Bullock et al (2001) demonstrated that dark halos have a universal angular momentum distribution that should also be characteristic for the infalling gas component. Van den Bosch et al. (2001) lateron showed that this angular momentum distribution is not consistent with the observed distribution of exponential galactic disks indicating that viscous angular momentum redistribution in galactic disks must have played an important role. The viscosity is likely driven by interstellar turbulence which is a result of stellar energetic feedback processes (see Fig. 2) or global disk instabilities (magneto-rotational instability or gravitational instability). Note, that viscous effects will increases the angular momentum problem substantially as viscosity in general removes angular momentum from the dominante mass component in the disk and transfers it to the outermost parts of the disk.
Figure 2. Gas surface density of a gravitationally unstable gas-rich galactic disk, embedded in a dark matter halo. The left panel shows the gas density distribution if star formation is suppressed. The disk forms a few massive gaseous clumps that spiral into the center by dynamical friction. The situation is however very different if star formation and stellar energy feedback is included. In this run, supernova explosions efficiently disrupt dense clumps before they can merge into giant cloud complexes while at the same time generating a highly turbulent and filamentary multi-phase interstellar medium (Burkert et al. 2009).
The viscous formation of exponential stellar disks from gas disks with various different surface density distributions has been studied e.g. by Slyz et al. (2002). Their numerical simulations show that exponential disks form if the star formation timescale is of order the viscous timescale. Genzel et al. (2008) derive a timescale for turbulent viscosity in galactic disks of
where is of order unity. visc 1010 yrs for disks like the Milky Way with 10-20 km/s and self-regulated low star formation rates. H integral field spectroscopy has however detected z ~ 2 star forming disk galaxies with large random gas motions of order 40 km/s to 60 km/s and viscous timescales of less than 109 yrs (Genzel et al., 2006, 2008, Förster-Schreiber et al. 2006). Interestingly, for these objects, the star formation timescales are again similar to the viscous timescale, leading to star formation rates of 100 M/yr and confirming that galactic disk gas turbulence, star formation and secular evolution are intimately coupled. The origin of the clumpiness and high turbulence in redshift 2 disks is not well understood yet. It seems likely that it is a result of substantial filamentary gas inflow (Dekel et al. 2008), combined with gravitational instabilites in the disk (Bournaud et al. 2007).
Turbulence seems to regulate star formation not only on large galactic scales but also on local cloud scales. Most of the molecular gas in the Milky Way is found in giant molecular clouds with masses of order 104 - 106 M, temperatures of order 10 K and average densities of order 100 cm-3. As their Jeans mass is of order 20 M which is much smaller than their total mass one would expect that molecular clouds should collapse and condense into stars on a local free-fall time which is of order 5 × 106 yrs. Adopting a total molecular mass of MH2 3 × 109 M and assuming that a fraction ηSF 0.1 of the molecular cloud's mass forms stars, the inferred mean star formation rate in the Milky Way is
which is an order of magnitude larger than observed. A possible solution of this problem is turbulence. Molecular clouds are observed to be driven and shaped by supersonic turbulence that might strongly affect their stability and star formation rate. The origin of this turbulent motion and its impact on the cloud's lifetime and star formation process is not well understood yet. It is however likely that large-scale disk turbulence is the seed for turbulence in molecular clouds which again affects the star formation rate that in turn drives again large scale disk turbulence and by this also the viscous secular evolution of galactic disks.