Bulge formation in disk galaxies with MOND

Observatoire de Paris, LERMA (CNRS: UMR 8112), 61 Av. de l’Observatoire, 75014 Paris, France
e-mail: francoise.combes@obspm.fr

Received: 15 September 2014
Accepted: 24 September 2014

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 bulgeless spiral galaxies. In the present work, the formation of bulges in very gas-rich galaxies, such as those in the early universe, is studied in Milgrom’s MOdified Newtonian Dynamics (MOND) through multigrid simulations of the nonlinear gravity, including gas dissipation, star formation, and feedback. Clumpy disks are rapidly formed, as in the equivalent Newtonian 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. We now show that gas-rich clumpy galaxies in the early universe do not form bulges. Disks with a low bulge fraction, which is compatible with the observations, are therefore a natural result in MOND. Since pseudo-bulges are formed by bars with a similar rate to those in the Newtonian equivalent systems, it can be expected that the contribution of pseudo-bulges is significantly higher in MOND.