Gas nuggets turn dwarf galaxy into star factory
Not too far from our own Milky Way galaxy is a tiny dwarf galaxy, known as WLM. You would expect such a tiny dwarf galaxy to produce few new stars. Yet, WLM contains young clusters of newly-born stars that are as crowded as similar star clusters in the Milky Way. Using ALMA, astronomers have now discovered how this is possible.
Stars are made out of gas – mainly hydrogen and helium. Clouds of gas can collapse under their own gravity to form new stars. But there’s a catch. A contracting cloud of gas begins to heat up. If it gets too hot, it won’t collapse into stars so easily. So during the collapse, the cloud needs to be cooled down somehow.
In large galaxies like our own Milky Way, molecules of carbon monoxide work as a coolant. There’s enough carbon monoxide around to allow for the formation of lots of new stars in the Milky Way – usually in large clusters. No riddle here.
But earlier measurements have revealed that dwarf galaxies like WLM have much lower levels of carbon monoxide. So how is it possible that they also contain clusters of young stars, more or less similar to the star clusters in the Milky Way?
The answer: the carbon monoxide coolant is not spread evenly through the gas. Instead, the new ALMA observations have revealed very small clumps – or ‘nuggets’ – of carbon monoxide gas hidden in the cores of much larger clouds of hydrogen and helium. Because of the pressure of the surrounding clouds, the carbon monoxide in these compact clumps gets concentrated enough to enable the birth of new star clusters.
ALMA discovered ten of those carbon monoxide ‘nuggets’ in WLM. They are only some ten to twenty light-years across. Without the super-sharp vision of ALMA, they would never have been detectable.
The discovery may also shed light on the formation of so-called globular clusters – large spherical collections of many hundreds of thousands of very old stars. Most astronomers think that these clusters were also formed in dwarf galaxies, billions of years ago.
WLM is a dwarf galaxy at a distance of about 3 million light-years. It is named after the three astronomers who first studied it a century ago: Wolf, Lundmark and Melotte. The dwarf galaxy is part of our Local Group of galaxies, which – apart from our own Milky Way – also contains the large Andromeda and Triangulum galaxies, and dozens of other dwarfs. WLM has an irregular, elongated shape. Even along its longest axis, it is more than ten times smaller than the Milky Way galaxy. It is located in the constellation Cetus the Whale, but it can only be seen with a very large professional telescope.
The ALMA observations of WLM were carried out by a team led by Monica Rubio of the Universidad de Chile in Santiago. Monica’s collaborators are Bruce Elmegreen of the IBM T.J. Watson Research Center in Yorktown Heights, New York; Deidre Hunter of the Lowell Observatory in Flagstaff, Arizona; Elias Brinks of the University of Hertfordshire in the United Kingdom, Juan Cortes of the Joint ALMA Observatory and National Radio Astronomy Observatory in Santiago, and Phil Cigan of the New Mexico Institute of Mining and Technology in Socorro. The published their results in the scientific journal Nature.