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Ancient Galaxies May Shed Light on Dark Matter

Using the combined power of numerous astronomical observatories in space and around the world, astronomers have discovered a treasure-trove of ancient massive galaxies that were not known before.

Ancient galaxies from the study are visible to ALMA (right) but not to Hubble (left). (Image credit: 2019 Wang et al.)

This multiple discovery is the first of its kind and an abundance of this form of galaxy challenges existing models of the universe. In addition, these galaxies are closely associated with the distribution of dark matter and also with giant black holes.

Although the Hubble Space Telescope provided unparalleled access to the formerly unseen universe, even this instrument cannot see some of the most underlying pieces of the cosmic puzzle.

At the University of Tokyo’s Institute of Astronomy, astronomers wanted to observe certain things which they believe may be out there but were not revealed by the Hubble Space Telescope. The latest generations of astronomical observatories have eventually exposed what they were searching for.

This is the first time that such a large population of massive galaxies was confirmed during the first 2 billion years of the 13.7-billion-year life of the universe. These were previously invisible to us. This finding contravenes current models for that period of cosmic evolution and will help to add some details, which have been missing until now.

Tao Wang, Researcher, The University of Tokyo

But How can Something as Big as a Galaxy be Invisible to Begin With?

The light from these galaxies is very faint with long wavelengths invisible to our eyes and undetectable by Hubble,” Professor Kotaro Kohno explained. “So we turned to the Atacama Large Millimeter/submillimeter Array (ALMA), which is ideal for viewing these kinds of things. I have a long history with that facility and so knew it would deliver good results.”

Although these galaxies were considered the largest of their time, the light emitted by them is weak and also stretched because of their significant distance. When the universe expands, the light traveling through it becomes stretched, and hence visible light turns out to be longer, ultimately becoming infrared.

The extent of stretching enables astronomers to compute the distance of an object that is located far away. This also reveals the time the light—that is being seen—was produced from the object in question.

It was tough to convince our peers these galaxies were as old as we suspected them to be. Our initial suspicions about their existence came from the Spitzer Space Telescope’s infrared data,” continued Wang. “But ALMA has sharp eyes and revealed details at submillimeter wavelengths, the best wavelength to peer through dust present in the early universe."

Wang added, "even so, it took further data from the imaginatively named Very Large Telescope in Chile to really prove we were seeing ancient massive galaxies where none had been seen before.”

There is another reason why these galaxies appear to be extremely weak: larger galaxies, even in the current day, are covered in dust, which conceals them more than their tinier galactic siblings.

And What Does the Discovery of These Massive Galaxies Imply?

The more massive a galaxy, the more massive the supermassive black hole at its heart. So the study of these galaxies and their evolution will tell us more about the evolution of supermassive black holes, too. Massive galaxies are also intimately connected with the distribution of invisible dark matter. This plays a role in shaping the structure and distribution of galaxies. Theoretical researchers will need to update their theories now.

Kotaro Kohno, Professor, The University of Tokyo

What is also fascinating is how these 39 galaxies are different from the Milky Way. If the solar system was located within one of these galaxies and individuals happen to look up at the sky on a clear night, they would observe something very different to the well-known pattern of the Milky Way.

For one thing, the night sky would appear far more majestic. The greater density of stars means there would be many more stars close by appearing larger and brighter,” Wang explained. “But conversely, the large amount of dust means farther-away stars would be far less visible, so the background to these bright close stars might be a vast dark void.”

Since this is the first time such a population of galaxies has been identified, the implications of their analysis are being realized only now. However, many surprises are expected to come.

These gargantuan galaxies are invisible in optical wavelengths so it’s extremely hard to do spectroscopy, a way to investigate stellar populations and chemical composition of galaxies. ALMA is not good at this and we need something more. I’m eager for upcoming observatories like the space-based James Webb Space Telescope to show us what these primordial beasts are really made of.

Tao Wang, Researcher, The University of Tokyo


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