The role of dark matter in the evolution of galaxies

The Big Deal About Dwarf Galaxies

  • The stellar stream in the halo of the nearby dwarf starburst galaxy NGC 4449 is resolved into its individual starry constituents in this exquisite image taken with the 8.2-meter Subaru Telescope and Suprime-Cam.

    Image credit: R. Jay GaBany and Aaron J. Romanowsky (UCSC) in collaboration with David Martinez-Delgado (MPIA) and NAOJ. Image processed by R. Jay GaBany
  • By: R. Jay GaBany

Dwarf galaxies shed light on dark matter

Every galaxy resides at the center of an enormous bubble of dark matter several times larger in diameter and many times greater in mass. Dwarf galaxies, the small cosmic Lego bricks that combined to form the galaxies we witness today, contain greater proportions of dark matter than large spirals. As a result, dwarf galaxies give astronomers a better opportunity to study dark matter and its effects.
  • NGC 4449 and its halo stream.

    Although the Cold Dark Matter Theory predicts the merger of dwarf galaxies, this image captures the first example to be studied in detail.
  • Photo credit: R. Jay GaBany

For most of human history, the night sky was thought to only contain the few thousand stars and handful of planets that could be seen with the naked eye. Then Galileo raised his telescope and discovered the Cosmos was far grander- sprinkled with new planets and showered with untold numbers of stars. For hundreds of years that followed, the heavens were assumed to be only be filled with the objects visible through a telescope. But the discovery of other wavelengths that reached beyond the limits of our vision revealed it was illuminated by radio waves, X-rays, infrared and ultraviolet radiation, too. From this we have uncovered ancient remnants of titanic stellar deaths, the existence of black holes at the center most galaxies, evidence from the birth of the Universe and much more.

So, there is more in the sky than our eyes can spy.

For example, until a few decades ago, galaxies were considered to only consist of stars, gas and dust. If you summed their masses, you could calculate the mass for any galaxy. But, the gravitation effect of the calculated mass was insufficient to explain the motion of its stars and other material obtained through direct observations. Apparently, galaxies contained more stuff than we could detect. We now call this missing mass dark matter.

You aren't alone if you find dark matter hard to believe. Early in the 20th century, astronomer Fritz Zwicky began to suspect the Universe must be filled with something invisible that was capable of tugging the members of galaxy clusters with its own gravity. Perhaps partially because he was a notorious science maverick, both brilliant and insufferable, his peers either ignored his ideas or considered them outlandish. Then during the nineteen-seventies, astronomer Vera Rubin noticed stars at the edge of galaxies traveled faster than expected and concluded their speed could only be explained by the presence of something massive but transparent.

Today, the existence of dark matter is firmly established within the scientific community. Although it remains mysterious and continues to elude direct detection, it correctly explains why galaxies spin without flying apart and how individual galaxies move within groups. In fact, galaxies have about ten times more dark matter as the material we can see. So, we live in a Universe in which every galaxy, including our own, is surrounded by a blob of dark matter that encases it like a bug suspended in clear amber.
  • The Magellanic Stream
    Spanning the sky behind the majestic Clouds of Magellan is an unusual stream of gas: the Magellanic Stream. Combined radio/optical image shows Milky Way, Magellanic Clouds, and the new radio image of the Magellanic Stream.
  • Photo credit: Nidever, et al., NRAO/AUI/NSF and Meilinger, Leiden-Argentine-Bonn Survey, Parkes Observatory, Westerbork Observatory, Arecibo Observatory

Dark matter may be made of a new subatomic particle, but even though there are many ideas about what those particles might be, no one has succeeded in identifying a dark matter particle either in outer space or in a laboratory. Regardless of its composition, dark matter throughout the Universe outweighs the stuff we can see by a factor of six.

Astrophysicists are convinced that dark matter came first in blobs of various sizes shortly after the event that gave birth to the Universe- the Big Bang. Those invisible masses then pulled in ordinary matter to make the galaxies. Not all galaxies were created equal, however. Some became giants while others remained diminutive.

The stature of dwarf galaxies

Miniature galaxies, called dwarfs, have even higher proportions of dark matter than larger galaxies because the motions of their stars cannot be fully explained by the sum of their stellar mass alone. Dwarf galaxies are also the most common type of galaxy throughout the Cosmos. They contribute significantly to the mass of the Universe, but very little is known about them partially because they are small and dimly lit.

The Milky Way galaxy is over 100 thousand light years in diameter and its mass is over a trillion times that of the Sun. Conversely, dwarf galaxies are only about one tenth the size of our home galaxy and most of them are smaller. They are typically found orbiting a larger parent galaxy, like a satellite, in elliptical or circular patterns similar to the way our planet travels around the Sun. So far, fourteen dwarf galaxies have been discovered circling the Milky Way. The Large and Small Magellanic Clouds are the two brightest and most massive examples.

According to the Cold Dark Matter (CDM) Theory, large galaxies grew and evolved over time in a hierarchical manner by assimilating small dwarf galaxies. Although the CDM theory predicts at least 100, and possibly as many as 500, dwarf galaxies should be in the Milky Way's neighborhood, only 38 have been found in the Local Group of galaxies to which we belong. This discrepancy is known as the "missing satellite problem" and it has puzzled astrophysicists for decades.

However, this dilemma has two possible solutions. One is that many dwarfs have a huge amount of dark matter but very few stars. This makes them inherently faint and difficult to detect. So, many dwarfs may have eluded discovery so far. For example, six dwarf satellites orbiting the Milky Way were recently discovered to be made of 99% dark matter and only 1% stars. The other solution may be that most of the missing dwarf galaxies have already been absorbed by larger galaxies.

Refugee and immigrant stars

Until recently, astronomers were convinced that virtually all stars formed within their resident galaxy. But today, we know the situation is more complicated.
  • Field of star streams
    A map of stars in the outer regions of the Milky Way Galaxy, derived from the SDSS images of the northern sky. Structures visible in this map include streams of stars.
  • Photo credit: V. Belokurov and the Sloan Digital Sky Survey

First, on rare occasions, massive galaxies collide and rip each other apart leaving trails of stars streaming into intergalactic space. These stars may eventually become assimilated into another nearby galaxy like war torn refugees.

Second, large galaxies have enormous gravitational fields that attract dwarf galaxies and eventually disrupt then assimilate their stellar constituents. This is predicted to happen with much greater frequency than large galactic collisions since dwarf satellite galaxies are small and have a harder time holding on to their mass. As a result, stars that once belonged to a dwarf satellite star system can immigrate into the outer halo of the larger parent galaxy they orbit and eventually become part of its surrounding starry shell.

Interestingly, since the early 20th century when telescopes became big enough to study galaxies in detail, astronomers have noticed most spiral galaxies exhibit distinctly different star populations. Bluer stars appear within the disk of the galaxy, whereas redder stars appear in a vast thin surrounding halo.

The star colors are significant. Spectroscopic analysis reveals the redder halo stars are composed of hydrogen and helium, while the bluer disk stars also contain heavier chemicals such as iron, carbon and oxygen. Astronomers refer to these elements as "metals". As a result, astronomers concluded that the Milky Way’s metal-poor halo stars must be the oldest in the galaxy, formed when the Universe was younger.

Dwarf galaxies, like the two dozen or so near the Milky Way, also have stars that are metal deficient. This has led astronomers to speculate that some of the stars in the Milky Way's halo came from long since destroyed dwarf galaxies that have been assimilated.

For example, the shredded remains of a dwarf galaxy was recently identified buried within our Milky Way galaxy as a massive stream of stars. The stellar stream was discovered by an international team located in Germany using a telescope situated in Australia by analyzing the movements of 12,000 stars throughout the Milky Way. During their review of the data, they noticed 15 stars moving at similar high speeds. These turned out to be part of a larger stream of stars originating from a small galaxy that the Milky Way had devoured about 700 million years ago.

Because most of the stars in the stream lay in the direction of the constellation of Aquarius, the group dubbed it the Aquarius Stream. This Stream covers an area of the sky about 1300 times the size of the full moon. Although they don't appear significant compared to other stars nearby, the fact they are moving together, in the same direction and at the same speed revealed they are a part of a stellar stream fossil.

Over 15 other star streams had been found in our galaxy and some astronomers speculate there may be close to 1,000 more waiting to be discovered.

However, not all stars in a dwarf galaxy become part of the halo surrounding their parent galaxy.
  • Sagittarius collision

    Time evolution of the Sagittarius satellite impact, from 2.65 billion years ago to the present-day. Note the emergence of spiral structure in the Milky Way disk as a response to the first disk crossing during infall.
  • Visualization credit: Erik Tollerud

Another team of scientists also believe they have uncovered how the Milky Way obtained some of its spiral structure. Their recent investigation studied one of the Milky Way's satellite galaxies named Sagittarius- a dwarf that was loaded with dark matter. Sagittarius has plowed through the Milky Way twice during the past two billion years and is on a collision course to do it again about 10 million years from now.

According to their computer simulations, when the dark matter in Sagittarius collided with the Milky Way, much of it was removed. Without the dark matter to hold the dwarf galaxy together, its visible stars began to be pulled apart by the Milky Way's huge gravitational field. Researchers found the collision initiated stellar density fluctuations within the disk of the rotating Milky Way. Since our galaxy rotates faster toward its center than at its edges, those instabilities were stretched and sheared, leading to the formation of spiral arms and other ring-like structures in the outskirts of our galaxy. In the process, the smaller galaxy was ripped apart with each encounter, releasing enormous amounts of its stars and dark matter into the new arms.

But, not all dwarfs orbit a spiral galaxy. Some remain independent and one was recently identified as having a star stream of its own.

NGC 4449, the unexpected parent

NGC 4449 is an small, irregular dwarf galaxy that's similar in size and composition to the Milky Way's Large Magellanic cloud. It's situated about 12.5 million light years from Earth and is a member of a group of galaxies located in the constellation Canes Venatici, the Hunting Dogs.

This dwarf star system contains thousands of hot blue stars and massive red regions, where new stars are being born, interwoven with darks thick strands of dust clouds. The prodigious amount of new star production indicates it's experiencing a star burst event where new stars are being produced at a dizzying rate- far greater than normally seen most galaxies.

Astronomers assumed the star burst was triggered by gravational interaction with neighboring galaxies within its group. However, new deep exposures produced with the Blackbird Observatory's half meter telescope over a series of nights spanning April 13 through June 10, 2010 and again between January 13 and January 28, 2011 along with high resolution images obtained through the 8.2 meter Subaru telescope/ Suprime Cam on Mauna Kea on January 5, 2011, has led an international team, spearheaded by researchers from the Max Planck Institute for Astronomy and the University of California Santa Cruz, to offer a different and unexpected explanation.

The new images reveal, for the first time, a stellar tidal stream and its individual starry constituents in the halo of NGC 4449 that represents the ongoing disruption of a smaller dwarf galaxy orbiting a larger dwarf. NGC 4449 is smallest parent galaxy where an ongoing merger has been identified and studied in detail.

Although the cold dark matter theory predicts mergers and interactions between dwarf galaxies, there is scant observational evidence that these types of mergers are still happening in the nearby local Universe. Interactions between dwarf galaxies invoke the possibility of exploring a very different merger regime. For example, research has shown that multiple dwarf galaxies with different stellar masses may exist in similar sized dark matter halos, hence what appears as a minor merger of stars could be a major dark matter merger. Studying interactions on a small scale, such as NGC 4449, provides unique insights on the role of stars versus dark matter in galactic merger events.
  • A star's eye view of the Subaru Telescope.
  • Photo credit: National Astronomical Observatory of Japan

It's also likely this interaction triggered the starburst in NGC 4449. The most interesting aspect of this finding is that it provides an example of interaction and accretion amongst dwarf galaxies. This is a topic that has not received widespread attention in scientific literature, but which may well be an important aspect of how galaxies assembled over time.

This may be quite similar to the interaction between the Large and Small Magellanic Clouds. New models for the gaseous Magellanic Stream suggest it may have been drawn from the Small Magellanic Cloud due to their interaction before the pair became part of the Local Group. So, both the newly discovered NGC 4449 Stream and the Magellanic Stream provide support for the hypothesis that dwarf-dwarf interactions can be a powerful force in the evolution of galaxies.

Shedding some final light

When we look on the face of a graceful spiral galaxy caught between the borders of a long exposure photograph, we not only see it as a snapshot caught in time, we now understand that we only see a very small portion of what's really there. The presence of dark matter, which makes up 95% of the mass of all galaxies including our own Milky Way, suggests the stars, planets, dust, gas and living creatures that call it home are somehow less significant. But, nothing could be farther from the truth.

Although dark matter is all pervasive and unlike anything we have discovered here on Earth, dark matter appears only to be a superstructure that holds everything together. It's the glue that binds the stars to the galaxy, the unseen rigging that supports the stage scenery, the mount that secures a gemstone to a finger ring and it's highly unlikely it will ever become something as subtle and curious as we ourselves.

We live in an age where science has become unfettered from examining the Universe with only our physical six senses. This has unlocked a profound new level of understanding, resolved ancient mysteries and unlatched a Pandora's chest filled with new questions begging for answers. We still have much to learn.

Also read:
- Dwarfs gobbling dwarfs: a tidal star stream around NGC 4449
- The Formation and Evolution of Galaxies
- A Burst of Starlight
- The Model Universe
- You Can't Believe Your Eyes!