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7.1. Formation of disk galaxies and spin alignment

The disk galaxy formation process originates from the conservation of angular momentum from collapsing gas in extended dark matter haloes: the collapse stops once the system becomes rotationally supported.

The dimensionless spin parameter of a protogalactic system is

Equation 50 (50)

where J, E and M are the total angular momentum, energy and mass of the system. The angular momentum of the gas arises in the same way as that of dark matter, from tidal torques from the surrounding large scale structure. Hoyle [102] was the first to propose that protogalaxies acquired their angular momentum via tidal torques from neighbouring perturbations during a period of gravitational instability.

Peebles [103] analyzed the tidal torque theory using linear perturbation theory and showed that it could account, roughly, for the angular momentum of the Milky Way. Tidal torque theory was first applied to the angular momentum of disks in [104] and reviewed in [105].

A topic that has recently seen a revival in interest is the study of correlations between the spin of dark matter halos and their large-scale environment, specifically the large-scale filamentary structure of the cosmic web. [106] investigated the alignment of the spin of dark matter halos relative to the surrounding filamentary structure, using a dark matter-only simulation which resolves over 43 million dark matter halos at redshift zero. She detected a clear mass transition: the spin of dark matter halos above the critical mass Mcrit ~ 5 ⋅ 1012 Modot tends to be perpendicular to the closest large scale host filament, whereas the spin of lower mass halos is more likely to be aligned with the closest filament, see fig. 20.

Figure 20

Figure 20. Excess probability of alignment between the spin and the direction of the closest filament. Different colors correspond to different mass bins as labeled. Figure from [106].

The proposed explanation is that low mass halos mostly form at high redshift within the filaments generated by colliding-collapsing walls, a process that naturally produces a net halo spin parallel to the filaments. In contrast, high-mass halos mainly form by merging with other halos along the filaments at a later time when the filaments are themselves colliding and/or collapsing. Therefore they acquire a spin which is preferentially perpendicular to these filaments. Hence, the correlations measured in [106] can be understood as a consequence of the dynamics of large-scale cosmic flows.

In the context of high redshift galaxy formation, this paper argued that galaxies form preferentially along filaments, and that the main nodes of the cosmic web are where galaxies migrate, not where they form. Consequently, these galaxies would inherit the anisotropy of their birth place as spin orientation.

7.2. Bulgeless disk galaxies

Giant pure-disk galaxies are a challenge to our understanding of galaxy formation. Reference [107] studied several pure disk galaxies, two of which are shown in fig. 21, concluding that, in the nearby field, much of the stellar mass is in pure disks. The problem is that is difficult to understand how, within a hierarchical halo growth model, such galaxies could have formed, without converting any preexisting stellar disk into a classical bulge. Angular momentum loss and spheroid formation is inevitably found in galaxy formation simulations. This problem may, with extreme feedback, be avoided in dwarf galaxies [108]. In fact, these massive galaxies only show a pseudo-bulge, which is presumably created by secular evolution of isolated galaxy disks. Massive pure disk galaxies are not rare enough to be explained as mergerless galaxies. It does not seems likely that physical processes such as energy feedback, are able to sufficiently delay star formation and thereby allow the halo to grow without forming a classical bulge, as might provide a solution to the problem [107].

Figure 21

Figure 21. Left: SDSS color image of NGC 5457,emphasizing how much this giant galaxy is dominated by its disk; the tiny, bright center is the pseudo-bulge. Right: Color image of NGC 6503, a slightly smaller galaxy, taken with the Hubble Space Telescope Advanced Camera for Surveys. The central tiny pseudo-bulge makes up 0.11% of the I-band light of the galaxy. Figure from [107].

In the Virgo cluster, more than 60% of the stellar mass is in elliptical galaxies and some additional mass is in classical bulges, whereas in Local-Group-like environments apparently the majority of galaxies with halo vcirc > 150 km s-1 form with no sign of the major merger that would have formed a classical bulge: [107] emphasizes that the problem of the formation of bulgeless giant galaxies appears to be a strong function of environment.

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