In bubbles too, correlation ≠ causation

The last couple of weeks saw the culmination for me of several months’ worth of hard work on a paper following up on our exciting new Milky Way Project catalogue. With the help of a number of MWP science teamers, I performed a statistical study looking at the correlation of the 5000-odd bubbles sample with a catalogue of known massive young stars detected by the infrared Midcourse Space Experiment (MSX) satellite. What we found was that these two types of sources are strongly associated with one another, which is not unexpected. But we also noticed that the largest of our bubbles appear to have a disproportionally large number of massive young stars around their edges, which is a more exciting find. It confirms results from a very recent study by UK/Australian/German colleagues.

From previous studies of smaller samples of infrared bubbles, we know that many of these beautiful sources form around massive young stars or clusters, which clear away their surrounding cloud material with powerful UV radiation and stellar winds. The resulting cavity filled with hot dust glows brightly in the infrared (the red stuff), and the complex carbon-based molecules in the rims are excited by the stars’ UV photons (the green stuff).

Studying the correlation between massive young stars on the one hand and bubbles on the other, can tell us two things: (i) can we detect the massive young stars at the centre of the bubbles in the young stars catalogue, and (ii) is there any evidence of triggered star formation happening on the outskirts of the bubbles? The answer to (i) was pretty easy to test with our methods and resulted in a statistically resounding “yes”: the data tell us that we find loads of massive young sources in bubble interiors – far more than we’d expect from chance alignments.

Triggering is a special mode of star formation that we think might occur when energetic events, such as supernova explosions or bubble expansions, shock and compress molecular gas around them, causing dense pockets to collapse and form new stars in regions where this would otherwise not have happened.

Many papers have been published in recent years studying this phenomenon in theoretical calculations and simulations, and showing tentative evidence in observations. Triggered star formation is a potentially important phenomenon, as it might allow star formation in galaxies to sort of “daisy-chain” through a galaxy, which each generation of young stars providing the energetic kick into the surrounding gas to set off the next.

So in this paper, I show how the correlation between our bubbles and the catalogue of MSX sources (called the RMS catalogue, curated by the Leeds astrophysics group) paints a picture that is possibly consistent with triggered star formation happening around the largest of the MWP bubbles.

The really important caveat to the work is that this association does not imply that triggering is really happening. With bubbles, like with everything, correlation does not equal causation. The analysis I performed looks at a simple 2D projection of these objects on the sky, ignoring the 3D structure of both the bubbles themselves and of the Milky Way Galaxy. And demonstrating this causal effect between one newly born cluster and new stars forming in the same area is a really tough challenge that I’d argue very few authors have convincingly overcome (though I haven’t read every single paper).

I presented this work at the joint UK/German National Astronomy Meeting in Manchester earlier this week, which Rob wrote about on the Milky Way Project blog (with photographic evidence). Rob & Chris’ Recycled Electrons podcast also throws some random thoughts on the work around in typical style, and Will Gater interviewed Rob and me for Sky at Night magazine.

Of course, there’s an awful lot more to this work than I can capture here or in a 15-minute talk. If you’re interested, check out the paper on astro-ph but note that it’s not actually been accepted for publication yet – so all findings should be considered preliminary. I also submitted my Python code to the journal so that should be made available once the paper gets published as well. All data I used for the analysis are publicly available from either the MWP webpages or the RMS database.

The Manchester NAM was excellent fun. I heard some great talks and met lots of interesting likeminded people.  Since becoming a conference organiser myself I really appreciate a smoothly run event – so big thanks to the organisers for that.

Here’s the details of the paper:

Sarah Kendrew, Robert J. Simpson, Eli Bressert, Matthew S. Povich, Reid Sherman, Chris Lintott, Thomas P. Robitaille, Kevin Schawinski, & Grace Wolf-Chase (2012). The Milky Way Project: A statistical study of massive star formation associated with infrared bubbles ApJ submitted arXiv: 1203.5486v1

Comments welcome!


  1. […] work I described in yesterday’s post has got me thinking lots about statistics. I used to hate statistics in school and at university. […]