Why Are Galaxies Smooth? Star Streams

Article Updated: 24 Dec , 2015

Look at the disk of any large spiral galaxy, and outwardly it appears smooth, with stars evenly distributed throughout. But when young stars are forming, they are clustered together in dense clouds of dust and gas. So what happens as the galaxy matures to allow for the smooth distribution seen in galaxies like the Milky Way? Using NASA’s Spitzer Space Telescope, an international team of astronomers has discovered streams of young stars flowing from their natal cocoons in distant galaxies. These distant rivers of stars provide an answer to one of astronomy’s most fundamental puzzles.

Astronomers know that the clusters where stars form begin to disappear when their ages reach several hundred million years. A few mechanisms are thought to explain this: some clusters evaporate when random internal motions kick out stars one by one, and other clusters disperse as a result of collisions among the clouds where they were born. Zooming out to mechanisms operating on larger scales still, shearing motions caused by the galaxy’s rotation around its center disperses the clusters of clusters of young stars.

“Our analysis now answers the grand puzzle. By finding a myriad of streams of young stars all over the disks of galaxies we studied, we see that the mechanism for pulling the clusters of young stars apart is shearing motions of the parent galaxy. These streams are the ‘missing link’ we needed to understand how the disks of galaxies evolve to look the way they do,” said team leader David Block of the University of the Witwatersrand in South Africa.

Crucial to this discovery was finding a way to image previously hidden young stellar streams in galaxies millions of light-years away. To do this the team used high-resolution infrared observations from the Spitzer.
Using infrared rather than visible light to look at the galaxies allowed the group to pick out stars at just the right age when the stars are just starting to spread out from their clusters.
Credit: NASA/ Spitzer team
“Spitzer observes in the infrared where 100-million-year-old populations of stars dominate the light,” noted co-author Bruce Elmegreen, from IBM’s Research Division in New York. “Younger regions shine more in the visible and ultraviolet parts of the spectrum, and older regions get too faint to see. So we can filter out all the stars we don’t want by taking pictures with an infrared camera.”

Infrared is also important because light in this part of the spectrum can penetrate the dense dust clouds surrounding the clusters where stars form.

“Dust blocks optical starlight very effectively,” said Robert Gehrz of the University of Minnesota, “but infrared light with its longer wavelength goes right around the dust particles blocking our view. This allows the infrared light from young stars to be seen more clearly.”

But even when the images are taken in the infrared, they are still dominated by the light from the smooth older disks of galaxies, not the faint tracks of young dispersing clusters. Special mathematical manipulations were needed to pick out the clusters, whose faint tracks can still be seen precisely because they are not smooth.

Team member Ivanio Puerari of the Instituto Nacional de Astrofisica, Optica y Electronica in Puebla, Mexico used a technique invented by mathematician Jean Baptiste Fourier in the early 1800’s. The technique is effectively a spatial filter that picks out structure on the physical scale where star formation occurs. “The structures cannot be seen on the original Spitzer images with the human eye,” noted Puerari.

“The combination of the Fourier filtering and infrared images highlighted regions of just the right size and the right age. To then unveil so many star streams in the disks of galaxies was unimaginable a year ago. This discovery continues to highlight the enormous potential of the Spitzer Space Telescope to make contributions none of us could have dreamed possible,” commented Giovanni Fazio from the Harvard-Smithsonian Center for Astrophysics, project leader for the Spitzer Infrared Array Camera team used to take the pictures, and co-author of the discovery.

“Galileo, as both astronomer and mathematician, would have been proud. It is a wonderful interplay between the use of astronomical observations and mathematics and computers, exactly 400 years since Galileo used his telescope to examine our Milky Way galaxy in 1609,” Fazio said

Source: Spitzer

4 Responses

  1. Dave Finton says:

    Aaaaaagh! It’s the Eye of Sauron!

    Just kidding, but that’s pretty cool.

  2. Jon Hanford says:

    I find it interesting that years after astronomers found star streams in our galaxy (Ursa Major Stream, Scorpius-Centaurus Stream), this clever study has now detected star streaming phenomena in other nearby galaxies. While dissolution of star clusters has long been proposed to account for the ‘smoothness’ we see in spiral galaxies, this research provides astronomers with direct evidence that this mechanism is indeed at work in galaxies other than our own.

  3. Manu says:

    @ Jon Hanford:
    I’m not sure these are the same streams.

    Milky Way streams such as Ursa Major are supposed to be remnants of swallowed dwarf galaxies, made of extra-Milky Way stars, whereas the streams the article is about result from shearing of the galaxies’ own clusters.

    In fact I think the authors might have found a better word than ‘stream’ to describe it.

  4. Jon Hanford says:

    @Manu, Sorry, the streams I was referring to are not those found recently by combing the SDSS data releases ( the so- called ‘Field of Streams”, etc.). I was referring to groups of stars found to be moving in roughly the same direction through measurement of their proper motions back in the 40s, 50s, and 60s. Granted, the ‘associations’ I mentioned are in an advanced state of dissolution compared to those studied in the current research, but their link to each other now seems reasonable. You are quite correct in your assertion that that the new ‘star streams’ found in the study are not the ‘star streams’ recently (since 1999) found in the analysis of the Sloan Digital Sky Survey data (for one, these SDSS streams are much more distant, being mostly out of the Milky Way galaxy as opposed to the ‘associations’ I mentioned, which are IN the Milky Way). Sorry for the confusion, I should have clarified which ‘star streams’ I was referring to. I hope this helps clear that up.

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