Newly Discovered Ultra-Diffuse Galaxies as Large as the Milky Way but with Only 1% of the Stars

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Comparing data from the Dragonfly Telephoto Array (which uses 14 centimetre state-of-the-art telephoto camera lens) and the celebrated 10m Keck I private telescope at the W. M. Observatory on Mauna Kea in Hawaii – both directed at the Coma Galactic Cluster, located over 300 light-years away from the Earth – astronomers have discovered 47 so-called ultra-diffuse (or “wispy”) galaxies that are almost as wide as the Milky Way (about 60.000 light-years across), yet harbour only 1% of the stars.

Astronomers have found a new type of galaxies, more similar to wisps of clouds than to a sea of stars that are near the size of our own native Milky Way, but contain only around 1% of the stars. Image credit: ESA/Hubble & NASA via spacetelescope.org, CC BY 3.0.

“If the Milky Way is a sea of stars, then these newly discovered galaxies are like wisps of clouds,” said Yale University astronomer and study lead author Pieter van Dokkum. “We are beginning to form some ideas about how they were born and it’s remarkable they have survived at all. They are found in a dense, violent region of space filled with dark matter and galaxies whizzing around, so we think they must be cloaked in their own invisible dark matter “shields” that are protecting them from this intergalactic assault.”
The discovery was made by separating the light from one of the objects (an ultra-diffuse galaxy called DragonFly44) – seen in the digital images produced by the DTA – into colors, which helped diagnose its composition, age and distance. Ultra-diffuse galaxies have the same number of stars as dwarf elliptical galaxies, but are spread out over a much larger region. The findings of the study were recently published in the Astrophysical Journal Letters.
“If there are any aliens living on a planet in an ultra-diffuse galaxy, they would have no band of light across the sky, like our own Milky Way, to tell them they were living in a galaxy. The night sky would be much emptier of stars,” noted team member Aaron Romanowsky of the San Jose State University.
Given what we know about galactic formation, these new galaxies, seen in the constellation Coma Berenices (situated near Leo), simply shouldn’t exist, prompting astronomers to revise some of their hypotheses.
“The big challenge now is to figure out where these mysterious objects came from. Are they “failed galaxies” that started off well and then ran out of gas? Were they once normal galaxies that got knocked around so much inside the Coma Cluster that they puffed up? Or are they bits of galaxies that were pulled off and then got lost in space?” speculated University of Toronto astronomer Roberto Abraham.
To answer these questions and better understand the new type of galaxies, the researchers will now have to pin down exactly how much dark matter they contain – a measurement that no doubt will prove to be even more complicated than the ones involved in the present work.