Astronomers have found a mathematical principle that they say surprisingly fits all galaxies, from the stately spiral-shaped ones to the messy “train wrecks.” This could reveal something deep about the evolution of the universe, they claim.

In a new study using the W.M. Keck Observatory in Hawaii, the scientists found that this commonality has linked all galaxies for eight billion years, nearly half the age of the Universe.

All galaxies, they said, follow a consistent relationship between their mass, or weight, and the velocities of the stars and gas clouds that compose them. “We were truly surprised at how well” the pattern fits a dizzying array of galaxy types, said Sandra Faber of the University of California, Santa Cruz, co-author of the study.

The report is to appear in a forthcoming special issue of the research publication Astrophysical Journal Letters.

Galaxies fall into three basic types: spiral or disk-like ones such as our own Milky Way; those shaped like roundish clouds, known as elliptical galaxies; and messy, bashed-up or oddball galaxies. These are usually thought to be remnants of galaxy collisions, and sometimes dubbed “train wrecks.”


An image of colliding galaxies captured in 1999 using the Hubble Space Telescope. The objects are part of a large galaxy cluster designated MS1054-03. It is about eight billion light-years away, hence shows itself to us as it looked that number of years ago. (Courtesy Pieter van Dokkum, Marijn Franx [University of Groningen/Leiden], ESA and NASA)

Astronomers noted years ago that for spirals and ellipticals, there were specific relationships between their masses and the velocities of their constituent stars. These rules are called the Tully-Fisher and the Faber-Jackson relations, respectively.

What wasn’t known, Faber and colleagues said, was that these two relationships—which seemingly work differently—are themselves related, as aspects of one overarching rule. And this principle also applies to the “train wrecks,” not previously known to respect any such law.

According to Faber’s group, in all galaxies, there is some orderly, regular rotation: the constituent stars and gas clouds revolve together about a common center. For train-wreck galaxies, this neat rotation is overlaid with a certain amount of mixed-up velocities.

The researchers devised a new measure of the components’ total velocity, which they called a “speed indicator.” It combines both the orderly rotation velocity and the random or disordered motion. This property turns out to be strictly related to the mass of galactic components, said Susan Kassin, a postdoctoral researcher at the university and the study’s lead author. “Surprisingly, if you use this new speed indicator to measure the motions of stars and gas in a galaxy, you can predict the mass in stars the galaxy has with pretty high accuracy.”

Galaxies like our Milky Way consist of billions of stars formed into a spiral disk along with some gas. Our galaxy also spins like a pinwheel at a few hundred kilometers (miles) per second.

Such elegant galaxies were scarce once, the scientists said. Astronomers can observe the ancient universe by looking extremely far away, because this means the light we see left those areas billions of years ago. Half of the age of the universe ago, many galaxies look more disheveled, as they were being assembled through galaxy collisions and the piling on of new gas and stars, Kassin’s team said.

The new findings in essence show that the mixed-up and orderly velocities are somehow related, said Ben Weiner of the University of Arizona in Tucson, Ariz., another co-author of the study. “The mixed-up velocities may settle down to orderly rotation over time as the universe ages,” he proposed.

The findings probably reflect an even deeper property of the cosmos, said Faber, one of the namesakes of the Faber-Jackson relation, which she helped develop in 1976. “Both of these relations were imprinted by the nature of fluctuations [in the universe] that made galaxies in the first place,” she said.

“This relation holds for all the galaxies, no matter what they look like,” Kassin remarked. “It ties together the Faber-Jackson relation with the Tully-Fisher relation and works for all kinds of oddball galaxies that are more common in the early universe.” The study involved 544 far-off galaxies of various types. Kassin said that makes it the largest study to date of the speed and movement of distant galaxies’ stars and other matter.

ABOUT THE AUTHOR

Science writer Jack Lucentini is founder and editor of the World Science science news webzine. He has worked as a staff writer at three daily newspapers, and as a freelance science writer for a range of publications including The Washington Post, Discover magazine and The Scientist magazine. He earned his bachelor's degree at Oberlin College in Oberlin, Ohio in 1993.