A nearby dwarf irregular galaxy called the Wolf-Lundmark-Melotte (WLM) poses an intriguing mystery: how is it able to form large star clusters without the dusty, gas-rich environments found in larger galaxies? According to an international team of astronomers from the U.S., Chile, and UK, the answer lies in an unexpected population of compact interstellar clouds hidden within the galaxy.
Tiny clouds of carbon monoxide in the dwarf galaxy Wolf-Lundmark-Melotte: the carbon monoxide emission is shown as black single contours inside the 1 arcmin x 1 arcmin white squares that outline the area mapped by ALMA. The inset in the top left is the full view of the galaxy. Image credit: Monica Rubio et al.
The newly-discovered interstellar clouds, which are nestled within a heavy blanket of interstellar material, help explain how dense star clusters are able to form in the tenuous environs of the WLM galaxy, which is thousands of times smaller and far more diffuse than our Milky Way Galaxy.
“For many reasons, dwarf irregular galaxies like WLM are poorly equipped to form star clusters. 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,” said Dr Monica Rubio of the University of Chile, who is the first author of a paper published in the journal Nature.
The Wolf-Lundmark-Melotte, also known as LEDA 143 or DDO 221, was discovered in 1909 by astronomer Max Wolf. Its nature as a galaxy was only established in 1926 by Knut Lundmark and Philibert Jacques Melotte.
It is a relatively isolated dwarf galaxy located in the constellation Cetus, about 3.13 million light-years away.
The galaxy is a remote member of the Local Group — the collection of galaxies that includes the Milky Way, the Magellanic Clouds, Andromeda, M33, and dozens of smaller galaxies.
By studying the WLM galaxy with the Atacama Large Millimeter/submillimeter Array (ALMA), Dr Rubio and co-authors were able to locate, for the first time, compact regions that appear able to emulate the nurturing environments found in larger galaxies.
These regions were discovered by pinpointing the almost imperceptible and highly localized millimeter wavelength light emitted by carbon monoxide molecules, which are typically associated with star-forming interstellar clouds.
Earlier, another team first detected carbon monoxide in the galaxy with the single-dish Atacama Pathfinder Experiment telescope.
These initial, low-resolution observations could not resolve where the molecules reside, but they did confirm that WLM contains the lowest abundance of carbon monoxide ever detected in any galaxy.
This lack of carbon monoxide and other heavy elements should put a serious damper on star formation, the astronomers note.
“Molecules, and carbon monoxide in particular, play an important role in star formation. As gas clouds begin to collapse, temperatures and densities rise, pushing back against gravity,” Dr Rubio said.
“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.”
This cooling effect enables gravity to continue the collapse until a star forms.
The problem previously was that in WLM and similar galaxies with very low abundances of heavy elements, astronomers simply didn’t see enough of this material to account for the new star clusters they observed.
The reason carbon monoxide was initially so difficult to see, the astronomers discovered, is that unlike in normal galaxies, the WLM galaxy clouds are very tiny compared to their overlying envelopes of molecular and atomic gas.
To become viable star factories, the concentrated carbon monoxide clouds need these enormous envelopes of transitional gas to bear down on them, giving the cores of carbon monoxide a high enough density to allow them to form a normal cluster of stars.
“Like a diver being squeezed at the bottom of a deep abyss, these bundles of star-forming gas are under tremendous pressure, even though the surrounding ocean of interstellar gas is much more shallow,” said co-author Dr Bruce Elmegreen of the IBM T.J. Watson Research Center.
“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.”
_____
Monica Rubio et al. 2015. Dense cloud cores revealed by CO in the low metallicity dwarf galaxy WLM. Nature 525, 218-221; doi: 10.1038/nature14901
