Cluster Filled with Dark Matter May House 'Failed Galaxies'

Excerpt space.com




A strange set of 48 galaxies appears to be rich in dark matter and

lacking in stars, suggesting that they may be so-called “failed”

galaxies, a new study reports.

The galaxies

in question are part of the Coma Cluster, which lies 300 million

light-years from Earth and packs several thousand galaxies into a space

just 20 million light-years across. To study them, Pieter van Dokkum of

Yale University and his colleagues used the Dragonfly Telephoto Array in

New Mexico.

The array’s eight connected Canon telephoto lenses allow the

researchers to search for extremely faint objects that traditional

telescope surveys miss. Often, such as when the researchers used the

array to search for the faint glow that dark matter might create, the

hunt comes up empty. 

But when van Dokkum and his colleagues looked toward the Coma Cluster, they found a pleasant surprise.

“We noticed all these faint little smudges in the images from the Dragonfly telescope,” van Dokkum told Space.com.

The mysterious blobs nagged at van Dokkum, compelling him to look into the objects further. Fortuitously, NASA’s

Hubble Space Telescope had recently captured one of these objects with

its sharp eye. 

“It turned out that they’re these fuzzy blobs that look somewhat like dwarf spheroidal galaxies around our own Milky Way,”

van Dokkum said. “So they looked familiar in some sense … except that

if they are at the distance of the Coma Cluster, they must be really

huge.”

And with very few stars to account for the mass in these galaxies, they must contain huge amounts of dark matter,

the researchers said. In fact, to stay intact, the 48 galaxies must

contain 98 percent dark matter and just 2 percent “normal” matter that

we can see. The fraction of dark matter in the universe as a whole is

thought to be around 83 percent. 

But before making this claim, the team had to verify that these blobs

really are as distant as the Coma Cluster. (In fact, the team initially

thought the galaxies were much closer.). But even in the Hubble image

the stars were not resolved. If Hubble — one of the most powerful

telescopes in existence — can’t resolve the stars, those pinpricks of

light must be pretty far away, study team members reasoned. 

Now, van Dokkum and his colleagues have definitive evidence: They’ve

determined the exact distance to one of the galaxies. The team used the

Keck Telescope in Hawaii to look at one of the objects for two hours.

This gave them a hazy spectrum, from which they were able to tease out

the galaxy’s recessional velocity — that is, how fast it is moving away

from Earth.

That measure traces back to the Hubble Telescope’s namesake. In 1929, American astronomer Edwin Hubble

discovered one of the simplest and most surprising relationships in

astronomy: The more distant a galaxy, the faster it moves away from the

Milky Way.

Today, astronomers use the relationship to measure a galaxy’s

recessional velocity and thus calculate the galaxy’s distance. In this

case, the small fuzzy blob observed with Keck was moving away from Earth

at 15.7 million mph (25.3 million km/h). That places it at 300 million

light-years away from Earth, the distance of the Coma Cluster.

So the verdict is officially in: These galaxies must be associated with

the Coma Cluster and therefore must be extremely massive.



“It looks like the universe is able to make unexpected galaxies,” van

Dokkum said, adding that there is an amazing diversity of massive

galaxies.

But the clusters still present a mystery: The team doesn’t know why they have so much dark matter and so few stars.

One possibility is that these are “failed” galaxies. A galaxy’s first supernova explosions

will drive away huge amounts of gas. 

Normally, the galaxy has such a

strong gravitational pull that most of the expelled gas falls back onto

the galaxy and forms the next generations of stars. But maybe the strong

gravitational pull of the other galaxies in the Coma Cluster interfered

with this process, pulling the gas away.

“If that happened, they had no more fuel for star formation and they

were sort of stillborn galaxies where they started to get going but then

failed to really build up a lot of stars,” said van Dokkum, adding that

this is the most likely scenario. 

Another possibility is that these galaxies are in the process of being

ripped apart. But astronomers expect that if this were the case, the

galaxies would be distorted and streams of stars would be flowing away

from them. Because these effects don’t appear, this scenario is very

unlikely.

The next step is to try to measure the individual motions of stars

within the galaxies. If the team knew those stars’ speeds, it could

calculate the galaxies’ exact mass, and therefore the amount of dark

matter they contain. If the stars move faster, the galaxy is more

massive. And if they move slower, the galaxy is less massive. 
However,

this would require a better spectrum than the one the team has right

now.

“But it’s not outside the realm of what’s possible,” van Dokkum assured. “It’s just very hard.”

The original study has been published in Astrophysical Journal Letters. You can read it for free on the preprint site arXiv.org.

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Cluster Filled with Dark Matter May House 'Failed Galaxies'