Astronomers solve mystery of how spiral galaxies get their arms

Delila James | Science Recorder | April 02, 2013

Astronomers solve mystery of how spiral galaxies get their arms

Astronomers discover why some galaxies are spiral-shaped.

Using sophisticated computer simulations, a team of researcher say they one step closer to understanding how we came to be. The study, published earlier this month, provides some surprising answers to long-standing questions about the origin and history of spiral galaxies.

Scientists from the University of Wisconsin-Madison and the Harvard-Smithsonian Center for Astrophysics say for the first time they can show that stellar spiral arms are not transient features, as many have long believed, but rather are “self-perpetuating, persistent and surprisingly long-lived.”

The mystery of how spiral galaxies have evolved over billions of years have long been a question without an answer to those in astrophysics. About 15 percent of all galaxies in the visible universe are spiral galaxies, including our own Milky Way galaxy. Nearly 80 percent of the galaxies surrounding our own Milky Way galaxy are thought to be spiral galaxies, adding to the mystery. Our solar system is located near one of the extended arms of the Milky Way, while the oldest and largest galaxies in the universe are ellipticals, hinting that spiral galaxies may be a relatively recent cosmic evolution.

For decades, astrophysicists have debated the question of whether the spiral arms in disk galaxies come and go over time, or are relatively stable, with variations in gravitational pull on the materials that makes up the arms — stars, gas, and dust —  which holds the material in place for long periods of time.

Now, computer simulations have allowed researchers to follow the motions of as many as 100 million “stellar particles,” providing a model that shows how gravity and other forces mold small material into massive spiral arms.

While a number of past studies have explored the likelihood of spiral galaxies with neighboring galaxies — such as those with a nearby dwarf galaxy, for example — developing spiral arms as the nearby galaxy pulled the disk of its neighbor, researchers say that seem to be only part of the story. The new study modeled stand-alone disk galaxies — those not influenced by another nearby galaxy or object — and the data derived from the new computer simulations suggest that the spiral arms first arise as a result of the influence of giant molecular clouds or star nurseries. The discovery not only gives birth to the spiral arms, but sustain them indefinitely, according to scientists.

“Past theory held the arms would go away with the perturbations removed, but we see that [once formed] the arms self-perpetuate, even when the perturbations are removed,” said D’Onghia, a UW-Madison professor of astronomy. “It proves that once the arms are generated through these clouds, they can exist on their own through [the influence of] gravity, even in the extreme when the perturbations are no longer there.”


According to co-authors Mark Vogelsberger and Lars Hernquist from the Harvard-Smithsonian Center for Astrophysics, the new computer simulations can be used to reinterpret observational data by looking at high-density molecular clouds and gravitationally induced “holes” in space as the mechanisms driving the formation of the emblematic arms of spiral galaxies.

The study may provide astronomers with better understanding of how galaxies around the universe form. By relying on models that show the grouping  “stellar particles,” researchers were able to better model how gravity influences the forming of galaxies.  The study could also provide insight into how our own solar system came to be.

The study comes just months after NASA’s Spitzer Space Telescope spotted one of the universe’s largest spiral galaxies earlier this year. Measuring tip-to-tip across its two outsized spiral arms, NGC 6872 spans more than 522,000 light-years, making it more than five times the size of our Milky Way galaxy. According to NASA scientists, the massive galaxy could  allow for a better understanding of the structure and dynamics of galaxies, bringing  us a step closer to placing cosmic events into their proper cosmological context.

The study is published in the latest edition of The Astrophysical Journal.

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