The star formation timescale of elliptical galaxies -- Fitting [Mg/Fe] and total metallicity simultaneously

Abstract

The alpha element to iron peak element ratio, for example [Mg/Fe], is a commonly applied indicator of the galaxy star formation timescale (SFT) since the two groups of elements are mainly produced by different types of supernovae that explode over different timescales. However, it is insufficient to consider only [Mg/Fe] when estimating the SFT. The [Mg/Fe] yield of a stellar population depends on its metallicity. Therefore, it is possible for galaxies with different SFTs and at the same time different total metallicity to have the same [Mg/Fe]. This effect has not been properly taken into consideration in previous studies. In this study, we assume the galaxy-wide stellar initial mass function (gwIMF) to be canonical and invariant. We demonstrate that our computation code reproduces the SFT estimations of previous studies where only the [Mg/Fe] observational constraint is applied. We then demonstrate that once both metallicity and [Mg/Fe] observations are considered, a more severe "downsizing relation" is required. This means that either lowmass ellipticals have longer SFTs (> 4 Gyr for galaxies with mass below 10 10 M ) or massive ellipticals have shorter SFTs (≈ 200 Myr for galaxies more massive than 10 11 M ) than previously thought. This modification increases the difficulty in reconciling such SFTs with other observational constraints. We show that applying different stellar yield modifications does not relieve this formation timescale problem. The quite unrealistically short SFT required by [Mg/Fe] and total metallicity would be prolonged if a variable stellar gwIMF were assumed. Since a systematically varying gwIMF has been suggested by various observations this could present a natural solution to this problem.1 Not to be confused with the downsizing of the galaxy age or star formation rate (SFR), which is the original meaning of downsizing (Cowie et al. 1996). See Fontanot et al. (2009 for a summary of the many manifestations of downsizing.

Very high redshift quasars and the rapid emergence of super-massive black holes

“…The gas in-flow and out-flows will be modulated by the accretion activity of the SMBH-seed(s), being dependent also on the progressive metal enrichment of the gas as the spheroid forms (e.g. Yan et al 2019a). The accretion of gas onto the BHs and the likely feedback effect are discussed in Secs 8.1.…”