How Galaxies Grow, How They Age [Video]

how spiral galaxies form, how elliptical galaxies age

How do spiral galaxies form so that they get their lovely pinwheeling arms? Why were the ancient red elliptical galaxies so much smaller ten billion years ago than they are today? Two teams from the Harvard-Smithsonian Center for Astrophysics (CfA) recently published their research on these questions.

In a late March “Weekly Science Update,” CfA astronomer Matt Ashby and The CANDELS project (Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey) noted that over 80 percent of galaxies fall into two broad categories. The first category holds the blue spiral galaxies like our Milky Way, which contain plenty of young stars. The other group is represented by the red elliptical galaxies, which contain older stars.

Today’s red elliptical galaxies are very large and sprawling. However, when we look back at distant galaxies nine to 12 billion light-years away, we’re also looking back in time to when the light left those earlier galaxies. As a result, we can see that the ancient red elliptical galaxies were much smaller. Why?

Galaxies like our Milky Way have grown at least in part through colliding with and joining other galaxies, so you would assume that redder (which means older) galaxies would always be larger.

However, Ashby’s team discovered that the ancient red elliptical galaxies ran out of fuel, causing them to stop growing and then to shrink. Poor babies. They’re not just getting older. They’re running out of steam.

The second study, published in late March in The Astrophysical Journal, used powerful simulation techniques to calculate how the spiral galaxies develop those lovely arms. CfA called the spiral galaxies “some of the most beautiful and photogenic residents of the universe,” and I certainly agree. In fact, I nominated lovely Messier 77 as the most beautiful galaxy in the universe, bar none.

A lot of other astronomy buffs would give that honor to the four-armed spiral Messier 106.

Be that as it may, beauty sometimes comes at a cost, and some previous astronomers had theorized that the spiral arms were rather temporary features in a galaxy’s life. However, the team led by University of Wisconsin-Madison’s Elena D’Onghia ran simulations which verified that the spiral arms are “surprisingly long-lived” — not a brief flash-in-the-pan moment of beauty that passes all too soon.

Their 2011 video simulation, which was created along the way as they performed their research, can be seen here:

Astronomers are clearly getting closer to learning the whole story of how galaxies are born — and how galaxies die.

[Messier 77 spiral galaxy photo courtesy NASA, ESA & A. van der Hoeven]