Galaxies evolve very quickly, in a time-scale short compared to the age of the Universe, and can change morphology drastically. It has already been established by numerical simulations that a bar in a galaxy can be destroyed by the interstellar gas concentration towards its galaxy center ; the type of a galaxy can then pass from barred spiral to unbarred and in the same time, the galaxy being more concentrated, its morphology goes up the Hubble classification towards the "early" types. New numerical simulations by two astronomers of Paris Observatory show for the first time how gas accretion in a galaxy can lead to the formation of a new bar, and how the galaxy then goes down the Hubble sequence towards the late types. Several bars can thus follow one another in the same spiral galaxy, and these bars are density waves which rotate increasingly fast in the galaxy.
Galaxies are distributed according to their morphology on the Hubble sequence, which is the famous tuning fork reproduced on Figure 1. The two branches of the tuning fork represent two varieties, the barred and unbarred spiral galaxies. Bars are one of the essential drivers in the evolution of galaxies, they exist in 2/3 of spiral galaxies (more precisely a third of the galaxies has a strong bar, of type SB, a third a intermediate bar, type SAB, and a third does not have any bar, type SA). The tuning fork in fact should be generalized with all possible intensities for the bar.
Figure 1. Hubble sequence of galaxies. From left to right, elliptical galaxies, then spiral galaxies called "early", and finally "late-type" spiral galaxies. The Sequence is Sa —> Sb —> Sc. Spiral galaxies are flattened discs, but contain also a spheroidal component in their center, or "bulge". The bulge-to-disk ratio increases towards the left. There are two branches of spiral galaxies : barred and unbarred. The first represent a little more than 2/3 of the spiral galaxies.
Bars are density waves which develop spontaneously in a galactic disc subject to its own gravitation. All the matter takes part in this gravitational instability, interstellar gas and stars. A disc is the more unstable as it is "cold", i.e. its velocity dispersion (or disordered motions) is small, and its rotational velocity (ordered motion) is high. The bar is a perturbation which breaks the axial symmetry of the galactic disc, and consequently creates tangential gravity forces. These forces result in gravity torques on the spiral arms of the galaxy, and help to transfer the angular momentum of the interstellar gas towards the outer parts, which allows a large gas mass to fall towards the center. The action of the bar is thus to trigger bursts of star formation in the centers of galaxies, when those are fueled with gas. Many works have shown in the last decade how the accumulation of mass towards the center destroys gradually the bar (e.g. Pfenniger and Norman 1990, ApJ 363, 391 ; Hasan et al.. 1993, ApJ 409, 91). Indeed, the mass concentration and the formation of a bulge stabilize the disc which is then less subject to its self gravity. Bars would be the agent of their own destruction, since it is under their action that the mass concentration takes place.
Figure 2. Evolution of the bar strength as a fonction of time, during the simulation of Figure 3. However the majority of galaxies are barred. It is thus necessary to find a mechanism which reforms the bars. In a recent work, Frederic Bournaud and Francoise Combes propose that the accretion of large quantities of gas by galactic discs triggers the formation of new bars, in a galaxy where the mass ratio between disc and bulge is rebalanced in favor of the disc, that becomes then more self-gravitating, and thus more unstable. The disc is then also more "cold", because the velocity dispersion of gas is quite lower than that of stars, the gas losing energy by radiation.
Figure 3. Evolution of a spiral galaxy, with gas accretion. The animated sequence is a self-gravitating simulation, taking into account stars and gas. New stars are constantly formed from gas, according to a star formation rate proportional to the local gas density. The old stars are in red, the young stars in blue, the gas is in yellow. After the development of a spiral, a bar is formed by disk instability in the first 500 million years. This bar soon will weaken, then disappear, but other bars will replace it, for example the second strong bar at about 7 billion years, at the end of the animation. The current barred galaxies could be experiencing their third or fourth barred episode. Reload the image, to restart the movie
Gas accretion could even lead to the formation of a series of successive bars, according to a self-regulation mechanism. As soon as sufficient gas mass has settled in the disc, a bar is formed, produces torques on the spiral arms, and drives an accumulation of mass towards the center which weakens or destroys the bar. In the presence of a strong bar, the torques which change sign outside the disc, prevent external gas from entering the disc. Once the bar has disappeared, the gas will be able to settle again in the disc, and the process will start again (see the evolution of the bar strength as a function of time on figure 2 and the film of figure 3).
Figure 4. Evolution of the rotation speed of the bar wave as a function of time, during the simulation of Figure 3, with gas accretion. After a natural decrease during the first barred episode, the accretion increases the rotation speed and the second bar is faster than the first.
One of the consequences of this process is that galaxies evolve on the Hubble sequence, not only from one branch of the tuning fork to the other, but they also slide down towards later types Sa —> Sb —> Sc, to some extent they are re-juvenated, whereas in natural evolution they concentrate their mass and go up the sequence Sc —> Sb —> Sa. Their long-term evolution would still be in this direction, but with some returns towards "younger" states. In the same way, the rotation speed of the bar wave, which normally decreases in the evolution, would grow again, as shown in simulations (cf figure 4). Galaxies are not isolated, but are systems in formation, which continue to be fed with gas from the intergalactic medium, more or less efficiently according to the environment. To some extent, gas accretion would be a Fountain of Youth for galaxies !
Référence
- Bournaud F., Combes F., 2002 "Gas accretion on spiral galaxies : bar formation and renewal" Astronomy and Astrophysics in press astro-ph/0206273