A modern view of galaxies and their stellar populations

A critical discourse is provided on the current status of the astrophysics of galaxies in view of open fundamental questions on the law of gravitation and the physics-driven variation of the galaxy-wide stellar initial mass function (GWIMF). The Einstein/Newtonian plus cold or warm dark-matter-based models face many significant but unresolved tensions (e.g. planes of satellites, the highly organised and symmetrical structure of the Local Group, the local Gpc-scale void). The accumulating nature of these indicates quite compellingly the need for a different theoretical framework. An example is the prediction, made in 1997, of the existence of satellite galaxies with near-exact properties to the Hercules dwarf spheroidal satellite galaxy discovered in 2007 if it is a tidal dwarf galaxy void of dark matter. Such results indicate that once the evidence is accepted that dark matter particles have no role in galaxies with gravitation being effectively Milgromian, then the resulting theoretical understanding of galaxies becomes much simpler and highly predictive with remarkable successes. Along with these scientific advancements, the recent observational data solidify the existence of one of the most important relations in star-formation theory, namely the mmax-Mecl relation. The data rule out stochastic star formation and confirm that the IMF becomes increasingly top-heavy with increasing star-formation rate density. Galaxies and their embedded-star-cluster building blocks are therefore self-regulated dynamical systems which are computationally accessible. Finding a theory for the formation of galaxies involves the development of a new cosmological model, which may differ substantially from the dark-matter based one. Very significant new opportunities have thus emerged for inquisitive and daring researchers which may appear risky now but almost certainly lead to major breakthroughs.