Can Faint Satellite Galaxies Solve Dark Matter Mystery?

We may have 150 faint satellite galaxies orbiting the Milky Way. Find out how this could help explain the mystery of dark matter in the universe.

When I bought my first serious quality telescope, I started out looking at the planets. I was able to spot Jupiter and its moons, Saturn and its rings and Venus with its phases. As I got a little more adventurous, I started taking an interest in deep-sky objects.

These included the Andromeda Galaxy, the Orion Nebula and the Hercules Galaxy Cluster. To be honest, they don’t look like much in a small, amateur telescope like mine. 

The satisfaction came from finding these faint, cloudy nebulae on my own, and from the awe-inspiring realization that light from another galaxy had travelled across millions of light-years of space and then ended up in my little eye. It’s part of my ongoing process of understanding the story of our universe and my place within it.

THE STORY OF THE UNIVERSE AND MY PLACE WITHIN IT

Probably the first astronomer to feel that sense of awe was the 18th-century French astronomer Charles Messier. In those days, Halley’s Comet had made its regular visit, which vindicated Newton’s laws of gravity.

Comet hunting became a big fad after that, and Messier became part of the craze. He came across a lot of cloudy objects that looked a bit like comets but didn’t move.

He called them “nebulae,” and he decided to make a catalogue of them to help people distinguish them from the comets they were after. Ironically, his inventory is far more valuable today than any comets he managed to discover.

MESSIER CATALOG FAR MORE VALUABLE THAN COMETS

The Messier Catalog is a comprehensive, but not exhaustive, list of the nebulous objects in the night sky for the northern hemisphere. Each object has an alphanumeric code, starting with M1 and ending with M110.

Some of the Messier objects are clouds of dust and gas, and others are galaxies like our own Milky Way. Edwin Hubble’s interest in the nebula M31, the Andromeda Galaxy, led him to discover that there were galaxies outside of the Milky Way and that they were all moving away from each other. Hubble became the father of modern cosmology.

Some smaller dwarf galaxies, such as the Large and Small Magellanic Clouds, orbit our Milky Way galaxy the same way the planets go around the sun. We discussed the Large Magellanic Cloud (LMC) a couple of weeks ago in our story Massive Stars: Can They Form on Their Own? 

SMALLER DWARF GALAXIES ORBIT OUR MILKY WAY

There’s a new study forthcoming in the journal Astrophysical Letters. It’s from the US Department of Energy’s SLAC National Accelerator Laboratory and the Dark Energy Survey.

The researchers predict that over 150 faint satellite galaxies orbit the Milky Way that we have yet to discover. They also found evidence that the LMC has faint dwarf satellite galaxies of its own.

Astronomers used to think that there were only about a dozen satellite galaxies orbiting the Milky Way. That number jumped to about 60 based on data from the Dark Energy Survey, the Sloan Digital Sky Survey and other sources.

IMPLICATIONS FOR OUR UNDERSTANDING OF DARK MATTER

That’s mind-blowing in itself. Its implications for our understanding of dark matter are even more intriguing.

Scientists can estimate the mass of galaxies in two ways. They can watch how galaxies behave, and they can inventory all the observable objects in a galaxy, assess their masses and add them all up.

The problem is that the results from the two techniques don’t match. We wouldn’t expect the two estimates to get precisely the same answer, but one of these estimates is about ten times larger than the other one. 

NINETY PERCENT OF A GALAXY’S MASS COMES FROM DARK MATTER

That’s led scientists to believe that about 90% of a galaxy’s mass comes from some mysterious, invisible missing mass that they call dark matter. Dark matter isn’t stars and planets, it doesn’t consist of baryons, like protons and neutrons, and it isn’t anti-matter. Yet, it’s there.

The research team used high-powered computer simulations of the behaviour of millions of possible universes.  These models showed the formation of the dark matter structure in our galaxy.

A second modelling exercise determined which galaxies are most likely to be visible to us. They based this on their location, brightness, size and distance.

LARGE MAGELLANIC CLOUD HAS SIX FAINT SATELLITE GALAXIES

The first thing the model showed was that the LMC should have six faint galaxies in orbit around it.  It also showed that the LMC first approached the Milky Way Galaxy about 2.2 billion years ago.

These predictions confirm estimates made in separate studies and observations. That gave the researchers confidence in the model’s accuracy.

They then told the computer model to set limits on dark matter halos and galaxy structures. They narrowed down the simulations to those similar to the history of the Milky Way and the LMC.

FAINTEST GALAXIES WE CAN OBSERVE HAVE ABOUT 100 SUNS

They found that the faintest galaxies that astronomers could observe have about 100 stars in them. The faintest galaxies we could ever see could form in halos that were 100 times less massive than these.

That means that there are at least twice as many ultra-faint satellite galaxies rotating around the Milky Way than scientists realize. If we could confirm that they were really out there, it would validate the team’s model on the relationship between dark matter and how galaxies form.

Confirmation of these faint satellite galaxies would probably tell us more about what dark matter is. That would be a landmark breakthrough in science because right now, cosmologists know more about what dark matter isn’t than what dark matter is.

SCORES OF FAINT SATELLITE GALAXIES ORBIT OUR MILKY WAY

It’s captivating to visualize scores of faint satellite galaxies floating in orbit around our Milky Way. Even more tantalizing is the possibility that they could teach what we’re missing in our understanding of matter in our universe.

Without a better grasp of dark matter, we can’t tell each other the story of how our galaxies formed. Their strange, self-organizing properties are still a mystery to us.

Risa Wechsler is the director of the Kavli Institute for Particle Astrophysics and Cosmology. She explained the new opportunity this way, “For the first time, we can look for these satellite galaxies across about three-quarters of the sky, and that’s really important to several different ways of learning about dark matter and galaxy formation.”

We always have more to learn if we dare to know.

Learn more:

SLAC National Accelerator Laboratory
Milky Way Satellite Census — II. Galaxy-Halo Connection Constraints Including the Impact of the Large Magellanic Cloud
Newborn Stars Bring Forth Solar Systems
Missing Matter Missing in 19 Dwarf Galaxies
Unfolding the Milky Way

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