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Starts With A Bang

Earliest, brightest galaxies shine a ghostly green in surprising new find

Only a few galaxies exhibit this green glow in the nearby Universe. At early times, it’s practically all of the brightest ones.

“The discovery that young galaxies are so unexpectedly bright–if you look for this distinctive green light–will dramatically change and improve the way that we study Galaxy formation throughout the history of the Universe.” 
Matthew Malkan

Here in the nearby Universe, 13.8 billion years since the Big Bang, galaxies come in great varieties.

A great variety of galaxies in color, morphology, age and inherent stellar populations can be seen in this deep-field image. Image credit: NASA, ESA, R. Windhorst, S. Cohen, M. Mechtley, and M. Rutkowski (Arizona State University, Tempe), R. O’Connell (University of Virginia), P. McCarthy (Carnegie Observatories), N. Hathi (University of California, Riverside), R. Ryan (University of California, Davis), H. Yan (Ohio State University), and A. Koekemoer (Space Telescope Science Institute).

Spirals, ellipticals, rings and irregulars, they glow blue, white or red, depending on their stellar populations.

Galaxies undergoing massive bursts of star formation expel large quantities of matter at great speeds. They also glow red covering the whole galaxy, thanks to hydrogen emissions. Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA), of the Cigar Galaxy, Messier 82.

The most violent star-forming galaxies and nebulae are so hot they turn red, as ultraviolet radiation ionizes neutral hydrogen.

The great Orion Nebula is a fantastic example of an emission nebula, as evidenced by its red hues and its characteristic emission at 656.3 nanometers. Image credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team.

When the ionized electrons recombine with the nuclei, they transition between energy levels, emitting a particular set of wavelengths of light.

This image from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 surrounding a dim and dying star located about 3300 light-years away. Image credit: ESO / FORS instrument.

But there’s another, green line that happens only when oxygen gets doubly ionized at the hottest temperatures of all: 50,000 K and above.

Modern ‘green pea’ galaxies have their doubly-ionized oxygen emission offset from the main galaxy; in the Subaru Deep Field, the galaxies themselves exhibit the strong emission. Image credit: NASA, ESA, and Z. Levay (STScI), with science by NASA, ESA, and W. Keel (University of Alabama, Tuscaloosa).

Only planetary nebulae, with super-hot young white dwarfs, and the ultra-rare “green pea” galaxies exhibit these features.

The Subaru Deep Field, containing thousands of distant galaxies exhibiting these oxygen lines. Image credit: Subaru telescope, National Astronomical Observatory of Japan (NAOJ); Image processing: R. Jay GaBany.

But by looking at the most active star-forming galaxies in the Subaru Deep Field (above), Matthew Malkan and Daniel Cohen found, that all galaxies from 11 billion years ago or more emit this green signature.

The strong green emission line (highest point) as shown in a sample of over 1,000 galaxies, spectrally stacked from the Subaru Deep Field. The other point “above” the curves is from hydrogen; the strong green oxygen line indicates incredibly intense radiation. Image credit: Malkan and Cohen (2017).

The unexpected brightness and hotness of these galaxies hints that the stars in the ultra-distant Universe are somehow hotter than the hottest stars today.

The merging star clusters at the heart of the Tarantula Nebula, which contains the hottest stars in the local group, are still below 50,000 K. Perhaps lower metallicities, higher masses, or even a top-heavy initial mass function among stars in the early Universe are responsible for the increased, high temperatures. Image credit: NASA, ESA, and E. Sabbi (ESA/STScI); Acknowledgment: R. O’Connell (University of Virginia) and the Wide Field Camera 3 Science Oversight Committee.

These early galaxies are likely the type that reionized the Universe.

The reionization and star-formation history of our Universe. The study hints that green, oxygen-rich galaxies may have been responsible for reionization. Image credit: NASA / S.G. Djorgovski & Digital Media Center / Caltech.

JWST, launching 2018, will find out for sure.

Mostly Mute Monday tells the story of a single astronomical phenomenon or object in mostly visuals, limited to no more than 200 words.

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