The integrated galaxy-wide stellar initial mass function over the radial acceleration range of early-type galaxies

TL;DR

This study examines the radial acceleration relations of early-type galaxies and demonstrates that the integrated galaxy-wide stellar initial mass function (IGIMF) theory better explains observed data, especially at high masses, than traditional models.

Contribution

It introduces the application of the IGIMF theory to explain the galaxy-wide stellar initial mass function's impact on radial acceleration relations in early-type galaxies.

Findings

MONDian RARs may work at low accelerations with non-equilibrium dynamics.

All RARs fail at high accelerations with a canonical IMF.

IGIMF theory improves agreement with observations at high masses.

Abstract

The observed radial accelerations of 462 Early-type galaxies (ETGs) at their half-mass radii are discussed. They are compared to the baryonic masses of the same galaxies, which are derived from theoretical expectations for their stellar populations and cover a range from $\approx 10^4 \, {\rm M}_{\odot}$ to $\approx 10^{11} \, {\rm M}_{\odot}$. Both quantities are plotted against each other, and it is tested whether they lie (within errors) along theoretical radial acceleration relations (RARs). We choose the Newtonian RAR and two Milgromian, or MONDian RARs. At low radial accelerations (corresponding to low masses), the Newtonian RAR fails without non-baryonic dark matter, but the two MONDian ones may work, provided moderate out-of-equilibrium dynamics in some of the low-mass ETGs. However all three RARs fail at high accelerations (corresponding to high masses) if all ETGs have formed their stellar populations with the canonical stellar initial mass function (IMF). A much better agreement with the observations can however be accomplished, if the theory of the integrated galaxy-wide stellar initial mass functions (IGIMFs) is used instead. This is because the IGIMF-theory predicts the formation of an overabundance of stellar remnants during the lifetime of the massive ETGs. Thus their baryonic masses today are higher than they would be if the ETGs had formed with a canonical IMF. Also the masses of the stellar-mass black holes should be rather high, which would mean that most of them probably formed when the massive ETGs were not as metal-enriched as they are today. The IGIMF-approach confirms downsizing.