Final Report Summary - LOCALSTAR (Modelling star formation in the local universe)
This project investigated the evolution of molecular clouds, and star formation within them, in the local universe. Molecular clouds are dense regions of gas in galaxies where star formation occurs. Three important questions we are trying to understand are how are these clouds formed, how long they survive, and how are they dispersed. We provided one of the first estimates of how long clouds survive from numerical simulations. We show that these clouds are actually quite short-lived, surviving only 5-20 Myr. We also confirmed previous theoretical work, that star formation proceeds, and clouds survive for approximately a crossing time (the size of the cloud divided by the typical velocity in the gas). Our work showed that cloud destruction is governed by shear, which breaks apart the cloud, and feedback, which also breaks the cloud up and transforms molecular gas back to atomic gas at lower densities.
Our project has produced the first simulations of star formation molecular clouds taken from whole galaxy simulations. Usually simulations of star formation use uniform turbulent spheres, but we showed that these miss some of the complexity, including the initial density field, and previous history of the gas and star formation. These are important for determining the physics of star formation, and characteristics of star forming regions. We also produced simulations of a section of spiral arm, which allowed us to resolve the full population of molecular clouds, and the internal properties of giant molecular clouds. Furthermore we highlighted the need for stellar feedback to reduce the star formation rates.
We have also translated our results back into observer style maps to see how observers would view our simulations, and follow the evolution of our clouds in observed tracers HI and CO. We showed that clouds form from both a mixture of atomic and molecular hydrogen, and in particular that precursors of molecular clouds are giant molecular filaments. Although these filaments are very long, only parts of them appear in CO, so although such filaments are already observed in the Milky Way they may be more abundant than we think.
We have focused our research on star formation in Milky Way and similar type spiral galaxies. One big question is what produces the spiral arms in such galaxies. We showed that for the Milky Way and Trapezium galaxies, the spiral arms are best reproduced by transient gravitational instabilities. So far though there is not significant evidence that the mechanism by which spiral arms form has a large impact on the properties of molecular clouds, or amount of star formation, in galaxies.
Our project has produced the first simulations of star formation molecular clouds taken from whole galaxy simulations. Usually simulations of star formation use uniform turbulent spheres, but we showed that these miss some of the complexity, including the initial density field, and previous history of the gas and star formation. These are important for determining the physics of star formation, and characteristics of star forming regions. We also produced simulations of a section of spiral arm, which allowed us to resolve the full population of molecular clouds, and the internal properties of giant molecular clouds. Furthermore we highlighted the need for stellar feedback to reduce the star formation rates.
We have also translated our results back into observer style maps to see how observers would view our simulations, and follow the evolution of our clouds in observed tracers HI and CO. We showed that clouds form from both a mixture of atomic and molecular hydrogen, and in particular that precursors of molecular clouds are giant molecular filaments. Although these filaments are very long, only parts of them appear in CO, so although such filaments are already observed in the Milky Way they may be more abundant than we think.
We have focused our research on star formation in Milky Way and similar type spiral galaxies. One big question is what produces the spiral arms in such galaxies. We showed that for the Milky Way and Trapezium galaxies, the spiral arms are best reproduced by transient gravitational instabilities. So far though there is not significant evidence that the mechanism by which spiral arms form has a large impact on the properties of molecular clouds, or amount of star formation, in galaxies.