It's a cosmic fairytale story: A lowly dwarf galaxy, poor in matter and with no prospects of stardom, inexplicably begins producing a burst of dazzling star clusters. But how did this galaxy go from rags to riches in its isolated environment without the help of larger dust-rich galaxies?
The answer seems to lie in previously unseen pockets of dense matter, dotted around the galaxy like a trail of breadcrumbs. ALMA (Atacama Large Millimeter/submillimeter Array) managed to spot these pockets of matter for the first time, hidden inside the dwarf galaxy. The results can be found in Nature.
Named Wolf-Lundmark-Melotte (WLM), this irregular galaxy is isolated, away from any other galaxies or stray interstellar dust – vital for forming stars. However, this didn't stop WLM from doing so, and it's due to previously unseen reserves of gas that prompted the formation of stars.
"For many reasons, dwarf irregular galaxies like WLM are poorly equipped to form star clusters," commented Monica Rubio, lead author on the study from the University of Chile. "These galaxies are fluffy with very low densities. They also lack the heavy elements that contribute to star formation. Such galaxies should only form dispersed stars rather than concentrated clusters, but that is clearly not the case."
The astronomers located these star-forming regions by looking for carbon monoxide (CO) signals. An abundance of CO is associated with star-forming clouds. Interestingly, WLM has the lowest levels of CO ever detected in any galaxy. This made its star formation even more unlikely since it also appeared to lack star-making ingredients.
B. Saxton (NRAO/AUI/NSF); M. Rubio et al., Universidad de Chile, ALMA (NRAO/ESO/NAOJ)
"Molecules, and carbon monoxide in particular, play an important role in star formation," said Rubio. "As gas clouds begin to collapse, temperatures and densities rise, pushing back against gravity. That's where these molecules and dust particles come to the rescue by absorbing some of the heat through collisions and radiating it into space at infrared and submillimeter wavelengths." The cooler cloud can then shrink that little bit further since hot, energetic particles are no longer jostling the cloud apart. This allows pressures high enough for stars to form.
The reason scientists struggled to see the CO previously was because the pockets of star-forming gas were like little pinpricks coated with large clouds of molecular and atomic gas.
"By discovering that the carbon monoxide is confined to highly concentrated regions within a vast expanse of transitional gas, we could finally understand the mechanisms that led to the impressive stellar neighborhoods we see in the galaxy today," commented Bruce Elmegreen, a co-author on the paper.
In the future, ALMA will continue to scan the skies. The astronomers hope that they will be able to ascertain how even larger clusters of stars may have formed in dwarf galaxies.
ALMA at night. C. Padilla (NRAO/AUI/NSF).