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Music

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Narrator: When we picture a galaxy, we tend to imagine a tranquil

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pinwheel of stars, spinning in the cosmic night.

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Astronomers imagined something similar, that disk galaxies, like our own Milky Way,

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had reached their present states billions of years ago.

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Now, a study led by Susan Kassin at NASA Goddard, has turned this thinking

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on its head. Susan: We find that disk-like galaxies

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become progressively more ordered with time. This was a surprise to people in the field

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because we thought that galaxies already 8 billion years ago were gonna

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be very much like galaxies today, whereas that's really not the case.

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Over this period of time galaxies spin faster, the

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amount of disordered motions that they harbor has decreased, and their

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total energies increase. Narrator: Over the past 8 billion years

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disk galaxies began as train wrecks and then evolved into the orderly

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systems we see nearby today. Susan: We found out how fast

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they were rotating and how much disordered motions they have from

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spectra from the Keck telescopes. And then in order to interpret the rotation

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measurements we needed images from the Hubble Space Telescope to tell us how the

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galaxies were oriented.

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So we find the mass of a galaxy plays a large role in how organized it is.

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The most massive galaxies are the most well organized at all times, and

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the least massive galaxies are the least well organized at all times.

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So on average, the percentage of galaxies which are settled increases

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with time. Here you're seeing it for the

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higher mass systems. It's also

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the case for the lower mass systems. The percentage of galaxies

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which are settled just increases with time, but the overall

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percentages for the lower mass systems are always lower than the higher mass systems.

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We've yet to figure our why this is.

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In our models of how galaxies evolve, we find that galaxies are

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possibly more disordered in the past because they're bombarded with more material.

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There are more small galaxies that accrete onto it, there are more major

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mergers of galaxies and there's more accretion of gas.

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From our models we expect that this constant bombardment should slow down with time.

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And this might be why we're finding this in the observations.

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And what we're finding might also be due to a decreasing amount of

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supernovae with time. However, the simulations as they are

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now are really only at the stage where they're giving us clues as to what's going on.

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In order to get the detailed measurements to really find out what's going on, we're going

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to need the James Webb Space Telescope.

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Narrator: This new picture tells us that disk galaxies like our own Milky Way

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experienced a rowdy past for a much longer time than previously imagined.

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A period that includes the formation of the solar system and the origin of life on

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Earth. (Music)