Stellar Dynamics and the 3 D Structure of Bars

Recent observational constraints restrict the strict applicability of stellar dynamics in spirals to a few rotation periods. However, stellar dynamics concepts such as periodic orbits are invaluable for understanding the various dynamical processes occurring during much more periods. A distinction of two instability types in stellar systems is pointed out, the first one being well illustrated by the bar instability, and the second one by the bar bending instability. In bars the third dimension brings essential dynamical effects which modify the views about the history of bulges and the spiral secular evolution. Bars may grow, bend, thicken, and dissolve into spheroidal bulges, and spirals may evolve along the Hub-ble sequence in the sense Sd→Sa. This leads to a much more dynamical picture of isolated galaxies than imagined before. Recent developments about spirals bring a quite different picture about their physical state. The discovery of their large far-infrared (FIR) flux by IRAS and COBE, comparable or sometimes superior to the optical one, and consistent with the evidences that their optical parts are semi-transparent, means that a substantial fraction of the stellar light is thermalized, and re-emitted by dust in the FIR. This is not negligible with respect to the the typical power that large-scale dynamical processes (spiral arms, bars) can exchange, the gravitational power (ratio of the gravitational energy to the dynamical time). For a system in near virial equilibrium, v 2 = GM/R, we have L grav ≈ GM 2 /R / (R/v) = v 5 /G. Replacing v by the typical rotation speeds of spirals we find powers surprisingly close to the the galaxy luminosities (e.g. 10 44 erg s −1 for the Milky Way). Since the IR Tully-Fisher relation (L H ∝ v 4−5) is nearly parallel to this equation, the match is close along the whole spiral sequence (for more references on the subject see Pfenniger 1992). This coincidence supports the proposition that a feedback mechanism relates both dynamical instabilities and stellar activity. Essential is that although a stellar population pours out energy over several Gyr, the first Myr of a star-burst is a quicker cooperative and intense reaction with respect to the dynamical time τ dyn. The idea of a feedback mechanism between dynamics and star formation has been proposed several times (e.g. Quirk 1972; Kennicutt 1989). With a light thermalization power of the order of the gravitational power, we just need to convert at an …