A 100-year-old mystery surrounding the “shape-shifting” nature of some galaxies has been solved, revealing that our Milky Way galaxy doesn’t always have its usual spiral appearance. Astronomer Alistair Graham used observations old and new to show how galaxies evolve from one form to another; This process is known as galaxy speciation. The study shows that collisions and subsequent mergers between galaxies are a form of “natural selection” that drives the cosmic evolutionary process.

This means that the Milky Way’s history of cosmic violence is not unique to our galaxy. And it’s not over yet. “This is survival of the fittest,” Graham said in a statement. “Astronomy now has a new anatomical sequence and finally an evolutionary sequence in which the speciation of galaxies occurs through the inevitable merger of galaxies driven by gravity.”

Galaxies have different shapes. Some, like the Milky Way, consist of well-ordered stellar arms orbiting a central condensation or “bulge” of stellar masses. Other galaxies, such as Messier 87 (M87), consist of an ellipse of billions of stars buzzing chaotically around an irregular central concentration.

Since the 1920s, astronomers have classified galaxies according to a diverse array of galactic anatomy called the “Hubble sequence.” Spiral galaxies like ours are at one end of this series, and elliptical galaxies like M87 are at the other end. Lying between these two galaxies are elongated spherical galaxies without spiral arms, called lenticular galaxies.

But what has been missing so far in this widely used system are evolutionary pathways that connect one galaxy form to another.

Reshaping Galactic Evolution

To examine the evolutionary paths in the Hubble array, Graham looked at 100 galaxies near the Milky Way in optical light images collected by the Hubble Space Telescope and compared them with infrared images from the Spitzer Space Telescope. This allowed him to compare the mass of all stars in each galaxy with the mass of central supermassive black holes.

This revealed the existence of two different types of alternating lenticular galaxies: one version is old and dust-free, and the other is young and dust-rich.

As dust-poor galaxies accumulate gas, dust, and other matter, the disk surrounding their central regions is distorted, creating a spiral pattern radiating from their hearts. This creates spiral arms, which are super-dense rotating regions that form gas clumps as they spin, causing them to collapse and form stars.

On the other hand, dust-rich lenticular galaxies are formed when spiral galaxies collide and merge. This is indicated by the fact that spiral galaxies have a small central sphere with large spiral arms made up of stars, gas, and dust. Young and dusty lenticular galaxies have significantly more prominent spheroids and black holes than spiral galaxies and dust-poor lenticular galaxies.

An unexpected consequence of this is that spiral galaxies like the Milky Way are actually between dusty and dust-poor lenticular galaxies in the Hubble array.

“Everything fell into place when it became clear that lenticular galaxies were not the only connected populations for which they had long been depicted,” Graham explained. “It redraws our favorite galaxy series, and more importantly, we now see evolutionary paths through the galaxy marriage series, or what business might call mergers and acquisitions.”