Dynamical Triggering of Starbursts

Abstract

Galaxy interactions/mergers, gravitational instabilities and density waves, such as bars, are frequently invoked to trigger starbursts. These mechanisms have been explored through numerical simulations, with the help of various star formation recipes. Gravitational instabilities are necessary to initiate star formation, but the main trigger might be the gas flows, to provide sufficient fuel in a short time-scale. Gas accretion is also acting on the dynamics, in favoring bars/spirals, which will drive the gas inwards. Large amounts of external gas accretion are required to explain the bar frequency, and this accretion rate can be provided by the cosmic filaments, as supported by cosmological simulations. Subsequent interactions can then trigger starbursts by driving this accreted gas inwards. OBSERVATIONAL EVIDENCE OF DYNAMICAL TRIGGERINGIt is now widely accepted that strong galaxy interactions can produce violent starbursts, and ultra-luminous infra-red galaxies (ULIRGs) are all mergers (e.g. Sanders & Mirabel 1996). But interacting galaxies in general do not show intense starbursts (Bergvall et al 2003), or only in their centers. The tidal parameter, depending on mass and distance of companions, is not correlated to any star formation tracer. Starbursts are rare in the local universe, and much rarer than galaxy interactions. The only perceptible effect is that interacting galaxies have more mass -gas, dust & young stars, and are more concentrated to the center (Bergvall et al 2003). One of the main conclusions that can be drawn for giant galaxy starbursts is that galaxy interactions is a necessary condition to trigger them, but not sufficient. Certainly, other parameters have to be taken into account, such as the gas content, the geometry/velocity of the encounter, and also the phases of the interaction, the starburst occuring only during about 10% of the interaction time, according to simulations (e.g. Mihos & Hernquist 1996).For dwarf galaxies, interactions are even not a necessary condition for triggering: starbursting dwarfs have no more companions (Brosch et al 2004), although it might be different for Blue Compact Dwarfs (BCD, Hunter & Elmegreen 2004). Dwarf galaxies with starbursts present asymmetries, and a possible trigger could just come from gas accretion: the gas infalling in dark-matter haloes may experience sloshing and oscillations favoring compression and instabilities. This mechanism might be relevant for primordial galaxies (Brosch et al 2004).One of the best evidence of triggering can be obtained by tracing the fossil records of the star formation history in nearby galaxies. In the Small Magellanic Cloud, the number of stars at a given age has been determined by Zaritsky & Harris (2004). They noticed that the derived SMC star formation history reveals some bursts corresponding to pericenters of its orbit around the Milky Way. A model taking the tidal triggering

The reversal of the star formation-density relation in the distant universe

“…Hence major mergers do not appear to strongly affect the efficiency with which galaxies are forming stars and therefore appear not to be the major cause for the reversal of the star formation-density relation at z ∼ 1. Several other mechanisms of galaxy interactions could play a role that remain to be identified, such as minor mergers (mass ratio <1/4), galaxy harassment during group formation, dynamical instabilities during tidal encounters (Combes 2005) or even the acceleration of intergalactic gas infall during group formation.…”

“…Hence the true fraction of major mergers can vary between ∼30 and 50% of the galaxies. Here we are only discussing the role of major mergers, because this is the only mechanism that we can hope to visually identify in the z ∼ 1 LIRG population, but there are many other ways through which the environment can act on galaxies such as minor mergers (mass ratio <1/4), galaxy harassment during group formation, dynamical instabilities during tidal encounters (Combes 2005) or even the acceleration of intergalactic gas infall during group formation.…”

The star formation history of luminous infrared galaxies

“…Our determination of a burst duration of 0.1 Gyear and a Scalo parameter of 4, seems to rule out the possibility that distant LIRGs are isolated spirals forming stars at a constant rate over a long duration. The starbursts may instead be triggered by tidal effects and minor mergers in regions of the universe where the local density of galaxies is enhanced, as suggested by Elbaz & Cesarsky (2003) or by the infall of intergalactic gas (Combes 2005). Further kinematical studies of distant galaxies (see Flores et al 2006;Puech et al 2006) will help to distinguish between the various scenarios (mergers, gas infall) discussed here.…”

Multiscale magnetic fields in spiral galaxies: evolution and reversals

“…Star formation can be triggered by different processes including, for example, gravitational instability, and the interaction of clouds and tidal forces in isolated and merging galaxies (Kennicutt et al 1987;Combes 2005). The rate of SN explosions is proportional to the SFR.…”