��˾��ֱ��, Yale astronomers discover 'see-through' galaxy with almost no dark matter

A Hubble Space Telescope image of the galaxy NGC1052-DF2. Distant galaxies are visible through DF2 due to its lack of stars and “ghostly” nature (image by Pieter van Dokkum; Robert Abraham; STScI, Space Telescope Science Institute)

Published: March 28, 2018

By Chris Sasaki

A team of astronomers has discovered a galaxy – the first of its kind – that appears to contain virtually no dark matter. It is an exceptional find since galaxies are commonly thought to contain more dark matter than the ordinary matter that makes up a galaxy’s stars, gas and dust.

Also, it is generally accepted that galaxies first formed from concentrations of dark matter that act like “galaxy starters.” They gravitationally attract ordinary, or baryonic, matter that eventually settles within the extant cloud of dark matter.

Finding a galaxy with no dark matter raises the question: If the galaxy has no dark matter, how did it form?

“We thought all galaxies were made up of stars, gas and dark matter mixed together, but with dark matter always dominating,” says , a professor in the University of Toronto's department of astronomy and astrophysics, and co-author of the Nature paper describing the discovery. “Now it seems that at least some galaxies exist with lots of stars and gas and hardly any dark matter. It is pretty bizarre.”

According to the paper’s lead author, Pieter van Dokkum, a professor of astronomy and physics at Yale University, “It challenges the standard ideas of how we think galaxies work. This result also suggests that there may be more than one way to form a galaxy.”

The paper appears in the March 29 issue of the journal Nature.

��˾��ֱ�� Professor Robert Abraham (left) and Yale Professor Pieter van Dokkum (right), with their team of graduate students from both universities. They are standing with one-half of the 48-lens Dragonfly array at its home site in New Mexico

The newly discovered galaxy is called NGC1052-DF2, or DF2 for short. In addition to the dearth of dark matter, DF2 is unusual in another way: It is roughly the size of our Milky Way Galaxy, but contains only 1/200 of the number of stars.

It was first identified as a peculiar object using the , a groundbreaking instrument for detecting very faint astronomical objects, conceived of and built by van Dokkum and Abraham.

Dragonfly is a multi-lens array that uses commercially available telephoto lenses with specially coated optical glass that reduces scattered light. Having grown from a three-lens array in 2013, it now has 48 lenses and is the world’s largest, all-refracting telescope. Previously, van Dokkum, Abraham and their collaborators used the array to discover a new class of galaxies known as Ultra-Diffuse Galaxies (UDG).

Once the team identified the unusual nature of DF2, they conducted followup observations. From the W.M. Keck Observatory, they measured the velocity of clusters of stars (called globular clusters) within DF2 and found they were moving slower than expected. Using these velocities, they calculated the galaxy’s mass and determined that the visible stars, gas and dust in DF2 accounted for most of the mass and that there was only 1/400^th the amount of dark matter expected.

Followup observations with the Hubble Space Telescope revealed DF2’s other unusual qualities. Unlike typical spiral galaxies, DF2 has no dense, central region, nor spiral arms or a disk. And unlike elliptical galaxies, it has no central black hole.

“I spent an hour just staring at the Hubble image,” says van Dokkum. “This thing is astonishing: a gigantic blob that you can look through. It’s so sparse that you see all of the galaxies behind it. It is literally a see-through galaxy.”

DF2 is located in a cluster of galaxies that is dominated by a giant elliptical galaxy designated NGC 1052. The astronomers speculate that the birth and formation of DF2 in the dynamic environment of the cluster could have been influenced by the giant galaxy. Or, a cataclysmic event within DF2 such as an eruption of star formation could have cleared the galaxy of all its gas and dark matter.

“We’re currently undertaking a survey to find more objects like DF2,” says Abraham. “Dragonfly is good at finding these sorts of objects. Maybe we’re only seeing the tip of the iceberg.”

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