Scientists use rare 'Einstein Cross' to learn about young galaxy with surprisingly old stars

Astronomers have discovered a rare "Einstein Cross" gravitational lens, revealing a young galaxy with shockingly mature stars.

The galaxy in question is J1453g, an elliptical galaxy that is the first gravitational lens at a large cosmic distance for which astronomers have been able to precisely "weigh." J1453g lenses the light from a more distant quasar, a region of space dominated by a ravenously feeding supermassive black hole, magnifying it and causing it to appear multiple times in the same image in the shape of a cross.

The galaxy is seen as it was around 8 billion years ago, when the universe was less than 6 billion years old. However, though it is a "primordial galaxy" in its early stages of development, J1453g is surprisingly similar to the Milky Way, our "mature" home galaxy. What this shows us is that the growth and evolution of galaxies could be much more complex than previously theorized.

"The discovery of this exceptional object has allowed us to accurately study the nature of the stars at the center of an elliptical galaxy in a remote era of the universe, when the galaxy was still young," team leader Quirino D'Amato, a researcher at the Italian National Institute for Astrophysics (INAF), said in a statement. "The fact that their composition is very similar to what we see today in the Milky Way, in a completely different environment and era, is surprising.

"This tells us that we are still far from fully understanding the processes of galaxy formation and evolution, and represents an important point for the development of future models."

What is gravitational lensing?

This research wouldn't have been possible with a quirk of the cosmos first posited by Albert Einstein in his 1915 magnum opus theory of gravity, general relativity.

General relativity suggests objects with mass give rise to a curvature in the very fabric of space and time, united as a four-dimensional entity called "spacetime." The bigger the mass of an object, the greater the curvature it generates, and we experience these warps in spacetime as gravity. Thus, the greater mass an object has, the greater its gravitational influence.

And when light passes through warps in spacetime, something fascinating happens. The usually straight path of light gets curved along the warp, with the degree of curvature dictated by how close to the object of mass the light passes.

That means when an object of great mass comes between Earth and a more distant object, light from that background object can arrive at our telescopes at different times. These intervening bodies can cause background objects to be magnified, or "gravitationally lensed." Indeed, this phenomenon is used to great effect by the James Webb Space Telescope (JWST) to see ancient and distant galaxies.

Every so often, the difference in arrival time can cause a background object to appear multiple times in the same image, too. These multiple manifestations of the same background body can take circular arrangements, or Einstein Rings, and can also appear as rarer Einstein Crosses.

In the case of this Einstein Cross, the gravitational lens is the galaxy J1453g in near-perfect alignment with Earth and a distant quasar, the active region at the heart of the galaxy, which is powered by a feeding supermassive black hole.

Gravitational lensing isn't just useful for seeing objects ordinarily way beyond our view; the lensing effect can tell scientists a great deal about the body doing the lensing as well. In this case, the team was able to use the cross-shaped manifestations of this quasar to determine the mass distribution of the stars J1453g to an unprecedented level of precision. That revealed something that defies what current models suggest.

Scientists usually expect the central bulges of elliptical galaxies to form rapidly and thus be dominated by low-mass stars. However, it appears that J1453g has a configuration like that of the Milky Way, which is a barred spiral galaxy, meaning some elliptical galaxies may form more slowly with higher mass stars at their hearts. Another possibility is that J1453g was transformed in its early history by a violent incident, such as a collision and merger with another galaxy.

As such, the team's results not only represent one of the most robust measurements of star birth in the adolescence of the universe, but also represent a new window on the formation and evolution of massive cosmic structures. Indeed, the research suggests a more dynamic and complex history for galaxies than was previously thought possible.

The team's research was published on Thursday (April 2) in the journal Nature Astronomy.