It’s both motivating and humbling to think about how much we still have to learn about the universe. My colleagues and I have solved one of the enduring mysteries of astrophysics: how massive elliptical galaxies can form. We now have compelling observational data that provide an answer for the first time. Our results were recently published Nature.

Galaxies in the modern universe fall into two broad categories. There are spiral galaxies, such as the Milky Way, that are rich in gas and constantly form stars in a rotating disk. There are also elliptical galaxies that are large and spherical rather than flat like a rugby ball. The latter does not create new stars, but is dominated by stars that formed more than 10 billion years ago.

The formation of elliptical galaxies has long been difficult to explain with cosmological models that describe the evolution of the universe from the Big Bang to the present day. One problem is that during the period when elliptical galaxies formed (10-12 billion years ago), star formation is thought to have occurred in large rotating disks similar to the Milky Way. So how did galaxies change their shape from flat disks to three-dimensional elliptical galaxies?

Observation with Reception

We identified the birthplaces of giant elliptical galaxies by analyzing data from the large millimeter/submillimeter Atacama sequence (Alma). We find that local elliptical galaxies can form through intense, short-lived periods of star formation in the early Universe, rather than starting as a rotating disk and becoming increasingly elliptical over time. Our study examined the distribution of dust in more than 100 distant galaxies that we know formed many stars 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 form, and allows us to examine regions of the galaxy where new stars are actively forming. Using a new observation technique, we found that the dust in these distant galaxies is extremely dense and not what we expected from flat disk galaxies. We were also able to extract the three-dimensional geometry of the dust-emitting regions.

This analysis shows that most early star-forming galaxies were actually spheroids rather than disks. In fact, they are very similar to the shape of elliptical galaxies found near us today.

We then used cosmological computer simulations to interpret the observations and understand the physical mechanisms that might have dragged dust and gas into the centers of these distant star-forming galaxies.

Our analysis suggests that the synchronous movement of cold gas streams from surrounding galaxies, combined with galaxy interactions and mergers, may drive gas and dust into the compact star-forming cores of these galaxies. Simulations also show us that this process was common in the early universe and provide an important explanation for the rapid formation of elliptical galaxies. Our findings add an important piece to this puzzle, improving our understanding of galaxy formation and evolution.

A new observation method

This discovery was made possible by a new method of analyzing ALMA observations. Alma’s data is different from the images we are used to seeing from optical telescopes. Receiving actually works by combining signals from multiple antennas working together like one giant telescope.

This method is known as interferometry, and while it allows sharp images of distant galaxies, data analysis is more complex than traditional optical imaging. Our new technique provides a significant advance in this field by allowing more precise measurement of powder distribution compared to previous methods.

For this study, we used publicly available Alma archival data accumulated over several years. It highlights the power of open source data, where scientists share their findings, and global collaboration to achieve scientific breakthroughs.

Future observations with the JWST and Euclid space telescopes will also map the distribution of stars in the distant ancestors of modern elliptical galaxies. And the Extremely Large Telescope, with a 39-meter-wide mirror, will provide unprecedented details of star-forming nuclei in distant galaxies. In addition, more precise observations of gas dynamics with ALma and the Very Large Telescope will show how gas moves towards the centers of galaxies, triggering star formation and shaping the galaxies we see today.