Young Stars in the Outskirts of Galaxies Finally Have an Explanation

Star formation is well understood when it happens in the populous centers of galaxies. From our vantage point on Earth, within the Milky Way, we see it happening all around us. But when newborn stars are birthed in the empty outskirts of galactic space, it requires a new kind of explanation. At the 243rd meeting of the American Astronomical Association yesterday, astronomers announced that they have observed, for the first time, the unique molecular clouds that give rise to star formation near the remote edges of galaxies.

“We didn’t expect to find star formation at the outskirts of galaxies, but about 18 years ago, surprisingly, the NASA GALEX satellite (Galaxy Evolution Explorer) found a lot of newborn stars at the outskirts of galaxies,” says Jin Koda, lead researcher on the project and associate professor at Stony Brook University. “Astronomers became interested in the environment for this star formation and looked for molecular clouds – the parental sites of star formation – but couldn’t find any. Until now.”

The normal path to star formation involves diffuse atomic gas slowly gravitating together into a cloud that eventually begins to shrink and collapse. As the density increases, the single diffuse atoms form bonds, becoming molecular gas, at which point the gas has transformed into the star-forming engine that astronomers call molecular clouds.

“Molecular clouds typically have very dense cores at their centers which we call the ‘hearts’ of molecular clouds, which are surrounded by less dense molecular gas. Star formation is actually happening in these hearts of molecular clouds: the dense cores,” says Koda.

When GALEX observations found newborn stars on the edge of Galaxy M83 back in 2005, it did not see any accompanying molecular clouds. That was strange.

With this new research, the reason astronomers couldn’t see the clouds has become clear.

The outer envelope of the molecular clouds in this region were invisible: the observations could only see the minuscule “hearts” of the clouds.

Research on the far edge of galaxy M83 reveals unusual star formation in an extreme environment. This area, outlined in yellow, is shown in data from several different instruments. From left to right: optical image from CTIO, ultraviolet image from GALEX, HI 21cm image from VLA and GBT, and CO(3-2) image from ALMA. In this last image, the star-forming “hearts” of molecular clouds, circled with white, are shown. Image Credit: Jin Koda.

The team discovered a total of 23 of these solitary molecular cloud hearts, each within a tiny region of Galaxy M83.

Now that researchers know the formation sites for these outskirt stars exist (albeit looking very distinct from their inner galactic counterparts), they can put one 18-year-old mystery to rest. But the research also raised a new mystery.

The same region that hosted the 23 molecular cloud hearts was also home to a surprisingly large concentration of atomic gas, compared to the amount of molecular gas observed.

“We don’t yet understand why this atomic gas has not converted to molecular gas efficiently in this environment,” says Amanda Lee, who was an undergraduate on the team, and is now at the University of Massachusetts Amherst.

For that answer, more research is necessary.

David Thilker, the astronomer responsible for the original discovery of newborn stars in M83’s outskirts using GALEX 18 years ago, says “It has been gratifying to see the search for dense clouds associated with the outer disk finally come to fruition, revealing a characteristically different observational fingerprint for the molecular clouds.”

Besides GALEX, observations for this research were carried out using a combination of instruments, including the Atacama Large Millimeter/submillimeter Array (ALMA), the Karl G. Jansky Very Large Array (VLA), the Green Bank Telescope (GBT), and the Subaru Telescope.

Learn More:

Mystery of Star Formation Revealed by Hearts of Molecular Clouds.” Green Bank Observatory.

Watch the press conference here.