The faint HI environment of galaxies: a laboratory for testing galaxy formation

The SKA is a unique instrument to open a window on many aspects of galaxy formation and evolution which can be examined in our Local Universe. Here I will focus on the outermost regions of galaxies which can be observed with sufficient sensitivity and resolution at 21-cm as to enlighten the interplay between galaxies and the intergalactic gas, the competing race between the local dark matter gravity and the external ionizing radiation field. Tracing the gas distribution out to large galactocentric radii will be complementary to QSO’s Ly$\alpha$ absorption studies for understanding the evolution of the dark and visible matter of the Universe.