“Galaxies are social animals,” according to astronomer Laerte Sodré Júnior, full professor at the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences (IAG-USP) in Brazil.
Scientists believe that there are approximately 100 billion galaxies in the visible universe, and none of them survive in isolation. “Lone galaxies are not to be found,” Sodré Jr. said.
They form duets, interact in triplets, combine in groups by the dozen, or band together in colossal clusters containing thousands of galaxies.
Investigating galaxy triplets to understand how their components interact is the focus of a study conducted by postdoctoral studentMarcus Vinícius Costa-Duarte and Sodré Jr., his supervisor.
They have recently published a paper co-authored by two Argentinian colleagues in Monthly Notices of the Royal Astronomical Society (MNRAS). The study is part of a Thematic Project supported by FAPESP.
“We know the evolution of a galaxy depends basically on two variables: its mass and its environment,” Costa-Duarte said. “The purpose of the study was to find out how galaxies are formed in triplet environments. We concluded that the most massive of the three plays a key role in the evolution of the other two and of the entire system.”
The galaxy sample that the astronomers analyzed was drawn from Data Release 10 of the Sloan Digital Sky Survey (SDSS-DR10), a public database of approximately 2 million galaxies. They used a computer algorithm to search for galaxy triplets in which the most massive galaxy had a mass equivalent to or greater than that of 10 billion stars, like our own Sun.
For the sake of comparison, it is worth noting that this is the mass of the Large Magellanic Cloud, the largest satellite galaxy of the Milky Way. Our own galaxy’s mass is estimated to be 850 billion Sun masses (not to be confused with the number of stars in the Milky Way, currently estimated to be 400 billion).
Galaxies also come in different shapes in addition to different sizes. Everyone recognizes the majestic spiral of galaxies such as the Milky Way, but giant ellipses are common, as are irregular shapes such as the Large Magellanic Cloud.
The shape of a galaxy is often a reflection of the types of matter of which it is made – stars, planets, interstellar dust, and vast clouds of primordial gas, as well as dark matter. The gas clouds comprise hydrogen and helium produced by the Big Bang almost 13.8 billion years ago. This primordial hydrogen is the raw material and fuel for the formation of new stars.
Spiral galaxies and irregular galaxies still contain clouds of primordial gas and hence continue to forge new generations of stars.
The largest galaxies in the Universe are mostly elliptical. They have used up all the primordial gas and no longer produce stars. Nevertheless, their huge masses exert tremendous gravitational attraction on neighboring regions. This is precisely where the triplets investigated by Costa-Duarte and Sodré Jr. come in.
The algorithm identified 80 galaxy triplets from the Sloan Digital Sky Survey database, all between 550 million and 1.37 billion light years from Earth. Again, for comparison’s sake, Andromeda is a neighbor of the Milky Way and is substantially closer, at 2.5 million light years from Earth.
Light from the galaxy triplets analyzed was emitted between 1.37 billion and 550 million years ago, at a time when multicellular life (the first animals) was only just starting to appear in Earth’s oceans.
“Among the 80 triplets investigated, there could have been a system with three galaxies of equal mass. But that’s not what we observed,” Costa-Duarte said.
“The largest galaxies in each system are the brightest, have the most stars and therefore the greatest mass. They’re almost always giant ellipses. These galaxies have ‘retired’. They formed stars very rapidly and used up all the available hydrogen. They aren’t forming stars any more. They’re made up almost exclusively of old stars.”
The main galaxy’s two companions in each triplet can be elliptical, irregular, or dwarf elliptical.
The study has shown that the smaller galaxies undergo gravitational interactions, owing to the attraction of the more massive main galaxy. The principle whereby the main galaxy attracts its companions is the same as the mechanism that makes the Moon orbit the Earth and the Earth orbit the Sun.
This attraction is strong enough to force the gas clouds away from their neighbors, thus leading to the formation of new stars. As the gas clouds move, their denser regions tend to attract and accumulate increasing amounts of gas, thereby triggering a process that culminates in a burst of star formation capable of simultaneously giving birth to hundreds of thousands of stars.
“Triplets are simpler structures for understanding the main characteristics of galaxy formation,” Sodré Jr. said. The next step in their research project consists of comparing the results of their triplet analysis with what is observed in galactic groups with several dozen members. “These are much more complex cosmic entities,” he added.