An international team of astronomers lead by scientists at the Cosmic Dawn
Center in Copenhagen have examined a galaxy observed about 11 billion years
ago using the Hubble Space Telescope. In contrast to common observations,
the galaxy was found by the light it absorbs rather than the light it emits.
Although the galaxy itself is evasive, it has at least one partner in close
proximity. These galaxies collectively make up an early group that might
eventually change to resemble our own Local Group.
Things that emit light, like the sun or a flashlight, or reflect light
generated by others, like the moon or a bicycle, are the reasons we see
them.
This is also often how galaxies, both nearby and far away, are located. All
of the electromagnetic spectrum is emitted by galaxies, and various
telescopes can identify various types of light from them.
However, there is also an alternative approach, a supplementary technique
that depends on a galaxy's capacity to take in light.
Light-blocking galaxies
A galaxy will absorb part of the light from a background source if it
happens to be situated in the line of sight to a more distant, brilliant
light source. The gas and dust particles that are in between the stars in
the galaxy are what are causing this absorption. However, the particles
prefer to absorb light at particular wavelengths rather than equally
effectively at all wavelengths.
Then, if we take a spectrum of the background "lighthouse," which is an
observation that reveals how much light we see at each wavelength, we can
clearly observe absorption "holes" in the spectrum, which suggests that
something is obstructing the light.
We may then infer different physical properties of the foreground galaxy
based on the precise wavelengths at which we observe the "holes," and the
precise amount of light missing.
The bright background source is often a quasar, which is the incredibly
brilliant centre of a galaxy with a supermassive black hole devouring its
environs. It may also theoretically be another galaxy or sometimes an
exploding star.
A firefly in front of the projector in the stadium
A galaxy's physical properties are partially revealed by the absorbed
light, but not entirely. We might try looking for light coming from the same
area of the sky to learn more about it.
The issue? It is positioned directly—or almost directly—in front of the
brilliant quasar. It is like to attempting to watch a firefly via a stadium
projector.
Johan Fynbo, an astronomy professor at Copenhagen's Cosmic Dawn Center,
loves to challenge himself with that, though.
"We first search for particularly red quasars in order to find absorbing
galaxies," says Fynbo. "If there is a dusty galaxy in the foreground, the
quasar will be reddened because star dust tends to absorb the blue light but
not the red."
Using this method, Fynbo and his associates have found several of these
absorbers. The next, and most difficult, step is to search closely for light
that the galaxy is emitting that is the source of the absorption.
A duplicate of our local galaxy cluster?
The team's most recent endeavor was searching for light from a specific
absorber that was chosen because it significantly reddens a background
quasar and was seen about 11 billion years ago. This absorber is unique
since it is far more light-absorbing than most others, indicating that it is
a rather mature galaxy, maybe along the lines of the Milky Way. The work is
presently accessible on the arXiv preprint service and has been accepted for
publication in the journal Astronomy & Astrophysics.
"The characteristics we discovered in the absent light provide insight into
the dust in the front-runner galaxy," explains Lise Christensen, an
associate professor at the Cosmic Dawn Center and study participant. "The
dust actually looks a lot like the dust that we see in the Milky Way and one
of our nearby galaxies."
Unfortunately, despite their best efforts, the crew was unable to find the
absorber's bright equivalent. It is most likely situated nearly directly in
front of the quasar. However, they also found another neighboring galaxy
that appears to be actively star-forming. There could even be more.
Because the galaxies are so near to one another, gravitational force holds
them together rather than the universe's expansion pulling them away. This
indicates that they will eventually develop into a "galaxy group," similar
to our own Local Group, which is made up of Andromeda, the Milky Way, and
several smaller satellite galaxies.
"This makes the galaxies even more interesting to study," says Fynbo, who
intends to return to the field in the future using various telescopes,
including the Nordic Optical Telescope at La Palma, in order to look for
additional members of the group and, ideally, identify the galaxy that is
causing the absorption.
Provided by
Niels Bohr Institute
