Revisiting the classics: Is [Mg/Fe] a good proxy for galaxy formation time-scales?

TL;DR

This paper questions the reliability of [Mg/Fe] as a proxy for galaxy formation time-scales, highlighting inconsistencies with recent IMF findings and emphasizing the need for further IMF measurements.

Contribution

It critically examines the traditional interpretation of [Mg/Fe] as an indicator of rapid galaxy formation, considering recent IMF variations and proposing the need for additional data.

Findings

High [Mg/Fe] and steep IMF imply unrealistically short formation timescales.

Current models face inconsistencies when combining [Mg/Fe] and IMF data.

Further IMF measurements are essential for accurate galaxy formation histories.

Abstract

In the local Universe, massive early-type galaxies exhibit enhanced [Mg/Fe] ratios, which has been traditionally interpreted as the result of a rapid ($\tau \lesssim 1$ Gyr) collapse. However, recent claims of a non-universal, steep initial mass function call for a revision of this standard interpretation. In the present work we show how the simultaneous consideration of a high [Mg/Fe] and a steep IMF slope would imply unreasonably short ($\tau \sim 7$ Myr) and intense (SFR $\sim 10^{5}$ Msun yr$^{-1}$) formation events for massive early-type galaxies. We discuss possible caveats and explanations to this apparent inconsistency, and we suggest that further IMF determinations, both in the local Universe and at high redshift, are necessary to better understand the problem.