Bulge formation in disk galaxies with MOND

TL;DR

This study uses simulations to show that in MOND gravity, gas-rich early universe galaxies form clumpy disks but do not develop classical bulges, differing from dark matter models.

Contribution

It demonstrates through simulations that MOND predicts less bulge formation in gas-rich galaxies, contrasting with standard dark matter models.

Findings

Clumpy disks form rapidly in MOND.

Bulges do not form before clumps are eroded.

Pseudo-bulges form at similar rates as in Newtonian systems.

Abstract

The formation of galaxies and their various components can be stringent tests of dark matter models and of gravity theories. In the standard cold dark matter (CDM) model, spheroids are formed through mergers in a strongly hierarchical scenario, and also in the early universe through dynamical friction in clumpy galaxies. More secularly, pseudo-bulges are formed by the inner vertical resonance with bars. The high efficiency of bulge formation is in tension with observations in the local universe of a large amount of bulge-less spiral galaxies. In the present work, the formation of bulges in very gas-rich galaxies, as those in the early universe, is studied in the Milgrom's MOdified Newtonian Dynamics (MOND), through multi-grid simulations of the non-linear gravity, including the gas dissipation, star formation and feedback. Clumpy disks are rapidly formed, as in their Newtonian equivalent systems. However, the dynamical friction is not as efficient, in the absence of dark matter halos, and the clumps have no time to coalesce into the center to form bulges, before they are eroded by stellar feedback and shear forces. Previous work has established that mergers are less frequent in MOND, and classical bulges are expected less massive. It is now shown that gas-rich clumpy galaxies in the early universe do not form bulges. Since pseudo-bulges are formed with a similar rate as in the Newtonian equivalent systems, it can be expected that the contribution of pseudo-bulges is significantly higher in MOND.