“It seems odd that we can describe the basics of star formation, but the details are obscure”
The centres of spiral galaxies are funny places. Never mind the supermassive black hole lurking at the centre, the flow of gas into and around the centre of the galaxy can have dramatic effects. This is particularly true in systems like the Milky Way, which have a long bar crossing the centre, and a recent paper by Mark Krumholz of the University of California Santa Cruz and Diederik Kruijssen of the Max Planck Institute for Astrophysics in Germany uses these unusual places to peer into the mysteries of how stars form.
I’ve always found it curious that star formation is mysterious. It seems odd that though we can describe the basic process easily – a clump of cold gas collapses under its own gravity until the point where nuclear reactions can start – the details are obscure. We don’t know what triggers star formation, we don’t know what controls the mass of a star and we don’t know how it is affected by the galaxy. It’s this latter question that this study addresses, attempting to distinguish between the sensible argument that a single forming star doesn’t know much about its surroundings and the equally plausible argument that the properties of the clouds from which stars form varies depending on the environment.
If surroundings do make a difference to stellar nurseries, then a galaxy’s centre is the place to look. It’s the place where things are most confused; in the Milky Way, gas in believed to be transported along the bar to the centre where it piles up. This sounds like it should favour star formation – the piling up of gas should increase the density overall, after all – but the computer simulations carried out by the team suggest things are complicated.
As the gas rushes inwards along the bar, it is much less likely to form stars. Its rapid movement actually prevents star formation, and it’s not until it accumulates closer to the centre that star formation can really get going. Even then, things are complicated. Stars can form as such a rate that what the authors describe as a ‘blowout’ occurs, with gas being expelled from the centre and star formation being placed on hold until sufficient fuel can once again accumulate.
This is a complex story, and given that it depends both on broad assumptions about what physics is important and on some nifty simulations, I’d be skeptical about it were it not for the pile-up of evidence presented. Most impressive is the fact that the authors see in their computerised galaxy the formation of a ring of gas a little more than 300 lightyears from the centre. Just such a ring is observed in the Milky Way, and if this work is right something like it should appear in every barred galaxy.
Even better, the model explains why we see all sorts of behaviour in the centre of nearby barred galaxies. Some are hives of activity, while others are quiet, behaviour that is a natural consequence of a model that switches star formation on and off. Further observations will help, and the authors end with an unusual plea – ALMA, up high in the Atacama Desert, could peer into the hearts of many nearby galaxies, but has not yet. They think it should, and I agree. The secrets of star formation are there for the taking.