Journal Article

Giant molecular clouds: what are they made from, and how do they get there?

C. L. Dobbs, J. E. Pringle and A. Burkert

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 425, issue 3, pages 2157-2168
Published in print September 2012 | ISSN: 0035-8711
Published online September 2012 | e-ISSN: 1365-2966 | DOI:
Giant molecular clouds: what are they made from, and how do they get there?

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We analyse the results of four simulations of isolated galaxies: two with a rigid spiral potential of fixed pattern speed, but with different degrees of star formation induced feedback, one with an axisymmetric galactic potential and one with a ‘live’ self-gravitating stellar component. Since we use a Lagrangian method, we are able to select gas that lies within giant molecular clouds (GMCs) at a particular time frame, and then study the properties of this gas at earlier and later times. We find that gas which forms GMCs is not typical of the interstellar medium (ISM) at least 50 Myr before the clouds form and reaches mean densities within an order of magnitude of mean cloud densities by around 10 Myr before cloud formation. The gas in GMCs takes at least 50 Myr to return to typical ISM gas after dispersal by stellar feedback, and in some cases the gas is never fully recycled. We also present a study of the 2D, vertically averaged velocity fields within the ISM. We show that the velocity fields corresponding to the shortest time-scales (i.e. those time-scales closest to the immediate formation and dissipation of the clouds) can be readily understood in terms of the various cloud formation and dissipation mechanisms. Properties of the flow patterns can be used to distinguish the processes which drive converging flows (e.g. spiral shocks, supernovae) and thus molecular cloud formation, and we note that such properties may be detectable with future observations of nearby galaxies.

Keywords: stars: formation; ISM: clouds; ISM: kinematics and dynamics; galaxies: spiral

Journal Article.  8018 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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