When the universe was about half of its age (z ∼ 0.7) and earlier, the morphology of spiral galaxies were significantly different from what we know today, in the Hubble sequence. Galaxies were much more clumpy, with clumps of gas and stars of kpc size (e.g. Elmegreen 2007). These very irregular morphologies are thought to result from the very high gas fraction of these early galaxies. Noguchi (1999) simulated the formation of galaxies from highly gaseous systems, and found that they form giant clumps, which by dynamical friction can spiral inwards to the center rather quickly to form a bulge. Bournaud et al (2007b) developed further the dynamical mechanisms, and showed that rather quickly, clumpy disks form an exponential disk, a bulge, and also a thick disk due to the stars formed in the turbulently thick gaseous disk. The disruption of the clumps by the feedback of star formation (supernovae, winds) is not yet well known, and can be adjusted to maintain the clumpy disks at the observed frequency (Elmegreen et al. 2008) The large increase of the gas fraction of spiral galaxies with redshift has been confirmed by direct observations of the molecular gas (e.g. Tacconi et al. 2010).
The very high efficiency of bulge formation through dynamical friction in clumpy disks might be a problem for the standard dark matter model, since bulge-less galaxies are quite frequent today (e.g. Weinzirl et al. 2009). Since dynamical friction occurs mainly against dark matter halos, it is expected that it will be much less important in the MOND dynamics, and the rapid bulge formation could be avoided. This was indeed demontrated in a recent paper, comparing formation of bulges in gas-rich clumpy galaxies, in the two gravity models, Newtonian with dark matter and MOND (Combes, 2014).
This work first computes the dynamical time-scale in an idealized situation, where the galaxy disks are purely stellar, to isolate the main dynamical phenomenon, from the more complex gas hydrodynamics, star formation or feedback. When several clumps are launched randomly in the disk, the dynamical friction efficiency is difficult to predict, since the wakes of the different massive bodies interfere (Weinberg 1989). With typical clump mass fraction (25-30%), in the Newtonian model, the dynamical time-scale for clumps to spiral into the center of a galaxy with baryonic mass 6 × 1010 M⊙ is 0.3 Gyr, and 1 Gyr for a galaxy with baryonic mass 6 × 109 M⊙. In the MOND regime, the clumps do not fall into the center before 3 Gyr. When the gas and star formation/feedback are taken into account, the simulated galaxy disks are rapidly unstable to clump formation, due to the gas fraction of 50%. In the Newtonian gravity with dark matter, previous results are retrieved, i.e. an increasing clump mass fraction in the first 200 Myr, and the coalescence of clumps towards the center, with a spheroidal bulge formation, in less than 1 Gyr (Noguchi 1999, Immeli et al 2004, Bournaud et al. 2007b). With MOND gravity, clumps form quickly too (cf Fig 7), but they maintain in the disk for the whole simulation of 3 Gyr, until the gas has been consumed in stars. The clump mass fraction does not decrease much, being just eroded through stellar feedback, and shear forces (Fig. 8). Bulges are clearly not formed in the early clumpy phase of galaxy formation, as in the Newtonian equivalent systems.
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Figure 7. All baryons (left) and gas (right) surface densities of the dwarf clumpy galaxy, simulated with MOND gravity, at epochs 0.5, 1 and 2 Gyr. Each panel is 60 kpc in size. The color scale is logarithmic and the same for all plots. From Combes (2014). |
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Figure 8. Evolution of the clump mass fraction for the giant galaxy, in the MOND gravity (left) and in the Newtonian gravity (right). |