Milky Way Project meets ATLASGAL

26 Feb 2016

In my last week as a postdoc in Oxford - my last week as a postdoc anywhere, in fact - I was very happy to have two new first-author papers accepted for publication in the space of 2 working days. They appeared on astro-ph together on Wednesday, here and here.

The first was a round-up of results from the work I've been doing on Milky Way galactic-scale star formation, using the data from the Milky Way Project (MWP) citizen science project. In the MWP, tens of thousands of volunteers helped us find and characterize interstellar medium bubbles in infrared images from the GLIMPSE and MIPSGAL surveys with the Spitzer Space Telescope. These surveys mapped the inner regions of our Galactic Plane with an unprecedented spatial resolution at infrared wavelengths, showing us a (then) unique view on the beauty and complexity (and chemistry!) of the Galaxy's interstellar medium (ISM).

These bubbles, first catalogued by Ed Churchwell and collaborators in 2006-2007, were found to be excellent tracers of where massive stars and clusters have recently formed. So we can use them to study the interaction between these stars and the surrounding cloud material. But while many authors have done this for one or a few bubbles - complementing the Spitzer IR images with CO observations of the molecular cloud, optical spectroscopy of the cluster stars, X-ray data, etc - the MWP catalogue of some 5000 bubbles allows us to study those interactions statistically, on a Galaxy-wide scale.

We first did this by correlating the locations of massive young protostars or young HII regions as detected by the RMS survey with those of the bubbles in our 2012 paper, finding evidence that we're more likely to find newly forming massive protostars in or on the outskirts of bubbles than in regions without bubbles. But there were some limitations to this study, and in the new paper we compare the distribution of cold massive dust clumps in the Galactic Plane to that of the bubbles to overcome some of those.

Our sample of cold dust clumps is taken from the ATLASGAL survey, a major survey led by Frederic Schuller of the Max Planck Institute for Radio Astronomy in Bonn (now actually working for ESO in Chile; see the rest of the team members here). The ATLASGAL survey, which was in fact just recently completed giving some nice media coverage, was carried out with the 12-m ESO APEX telescope in Chile, observing at submillimeter wavelengths (870 microns). The data I used was from a catalogue published in 2014 by team member Timea Csengeri and her collaborators, also at MPIfR (do check out Timea's interesting recent papers here and here on ATLASGAL). The clumps in this catalogue represent dense condensations in the ISM, cold, massive and dense enough to form massive stars. Some of them may contain such young stars, others are quiescent (for now). We also used a set of follow-up observations of a subset of the ATLASGAL clumps, where ammonia (NH₃) spectroscopy with the 100-m Effelsberg radio telescope was used to derive physical properties of the clumps, such as their temperature and turbulence. These data were published in 2012 by Marion Wienen (MPIfR) and collaborators.

By studying the locations of these clumps as compared with the bubbles, we found the following:

• Around half the ATLASGAL clumps in our sample lie near an IR bubble, and a quarter appear projected towards the rim of a bubble;
• As the bubbles grow in size under the pressure of the young stars' radiation and of the hot ionized gas, they are sweeping up the dense material out of which the stars formed. A statistical correlation analysis between bubbles of increasing size and the clumps, shows the signature of this reordering of material on a Galactic scale.
• The observed correlation shows that it's standard for star forming regions in the Galactic Plane to harbour young stars with a wide range of ages; in other words that star formation proceeds in a phased way rather than in an instantaneous burst.
• From the ammonia measurements, we find that the clumps that are found near bubbles are hotter and more turbulent than their counterparts elsewhere, and this is circumstantial evidence that these clumps are more likely to be forming stars. That finding agrees well with those of our 2012 paper. Interestingly those different properties persist to quite large distances from the bubble. If the radiation from the massive young stars driving the bubbles expansion is responsible for this, it might indicate that a substantial amount of energy is able to leak out of the bubble and into the ISM, where it could contribute to the destruction of the cloud; however there could be underlying reasons for this connection.
• Importantly, our finding that massive star formation seems to be enhanced near the rims of bubbles may be indicative of the bubble expansion driving (''triggering'') a second generation of star formation, but our data cannot prove such a causal link. Jim Dale in Munich has done some excellent work on the difficulties of proving causality in this scenario, do check out his papers if you are interested in the phenomenon of triggered star formation.

The full paper is freely available on astro-ph and will be published in the Astrophysical Journal fairly soon. I had a lot of fun with this paper, the research and presenting it to others, and had great input from my co-authors as well as numerous colleagues at conferences. And of course it wouldn't have been possible without the tens of thousands of MWP volunteers who gave up their time in the name of science. The MWP catalogue is a unique and hugely valuable dataset, and it's great to see our bubbles making an impact.