One of my favorite things about the universe is that, like a well-trained improv actor, it’s response always seems to be “Yes, and”. 

In putting together today’s episode, I learned about a new way to form galaxies. In grad school, we were taught that galaxies either formed through the bulk collapse of material into massive systems or through the collapse of material into smaller systems that merged into bigger systems over time. In recent years, we’ve learned there is no “or”. Both these things are happening and it is awesome. 

Today I learned, there is yet another possible way to get galaxies! During the massive galaxy collisions, streams of material can get torn out of the merging systems, and this debris can form entirely new galaxies, called tidal dwarf galaxies, or TDGs. These are new systems made of old material and are capable of star formation. In a new paper in Astronomy & Astrophysics, a team led by Miguel Querejeta, researchers use ALMA to study TDG J1023+1952.

According to co-author Carole Mundell: The little galaxy we’ve been studying was born in a violent, gas-rich galactic collision and offers us a unique laboratory to study the physics of star formation in extreme environments.

The star formation they found wasn’t entirely like we’re used to seeing in our galaxy. Here, our gas is generally tied up into giant molecular clouds that can collapse into star-forming regions. In this tidal dwarf galaxy, they also found diffuse gas throughout the system, and that gas is somehow forming stars.

And this is another set of “ands”: galaxies form through the collapse of material, they form through mergers, and they form from the material discarded in mergers. Stars form in giant molecular clouds, and they can form in diffuse gas permeating these special systems.

The science we had was all on track – it just lacked the creativity needed to anticipate all the crazy improv our universe seems ready to do.

More Information

University of Bath press release

“ALMA resolves giant molecular clouds in a tidal dwarf galaxy,” M. Querejeta et al., 2021 January 22, Astronomy & Astrophysics