Recent N -body simulations show that the formation of a present-day , galaxy sized dark matter halo in the cold dark matter cosmogony in general consists of an early fast collapse phase , during which the potential associated with a halo is established , followed by a slow accretion phase , during which mass is added rather gently in the outer region .
In this paper , we consider the implication of such a halo assemble history for galaxy formation .
We outline a scenario in which the fast collapse phase is accompanied with rapid formation of cold clouds and with starbursts that can eject a large amount of gas from the halo centre .
The loss of orbital energy of the cold clouds to the dark matter and the ejection of gas from the halo centre by starbursts can significantly reduce the halo concentration .
The outflow from the starburst can also heat the gas in the protogalaxy region .
Subsequent formation of galaxies in the slow accretion regime is therefore in halos that have been pre-processed by these processes and may have properties different from that given by N -body simulations .
This scenario can help to solve several outstanding problems in the standard \Lambda CDM model of galaxy formation without compromising its success in allowing structure formation at high redshift .
The predicted rotation curves can be significantly flatter than those based on the halo profiles obtained from N -body simulations , alleviating the discrepancy of the Tully-Fisher relation predicted in the standard \Lambda CDM model with observations .
The flattened galaxy halos allow accreted minihalos to survive in their central regions longer , which may be helpful in producing the flux anomalies observed in some gravitational lensing systems .
The preheating by the early starbursts effectively reduces the amount of gas that can be accreted into galaxy halos , which may explain why the baryon fraction in a spiral galaxy is in general much lower than the universal baryon fraction , f _ { B } \sim 0.16 , in the standard \Lambda CDM model .