New class of galaxies: small, green, and bursting with new stars

(Carolin Cardamone and the Sloan Digital Sky Survey)
The top three images show "green pea" galaxies, compared with a more typical galaxy in the bottom image. The new galaxy type is intriguing: the green peas are far smaller and less massive than the Milky Way; but they are producing stars at a far faster rate than the Milky Way. For some astronomers, these nearby peas could open a window on processes taking place in the universe's very first galaxies.

Finally, people have some cosmic objects to talk about that are little and green but don't invoke little green men.

Astronomers -- and we're including some everyday folks in this category -- have discovered what appears to be new class of galaxies they've dubbed green-pea galaxies. Why? Because in images from the Sloan Digital Sky Survey, they look like, well, little green peas.

A formal report of the discovery has been accepted for publication in the Monthly Notices of the Royal Astronomical Society in Britain. You can find a preprint of that formal paper here and a plain-English description here.

The discovery is another feather in the cap for "citizen science." The galaxies emerged as volunteers with the Galaxy Zoo project helped professional astronomers classify galaxies, as in: this one's a spiral, this one's elliptical, this one's weird.

The green-pea galaxies galaxies were, if not weird, at least interesting enough to generate lots of comments from different Galaxy Zoo volunteers picking them out from different sets of galaxy images.

The galaxies' green color stems from their penchant for emitting a lot of light at green wavelengths peculiar to highly energized oxygen atoms.

What makes the galaxies so intriguing is their small size, the rapid pace at which they form new stars, and their proximity to Earth-bound observers.

On average, the galaxies appear to be 10 times smaller than the Milky Way and tip the scales at about 1/100th the Milky Way's mass. And they are generating new stars some 10 times faster than the Milky Way.

These galaxies also are relatively close; the 250 green-pea galaxies identified so far appear to be some 1.5 billion to 5 billion light-years away.

And that's a bonus for people like Carolin Cardamone, who study galaxy evolution.

Ms. Cardamone is working on her PhD at Yale University and is the lead author of the formal paper reporting the results on behalf of herself and a team of US and British astronomers and astrophysicists.

"These things are so local!" she enthuses during a phone chat. "We can study them in greater detail" than similar objects at far greater distances.

Windows on the past?

Those distances translate into far earlier times in the universe's history. That means that such objects also are far more difficult to observe in any detail.

The pea-green galaxies, by contrast, are close enough that with instruments like the Hubble Space Telescope, astronomers can tease out shapes, structures, and gather spectra that allow them to make inferences they can't make if they were observing the green peas' more-distant cousins.

For instance, researchers might be able to use infrared imaging to begin asking whether green-pea galaxies have an underlying population of older stars.

That information can help establish the ages of these galaxies, notes Henry Furgeson, deputy head of the science-mission office at the Space Telescope Science Institute in Baltimore and a specialist in galaxy evolution.

And they can begin to address the question of why these galaxies -- which appear in regions of space with a relative dearth of other galaxies -- display such high rates of star formation.

Astronomers have noted high rates of star birth in another type of small galaxy known as blue compact galaxies. But these too are in relatively rarefied regions of space where interactions with other galaxies just don't happen. Such interactions have been seen to trigger bursts of star formation in more galaxy-rich parts of space as galaxies collide or swing close by one another.

Galaxy Zoo project

The green-pea discovery is the latest to demonstrate the value of many eyes on a virtual stack of images no single human astronomer or small group of astronomers, could hope to plumb.  All you need is a computer and good pattern-recognition skills.

Indeed, the formal research paper tips its rhetorical hat to 10 of these Galaxy Zoo individuals, who made a particularly strong contribution to the research.

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