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Title:
When is star formation episodic? A delay differential equation `negative feedback' model
Authors:
Quillen, Alice C.; Bland-Hawthorn, Joss
Affiliation:
AA(Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA), AB(Institute of Astronomy, School of Physics, University of Sydney, NSW 2006, Australia)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 386, Issue 4, pp. 2227-2234. (MNRAS Homepage)
Publication Date:
06/2008
Origin:
MNRAS
MNRAS Keywords:
ISM: evolution , galaxies: ISM
DOI:
10.1111/j.1365-2966.2008.13193.x
Bibliographic Code:
2008MNRAS.386.2227Q

Abstract

We introduce a differential equation for star formation in galaxies that incorporates negative feedback with a delay. When the feedback is instantaneous, solutions approach a self-limiting equilibrium state. When there is a delay, even though the feedback is negative, the solutions can exhibit cyclic and episodic solutions. We find that periodic or episodic star formation only occurs when two conditions are satisfied. First the delay time-scale must exceed a cloud consumption time-scale. Secondly, the feedback must be strong. This statement is quantitatively equivalent to requiring that the time-scale to approach equilibrium be greater than approximately twice the cloud consumption time-scale. The period of oscillations predicted is approximately four times the delay time-scale. The amplitude of the oscillations increases with both feedback strength and delay time.

We discuss applications of the delay differential equation (DDE) model to star formation in galaxies using the cloud density as a variable. The DDE model is most applicable to systems that recycle gas and only slowly remove gas from the system. We propose likely delay mechanisms based on the requirement that the delay time is related to the observationally estimated time between episodic events. The proposed delay time-scale accounting for episodic star formation in galaxy centres on periods similar to P ~ 10 Myr, irregular galaxies with P ~ 100 Myr, and the Milky Way disc with P ~ 2 Gyr, could be that for exciting turbulence following creation of massive stars, that for gas pushed into the halo to return and interact with the disc and that for spiral density wave evolution, respectively.

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