How Young, Massive, Compact Galaxies Evolve into Their Red, Dead Elders

Scientists have uncovered young, massive compact galaxies that are coming to an end of their intense bouts of star formation. Using NASA's Hubble Space Telescope and Chandra X-ray Observatory, the scientists have found that these galaxies may be on the path of becoming so-called "red and dead galaxies," composed only of aging stars.

For decades, astronomers have debated how massive galaxies rapidly evolve from active star-forming machines to star-starved graveyards. Now, they may have their answer. Scientists analyzed 12 merging galaxies at the end of their star-birthing frenzy. In the end, they found that the stars themselves are turning out the lights.

The researchers identified the galaxies from the Sloan Digital Sky Survey as post-starburst objects spouting gaseous fountains. These outflows arose from the most compact galaxies yet found. In such small regions of space, the galaxies form a few hundred suns per year, as opposed to the one "sun" created by our Milky Way. Yet as the gas rapidly heats up, it becomes too hot to contract under gravity to form stars. Another possibility is that the star-birthing frenzy blasts out most of the star-making gas with powerful stellar winds.

"Before our study, the common belief was that stars cannot drive high-velocity outflows in galaxies; only more powerful supermassive black holes can do that," said Paul Sell, the lead author of the new study, in a news release. "Through our analysis we found that if you have a compact enough starburst, which Hubble showed was the case with these galaxies, you can actually produce the velocities of the outflows we observed from the stars alone without needing to invoke the black hole."

It's likely that the star-forming truly begins when two gas-rich galaxies collide, funneling a torrent of cold gas into the merging galaxies' compact center. The large amount of gas compressed into the small space ignites the burst of numerous stars. Then, the energy from the stellar firestorm blows out the leftover gas, quenching further star formation.

The findings reveal a bit more about star formation and the evolution of galaxies. This, in turn, shows scientists a bit more about compact galaxies.

The findings are published in the journal Monthly Notices of the Royal Astronomical Society.