The second problem is that of the angular momentum distribution. Even if angular momentum is assumed to be conserved, one cannot explain the exponential form of disc galaxies. Using cold dark matter (CDM) numerical simulations (Bullock et al. 2001), found that if discs are formed from gas with an angular momentum distribution similar to that of dark matter, this results in excess mass near the centre, in comparison with an exponential disc. Specifically, the models predict too much material with low angular momentum, and this makes it particularly difficult to explain the origin of bulgeless dwarf galaxies (van den Bosch, Burkert & Swaters 2001). Responding to this difficulty Maller & Dekel (2002) proposed a model in which the angular momentum is built up by a sequence of mergers. While the acquisition of angular momentum in dissipationless N-body simulations has been well studied (Bullock et al. 2001; Maller, Dekel & Somerville 2002; Vitvitska et al. 2002; Avila-Reese et al. 2005; Bett et al. 2007, 2010; D'Onghia & Navarro 2007), our understanding of the extent to which gas behaves similarly has only recently started to be explored via computations (Chen, Jing & Yoshikaw 2003; Sharma & Steinmetz 2005; Brook et al. 2011; Kimm et al. 2011; Pichon et al. 2011; Sharma & Nath 2012). In the last decade galaxy redshift surveys (see e.g. Colless et al. 2001) have enabled us to map the universe on increasingly large scales, and to study systematically the properties of large-scale structures. These surveys show that the galaxies are not distributed evenly, but develop within a pattern of filaments and sheets which are separated by immense voids. The latter are defined observationally as large regions with very low galaxy number density, which occupy some 40 per cent of the overall cosmic volume (Hoyle & Vogeley 2004). It is generally believed that the voids originate from the local minima of the primordial density fluctuations, and as their mean density is lower than that of their surroundings, they expand more quickly. If tidal shear plays a significant role in their development, it should not only cause them to depart from sphericity, but should entail their acquisition of spin angular momentum (Lee & Park 2006). The specific angular momentum of cold dark matter haloes in a ΛCDM universe depends strongly on their merging histories (D‘Onghia & Burkert 2004). Haloes with a quiet merging history, dominated until the present epoch by minor mergers and accretion, acquire by tidal torques an average of only 3 per cent of the angular momentum required for their rotational support. This is in direct conflict with the observational data for a sample of late-type bulgeless galaxies, which indicate that these galaxies reside in dark haloes with exceptionally high values of specific angular momentum. Although minor mergers and accretion preserve or slowly increase the specific angular momentum of dark haloes as a function of time, this mechanism is not efficient enough to explain the observed spin values, especially for late-type dwarf galaxies. Energetic feedback processes have been invoked to solve the problem implied by the loss of a large fraction of its specific angular momentum by the gas during infall. This is predicated on the assumption that dark haloes which host bulgeless galaxies acquire their mass via quiescent accretion. D‘Onghia & Burkert (2004) find an even more serious version of this problem: the specific angular momentum gained during the formation of these objects is not enough to provide rotational support even if no angular momentum is lost during gas infall.
Citation: Maller, Dekel & Somerville 2002
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