RAM-PRESSURE ACCRETION OF INTERGALACTIC GAS CLOUDS BY GALAXIES

Intergalactic gas clouds are captured and accreted by an encounter wit h gaseous halos of galaxies due to the ram-pressure friction. If cloud s are captured by an elliptical galaxy embedded in a hot halo without rotation, they first form a circum-galactic band of cool gas and then fall toward the central region along an ''accretion spiral'', and prob ably act to fuel the AGN. If clouds are captured by a spiral galaxy wi th a rotating disk and halo, they form stochastic spiral arms in the o uter region of the galaxy, while they make a ring feature in the inner region. The accretion rate is higher when clouds' encounter is retrog rade with respect to the galaxy's rotation, while it is lower when the encounter is direct. This asymmetric accretion about the rotation axi s results in a significant loss of angular momentum of the disk and ha lo, leading to a subsequent contraction of the galactic disk. When the encounter is off the galactic plane, retrograde clouds are captured a nd further accreted into the nuclear region along polar orbits. This a ccretion toward the central region feeds cool gas to the nucleus and w ill trigger starburst. Such an external fueling will act as the trigge ring mechanism for starburst in isolated spiral galaxies. The accretio n of intergalactic clouds may also explain some peculiar kinematics of gas such as the counter-rotation in the central regions of some early -type galaxies. We point out that such a gas-dynamical process of accr etion of intergalactic gas clouds, besides the gravitational effect, w ould have played an important role in the evolution and dynamics of pr imeval galaxies in the early universe, when the intergalactic space wa s rich in protogalactic gas clouds.