Dark Matter Sheds New Light on our Understanding of Starburst Galaxies

A Burst of Star Light

  • The central region of NGC 253, also known as the Silver Dollar Galaxy, is experiencing a starburst event. Starburst galaxies produce hundreds or thousands of new stars each year whereas most other galaxies only produce a handful during the same period.
    Image credit: R. Jay GaBany
  • By: R. Jay GaBany

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Starburst galaxies during the early Universe

Gravity can stimulate galaxies to produce a burst of new stars at exceptionally high rates compared to the average stellar population created by most star systems. These events can occur when galaxies experience a close encounter or collide with one another. Sometimes they are provoked by gravitational density waves within the galaxy. Starbursts were also common in the early Universe and help shed light on how dark matter contributed to cosmic evolution.

For centuries, astronomy was principally focused on objects that could be observed directly with telescopes. Some of that changed in the 70's, when astronomer Vera Rubin, with the help of her associate Kent Ford, made one of the most important astronomical discoveries of the last half of the twentieth century. Her breakthrough revealed there's far more out there than was previously suspected and it sheds light on why many galaxies were stellar overachievers during their youth.
  • In the 1970s, astronomer Vera Rubin found evidence of a hypothetical type of invisible material now called dark matter.
  • Image credit: Princeton University

While measuring the speed of stars in the outskirts of the Andromeda galaxy, Rubin anticipated to find them orbiting more slowly than those located near the star system's central region. Since the middle of a spiral galaxy has the highest concentration of visible stars, astronomers assumed its mass and gravity would also be concentrated there. As a result, stars near the middle were expected to move faster than those located at the edge just as the inner planets of our solar system travel faster about the Sun than the outer ones.

Instead of confirming her expectations, Rubin's measurements indicated the stars moved at similar speeds throughout the galaxy, distance from the core had little effect on their velocity. When she subsequently measured stars in 200 other galaxies, she obtained similar results.

Interestingly, the Swiss astrophysicist Fritz Zwicky encountered a similar problem during the early 1930's when he noticed the speed of galaxies in clusters moved faster than predicted and speculated something that didn't absorb, emit or reflect light must hold them together. He called this missing stuff "dark matter" and estimated there was ten times more of it than the material we can see. Zwicky’s conclusion was correct even though his colleagues demurred.
  • Astronomer Fritz Zwicky first predicted the existence of dark matter in the 1930s following his observations of the Coma galaxy cluster.
  • Image credit: zwicky-stiftung.ch

Rubin recalled Zwicky's observations and realized the missing mass he predicted could also explain the unexpected star velocities she observed. Initially, many astronomers disbelieved her, too. However, her observations were repeatable and the explanation she offered was completely unambiguous so, within a few years, Rubin's findings became an established concept in modern astrophysics.

Cosmic glue

Simply put, the stars we observe are only the visible vestiges of a much larger spherical-shaped mass of dark matter that surrounds galaxies like an enormous halo. Dark matter is now assumed to comprise 22 percent of the Universe, vastly outweighing all the normal material we can detect with our instruments or see with our eyes. Most scientists also believe another 74 percent of the Universe is made of dark energy. This invisible force is said to be responsible for accelerating the Universe's expansion. That means only 4 percent of the universe is composed of observable stuff!

Nevertheless, most astronomers are convinced dark matter must be there because of the gravitational pull it seems to exert on everything else- in fact, it's probably in the room with you right now. As Rubin's observations discovered, without dark matter, galaxies would fly apart as they rotate. Dark matter is the glue that holds the Universe together.

These dark matter halos are now assumed to be the sites where galaxies were born. According to sophisticated computer models that simulate how the Universe evolved and ticks, tiny fluctuations occurred in the very early Universe shortly following the Big Bang. These perturbations grew under the influence of gravity and formed a complex network of dark matter sheets and filaments known as the cosmic web. Later, normal matter was drawn inside the densest knots of the web, aggregated into clumps where the first stars formed then later assembled into galaxies.

Since the cosmic web forms the skeleton that supports stars and galaxies, the distribution of galaxies is anticipated to trace the location of dark matter throughout the Universe. Therefore, all galaxies are assumed to reside within a dark matter halo. But, galaxies aren't created equal. Some are runts, some are goliaths while others burst with a frenzy that produces thousands of new suns every year!

Ancient stellar zealots
  • The Herschel infrared space observatory being prepared for launch.
  • Image credit: ESA

For well over a decade, astronomers have puzzled over starburst galaxies that filled the early Universe. They have been challenging to detect individually because of their small apparent size and the difficulty of observing them in the far infrared light they emit. To help answer these and other questions, ESA launched the Herschel infrared space observatory in 2009. Its 3.5 meter mirror is the largest lifted into space and its instruments are capable of studying faint starlight in far infrared wavelengths.

Using recent Herschel observations, researchers now believe the creation of early starburst galaxies was based on having a dark matter knot with correct proportions. For example, oversized knots allowed their hydrogen atoms to form multiple, serene galaxies whereas, at the other end of the scale, knots with too little mass formed stars so quickly their heat eventually stifled further new star creation. But, with just the right amount of dark matter, early galaxies became prolific stellar breeders.

But, dark matter isn't the only reason galaxies burst at their seams.

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