An international team of astronomers, led by researchers from the Cosmic Dawn Center in Copenhagen, investigated a galaxy seen about 11 billion years ago using the Hubble Space Telescope. Contrary to popular observation, the galaxy was discovered by the light it absorbed, not by the light it emitted. The galaxy itself cannot be observed, but it has at least one neighbor. 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 either because they emit light (like the sun or a flashlight) or because they reflect light emitted by something else (like the moon or a bicycle).

This is also how we often find nearby and distant galaxies. Galaxies emit light across the entire electromagnetic spectrum, and different telescopes can detect different types of light. But actually there is another way; An additional method based on the galaxy’s ability to absorb light.

Galaxies block light

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

If we then take a spectrum of the background “signal” (i.e., an observation of how much light we see at each wavelength), we see clear absorption “holes” in the spectrum, indicating that something is blocking the light.

Depending on the exact wavelengths at which we see the “holes” and how much light is missing, we can make inferences about different physical properties of the foreground galaxy. In principle, the source of the bright background can be another galaxy or sometimes an exploding star, but more often it is a quasar; An extremely bright galactic core with a supermassive black hole eating away at its surroundings.

A firefly in front of the stadium floodlight

The absorbed light reveals some, but not all, of the physical properties of the galaxy. If we want to learn more about this, we can try looking for light emitted from the same region of the sky.

Problem? It is located directly or almost directly in front of the bright quasar. It’s almost like watching a firefly in front of a stadium floodlight. Still, this is a topic on which Johan Fynbo, professor of astronomy at the Cosmic Dawn Center in Copenhagen, likes to challenge himself.

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

This approach led Finbo and colleagues to identify several such sinks. The next and most difficult step is to look carefully at the light emitted from the galaxy and causing absorption.

Twice as big as our local galaxy group?

The team recently set out to look for light from a particular absorber seen about 11 billion years ago, chosen because it caused a fairly significant reddening of the background quasar. This absorber is excellent in that it absorbs significantly more light than others; an indication that this is a fairly mature galaxy, perhaps similar to the Milky Way. The article has been accepted for publication in the journal Astronomy and Astrophysics and is currently available on the preprint server arXiv.

“The features we find in the missing light tell us about dust in the foreground of the galaxy,” says Lisa Christensen, an associate professor at the Cosmic Dawn Center who participated in the study. “In fact, the dust is similar to the dust we see locally in the Milky Way and one of our neighboring galaxies.”

Unfortunately, despite their best efforts, the team was unable to detect a glowing analogue of the absorber. It is most likely located almost directly in front of the quasar. On the other hand, they discovered another galaxy nearby that appears to have a high rate of star formation. And there could be more.

Galaxies are located so close together that they are constrained by gravity rather than being stretched by the expansion of the universe. This means that in the future they will form a “group of galaxies” similar to our Local Group, which consists of the Milky Way, Andromeda, and many smaller satellite galaxies.

“This makes studying galaxies even more interesting,” says Finbo, who plans to revisit galaxies in the future with the Northern Optical Telescope at La Palma and other telescopes, hoping to both find other members of the group and explore the galaxy. , which caused absorption. Source