio for NGC 2005 is obtained.…”

Gas and dust from extremely metal-poor AGB stars

“…The bulk of the stars in the UFDs have metallicities between [Fe/H] = −3.2 and [Fe/H] = −2.0 (Simon 2019). This piece of observational evidence, combined with theoretical expectation that low-and intermediate-mass star formation is favoured in UFDs at the expenses of massive stars (Yan et al 2020) makes it clear why detailed modeling of extremely metal-poor AGB stars is desperately needed.…”

BiPoS1 -- a computer programme for the dynamical processing of the initial binary star population

“…The mathematical model of the IMF can be constructed from star counts by statistically accounting for all stars formed in one embedded cluster. The IMF shows an insignificant variation with physical conditions in the present-day star-forming activity observed in the Local Group (Bastian et al 2010;Kroupa et al 2013;Offner et al 2014), but has been found to vary systematically with metallicity and density (Dabringhausen et al 2009(Dabringhausen et al , 2012Zhang et al 2018;Yan et al 2020).…”