A team of astronomers is investigating a group 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, has investigated a galaxy that was seen about 11 billion years ago. Contrary to what is commonly observed, the galaxy is not found by the light it produces, but by the light it absorbs. The galaxy itself eludes detection, but it has one nearby companion. Together, these galaxies comprise a primordial group that may later evolve to resemble the Local Group we live in.

When we see things, we see them because they emit light—like the sun or a light bulb—or because of light emitted by someone else—like the moon or a bicycle.

This is also how we find galaxies near and far. The galaxy emits light across the entire electromagnetic spectrum, and different telescopes can see different types of light.

But in fact there is another way, a parallel way that depends on the power of light absorption.

The galaxies that block the light

If a galaxy is located along the line of sight to a more distant, bright light source, the galaxy will absorb some of the background light. This absorption is caused by the gas and dust particles between the stars of the galaxy. Particles do not absorb all wavelengths equally, however, but tend to absorb light at certain wavelengths.

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

Depending on the exact height at which we see the “holes,” and how much light is lost, we can detect various features of the foreground galaxy.

The bright background source can in principle be another galaxy, or sometimes an exploding star, but is usually a quasar; the bright core of a galaxy with a supermassive black hole eating away at it.

Fireflies in front of the stadium projector

Absorbed light reveals most of the visual properties of an object, but not everything. If we want to know more about it, we can try to look for light from this region of the sky.

The problem? It is positioned precisely, or almost precisely, in front of a bright quasar. It’s like trying to look at a star in front of a stadium.

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

Fynbo says: “In order to find absorbing galaxies, we first look for particularly red quasars. “Because interstellar dust absorbs blue light but not red, if there is a dusty galaxy in front, the quasar will be red.”

This approach has led Fynbo and his colleagues to discover many such absorption properties. The next, and difficult, step is to carefully look for the light emitted from the galaxy causing the absorption.

Is it a doppelgänger of our local galaxy?

Recently, the team undertook a mission to search for light from a particular absorber, which was observed about 11 billion years ago, and was chosen because it causes the reddening of the background quasar. This absorber is amazing in the sense that it absorbs more light than most; a sign that it is a mature galaxy, perhaps similar to the Celestial Galaxy. The article has been accepted for publication in the journal Astronomy and Astrophysics and is now available on arXiv pre-print server.

“The features we found in the stray light tell us something about the dust in the galaxy,” says Lise Christensen, an assistant professor at the Cosmic Dawn Center who also participated in the research. “In fact, the dust appears to be similar to the dust we see locally in the Milky Way and some of the neighboring galaxies.”

Alas, despite their efforts the team was unable to spot the absorber’s luminous companion. It is likely to be located almost directly in front of the quasar. On the other hand, they discovered another galaxy nearby—a galaxy that appears to be forming many stars. And more than that.

The galaxies are so close together that they are bound by gravity, not pulled by the expansion of the universe. This means that, in the future, they will form a “galaxy group,” unlike our own Milky Way, which includes the Milky Way, Andromeda, and a large number of small satellite galaxies.

“This makes the galaxies more interesting to study,” says Fynbo, who in the future plans to revisit the region with the Nordic Optical Telescope at La Palma and other telescopes—both to search for other members of the group, and in the hope of uncovering the galaxy causing the absorption.