It is as humbling as it is motivating to think about how much we still have to learn about the universe. My collaborators and I have just tackled one of astrophysics’ enduring mysteries: how massive elliptical galaxies can form.

Now, for the first time, we have solid observational evidence that provides an answer. Our results have recently been published in Nature.

Galaxies in the present-day universe fall into two broad categories. There are spiral galaxies, like our Milky Way, which are rich in gas and continuously form stars in a rotating disc. There are also elliptical galaxies, which are large and spherical rather than flat, similar to a rugby ball. The latter don’t produce new stars but are dominated by stars formed more than 10 billion years ago.

The formation of elliptical galaxies has long been difficult to explain with cosmological models describing the evolution of the universe from the Big Bang to now. One of the challenges is that star formation during the era when elliptical galaxies formed (10 billion to 12 billion years ago) was thought to occur within large rotating discs, similar to our own Milky Way.

So how did the galaxies transform their shape from flat discs to three-dimensional elliptical galaxies?

Observations with Alma

By analysing data from the Atacama Large Millimeter/submillimeter Array (Alma), we identified the birth sites of giant elliptical galaxies. We discovered that local elliptical galaxies can form through intense and short-lived star formation episodes early in the universe, as opposed to starting out as a rotating disc and becoming more elliptical over time.

Three Alma antennas on the 5000-metre altitude plateau of Chajnantor in Chile. — Three Alma antennas on the 5km altitude plateau of Chajnantor in Chile. wikipedia, CC BY-SA

Our study examined the distribution of dust in more than 100 distant galaxies, which we know were forming lots of stars back when the universe was between 2.2 billion and 5.9 billion years old. Dust indicates the presence of gas — the material from which new stars are formed — and enables us to study the regions within a galaxy that are actively forming new stars.

Using a novel observational technique, we found that the dust in these distant galaxies is extremely compact and isn’t what we expected from flat disc-shaped galaxies. Furthermore, we were able to infer the three-dimensional geometry of the dust-emitting regions. This analysis indicates that most of the early star-forming galaxies were actually spherical rather than disc-shaped. In fact, they closely resemble the shape of elliptical galaxies near us today.

We then used cosmological computer simulations to interpret the observational results and understand the physical mechanisms that may have caused dust and gas to sink into the centres of these distant, star-forming galaxies.

Our analysis reveals that the simultaneous action of cold gas streams from surrounding galaxies along with galaxy interactions and mergers can drive gas and dust into compact, star-forming cores within these galaxies. The simulations also show us that this process was common in the early universe, providing a key explanation for the rapid formation of elliptical galaxies.

Our findings add a crucial piece to this puzzle, advancing our understanding of galaxy formation and evolution.

A novel observational technique

This discovery was made possible by a novel technique for analysing ALMA observations. Alma data are different than the images we are used to see from optical telescopes. In fact, Alma operates by combining signals from multiple antennas that work together as a single, giant telescope.

This technique is known as interferometry, and while it allows to obtain sharp images of distant galaxies, the data analysis is more complex than for traditional optical images. Our new technique enables more precise measurements of dust distribution compared to previous methods, offering a significant advancement in this field.