Wednesday, August 16, 2006

G. Hensler on "Feedback from Star Formation"

As mentioned before, the energy release from various sources (mainly stars and AGN) is called feedback and it can act positively, enhancing SF even more, or negatively by shutting down star formation (SF). To understand all this, one needs to understand how stars work and how the ISM behaves.

Feedback takes place at all scales, from small (e.g. HII regions) to large, for example global outflows from galaxies and galactic winds. By feedback, SF is self-regulated in the sense that little SF can trigger more SF, but once you get too much, positive turns into negative feedback, preventing further SF.

This self regulation manifests itself in the Schmitt-law (famous paper by Kennicutt), stating that SF is proportional to the surface density of the gas to the power of around 1.5. The Toomre-criterion must be fulfilled to have SF at all and this accounts for a cut-off of SF at low surface densities. This self-regulation comes automatically, even in simulations and the Schmitt-law is nicely reproduced.

The efficiency of SF, i.e. what fraction of gas is turned into stars, depends strongly on the density of the ISM and in starbursts both can be very high. One uncertainty in studying feedback is how much if the released energy (e.b. by stellar winds) is actually deposited in the ISM and how much escapes by radiation. The values for this effficiency vary between one percent and one permille.


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