The role of bars

Secular evolution and fueling is driven by bars in spiral galaxies, and the related dynamical processes are reviewed. It is shown that gravity torques dominate over viscous torques, and produce gas infall to the center. In this infall, the bar wave accepts the angular momentum, which produces its destruction. In the end, a central mass concentration is build, which also contributes to this destruction. While gas can be stalled at ILR for a while, secondary bars then take over the fueling. In a galaxy life-time, several bar episodes can successively develop. Through external gas accretion, disks are replenished by intermittence, between two bar phases. While the general trend for a galaxy is to evolve towards early-types, its morphology can oscillate, and turn back transiently to later-types. 1. The problem of bars and AGN fueling Disks of spiral galaxies can be considered as accretion disks: matter tends to concentrate to reach the least energy state, but the angular momentum has to be transfered outwards. Since viscous torques are not sufficient over a Hubble time, gravity torques play the main role, through formation of bars and spirals. Numerical simulations reveal that bars are very efficient to drive the gas of a galaxy inwards, however there is no clear correlation between nuclear activity and bars in the observations (e.g. Mulchaey & Regan 1997). There could even be a lower fraction of strong bars in Seyferts (Shlosman et al 2000). The only positive signs of the bar action is the observation of more outer rings in Seyfert galaxies (Hunt & Malkan 1999, and Agüero et al, this meeting), and more gas concentration in barred galaxies (Sakamoto et al 1999). Also, there are always circumnuclear dust in AGN, while not always in normal galaxies (Martini et al 2003). The main reason for this lack of correlation could be the different time-scales: the duty cycle for nuclear activity is of the order of 40 Myr, much shorter than the dynamical time-scales of gas inflow (at least 300 Myr). The case of galaxy interactions is similar, since internal bars are then also the motor of gas inflow. Nuclear starbursts are often simultaneously observed with nuclear activity (cf Maia, Lim et al, Storchi-Bergman et al, this meeting). 2. Gas flows in barred galaxies Gas behaviour in a barred potential has been studied for a long time (e.g. Sanders & Huntley 1976), but the larger spatial resolution reached recently have allowed to better estimate the influence of viscosity (Maciejewski et al 2002). Already Englmaier & Gerhard (1997) revealed the large influence of the sound speed in the gas behaviour. The larger the velocity dispersion in the gas, the larger the mean free path of the particles, and the larger the distance over which they can exchange angular momentum, so that shear is more important. With large velocity dispersion, the gas that was previously trapped at ILR can further fall inwards (Maciejewski et al 2002, cf also Ann, this meeting). The role of bar characteristics (strength, axis ratio, etc..) on gas inflow has been examined in