A team of astronomers is investigating a cluster of galaxies in the early universe

Using the Hubble Space Telescope, an international team of astronomers led by researchers at the Cosmic Dawn Center in Copenhagen, studied a galaxy seen nearly 11 billion years ago. Contrary to typical observations, the galaxy is not discovered by the light it emits, but by the light it absorbs. The galaxy itself evades observations, but it has at least one nearby companion. Together, these galaxies form an early group that may later evolve to resemble the Local Group in which we live.

When we see objects, we see them because they either emit light – such as the sun or a flashlight – or because they reflect light emitted by someone else – such as the moon or a bicycle.

This is usually how we find galaxies near and far. Galaxies emit light across the full electromagnetic spectrum, and different telescopes can then detect different types of light.

But there is actually another way, a complementary way that relies on the galaxy’s ability to absorb light.

Galaxies block light

If the galaxy lies along the line of sight of a bright light source farther away, the galaxy will absorb some of the light from the background source. This absorption is caused by gas and dust particles found between the stars of the galaxy. However, particles do not absorb equally well at all wavelengths, but tend to absorb light at specific wavelengths.

If we then take a spectrum—that is, an observation showing how much light we see at each wavelength—of the background “beacon,” we see distinct absorption “holes” in the spectrum, indicating that something is blocking the light.

Depending on the exact wavelengths at which we see the ‘holes’, as well as exactly how much light is lost, we can then infer different physical properties of the foreground galaxy.

The source of the bright background may in principle be another galaxy, or sometimes an exploding star, but is often a quasar; The extremely luminous core of a galaxy with a supermassive black hole devouring its surroundings.

Firefly in front of a stadium projector

The absorbed light reveals many physical properties of the galaxy, but not all. If we want to know more about it, we can try looking for light emanating from the same area in the sky.

the problem? It was positioned exactly, or almost exactly, in front of the bright quasar. It’s almost like trying to observe a firefly in front of a projector in the stadium.

However, this is what Johan Finnbo, professor of astronomy at the Cosmic Dawn Center in Copenhagen, likes to challenge himself with.

“To find absorbing galaxies, we first look for quasars that are particularly red,” Finbo explains. “Because stardust tends to absorb blue light and not red, if there is a dusty galaxy in the foreground the quasar will turn red.”

This approach led Finbo and his collaborators to discover several such absorbers. The next, and more difficult, step is to carefully look for the light emanating from the galaxy that is causing the absorption.

Is it similar to the Local Group of galaxies?

Recently, the team took on the task of searching for light from a particular absorber, seen approximately 11 billion years ago, chosen because it causes a very significant reddening of the background quasar. This absorber is great in the sense that it absorbs much more light than most other materials; A sign that it is a fairly mature galaxy, perhaps similar to the Milky Way. The article has been accepted for publication in the journal Astronomy and astrophysics It is currently available on arXiv Advance print server.

“The features we found in the missing light tell us something about the dust in the foreground galaxy,” says Lise Christensen, an associate professor at the Cosmic Dawn Center who was also involved in the study. “In fact, the dust appears to be similar to the dust we see locally in the Milky Way and one of our neighboring galaxies.”

Unfortunately, despite their efforts, the team was unable to detect the absorber’s luminous counterpart. Most likely it is located exactly in front of the quasar. On the other hand, they discovered another galaxy nearby, one that appears to be actively forming stars. And there may be more.

The galaxies are located so close to each other that they are bound by gravity, and are not being pulled apart by the expansion of the universe. This means that in the future they will form a “galactic group”, not unlike our Local Group, which consists of the Milky Way, Andromeda and a large number of smaller satellite galaxies.

“This makes the galaxies more interesting,” says Finbo, who in the future plans to revisit the field using the Northern Optical Telescope at La Palma and other telescopes, with the goal of searching for other members of the group and, hopefully, uncovering the galaxy that gave rise to it. Absorption.