On the importance of feedback in the stream-fed high redshift universe

Cosmological hydrodynamic simulations have shown that galaxies are fed by dense, cold gas streams at high redshift. However, the presence of such gas has never been observationally confirmed. Using the Horizon- MareNostrum simulation, I examined whether cold flows are detectable with low-ionisation metal absorption lines, such as C II 1334. It is concluded that due to their low metallicity and density, it is extremely difficult to prove/disprove the presence of cold flows using the metal absorption lines. Revisiting the acquisition of angular momentum in disc galaxies using high resolution simulations, I found that at the time of accretion, gas and dark matter do carry a similar amount of specific angular momentum which is systematically and significantly higher (at minimum by a factor of 2) than that of the dark matter halo as a whole. Whereas cold streams directly deposit this large amount of angular momentum within a sphere of radius r~0.1 Rvir, dark matter particles easily pass through the central region, depositing their angular momentum over a much more spatially extended region. As a result, in our simulations neither the total specific angular momentum of the baryons nor its radial profile ever follows that of the virialised dark matter halo, contrary to what is typically assumed in the standard theory of disc galaxy formation. In order to better understand the formation of disc galaxies and the missing baryon problem in a LCDM universe, continuous, collective galactic winds are implemented. It is demonstrated that stellar feedback processes are able to suppress star formation by ~30% at z=3, compared to that from the run without feedback sources, but it still produces an unrealistic central peak in the rotation curve. Although inclusion of hypernovae further suppresses star formation, it is unable to quench the formation of low-angular momentum stars enough to remove the peaked rotation curves at high redshift. Finally, feedback from active galactic nuclei turns out to be effective at suppressing star formation in massive galaxies at 1<z