Using the Hubble Space Telescope, astronomers led by the Cosmic Dawn Center in Copenhagen investigated a galaxy that was seen almost 11 billion years ago.
Contrary to common observations, the galaxy was discovered not by the light it emits, but by the light it absorbs. The galaxy itself cannot escape observations, but it has at least one close companion. Together, these galaxies form an early group that may later evolve to look like the Local Group we live in.
When we see things, we see them because they emit light (like the sun or a flashlight) or because they reflect light emitted by someone else (like the moon or a bicycle).
This is usually how we find galaxies both near and far. Galaxies emit light across the electromagnetic spectrum, and different telescopes can detect different types of light.
But in fact, there is another way, a complementary way based on the ability of a galaxy to absorb light.
If a galaxy is located along the line of sight of a more distant bright light source, the galaxy will absorb some of the light from the background source. This absorption is caused by the gas and dust particles found between the stars in the galaxy. However, particles do not absorb all wavelengths equally well, but tend to absorb light at specific wavelengths.
If we take a spectrum (that is, an observation that shows how much light we see at each wavelength) of the background “lamp,” 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 the exact amount of light lost, we can determine many physical characteristics of the foreground galaxy.
The bright background source may initially be another galaxy or sometimes an exploding star, but more often it is a quasar; the extremely bright core of a galaxy with a supermassive black hole engulfing its surroundings.
The absorbed light reveals some, but not all, of the physical characteristics of the galaxy. If we want to know more about it, we can try to find the light emitted from the same region of the sky.
The problem? It is located exactly, or almost exactly, in front of the bright quasar. It’s almost like trying to see a firefly in front of a stadium projector.
“To find absorbing galaxies, we first look for especially red quasars,” explained Johan Fynbo, professor of Astronomy at the Cosmic Down Center, in a statement. “Because stardust absorbs blue light but not red light, if there is a dusty galaxy in front, the quasar will be red.”
This method led Fynbo and his colleagues to identify several sorbents of this class. The next step, and the most difficult, is to carefully find the light emitted by the galaxy that caused the absorption.
Recently, the team began to look for light from a particular absorber, which was seen almost 11 billion years ago in time, and was chosen because it causes enough reddening of a background quasar. This absorber is unique because it absorbs more light than most; a sign that it is a relatively mature galaxy, perhaps similar to the Milky Way. The paper has been accepted for publication in the journal Astronomy & Astrophysics and is currently available on the arXiv preprint server.
“The features we found in the missing light tell us about the dust in the foreground galaxy,” said Lise Christensen, associate professor at the Cosmic Dawn Center, who also participated in the study. “In fact, the dust looks similar to the dust we see locally in the Milky Way and in one of our neighboring galaxies.”
Unfortunately, despite their efforts, the team was unable to detect a luminous counterpart to the absorber. It is probably located almost in front of the quasar. On the other hand, they discovered another nearby galaxy, one that seems to have a high level of star formation. And there may be more out there.
Galaxies are so close to each other that they are bound by gravity and are not separated by the expansion of the universe. This means that, in the future, they will be a “galaxy group”, similar to our Local Group, consisting of the Milky Way, Andromeda and a large number of small satellite galaxies.