Like news ripped from a Hollywood tabloid, this saga includes an encounter between two individuals; one aging, and thought to be past its prime, the other youthful and vigorous. And for good measure, thrown in on this story are cannibalism and even zombies. The result of the meet-up? Babies. Baby stars, that is, and the individual galaxies in this tale ended up, seemingly, living together happily-ever-after. The Hubble Space Telescope’s Wide Field Camera 3 (WFC3) captured images of NGC 4150, an aging elliptical galaxy, and at the core of the galaxy was some vigorous star birth. The star-making days of this galaxy should have ended long ago, but here was active star birth taking place. This isn’t the first time astronomers have seen something like this, so they took a closer look.
Near-ultraviolet images of this old elliptical galaxy taken by WFC3 revealed streamers of dust and gas and clumps of young, blue stars, significantly less than a billion years old. But also there was evidence of an encounter — dark strands of dust in the center provided tentative evidence of a recent galaxy merger.
“Elliptical galaxies were thought to have made all of their stars billions of years ago,” said astronomer Mark Crockett of the University of Oxford, leader of the Hubble observations. “They had consumed all their gas to make new stars. Now we are finding evidence of star birth in many elliptical galaxies, fueled mostly by cannibalizing smaller galaxies. These observations support the theory that galaxies built themselves up over billions of years by collisions with dwarf galaxies, and NGC 4150 is a dramatic example in our galactic back yard of a common occurrence in the early universe.”
And so, a new study by Crockett and his team helps bolster the emerging view that most elliptical galaxies have young stars, and that galaxy mergers bring an old, dying galaxy back to life. The inset on the top image shows a magnified view of the chaotic activity inside the galaxy’s core. The blue areas indicate a flurry of recent star birth. The stellar breeding ground is about 1,300 light-years across.
Over the past five years ground- and space-based telescopes have offered hints of fresh star formation in elliptical galaxies. Ground-based observatories captured the blue glow of stars in elliptical galaxies, and satellites such as the Galaxy Evolution Explorer (GALEX), which looks in far- and near-ultraviolet light, confirmed that the blue glow came from fledgling stars much less than a billion years old. Ultraviolet light traces the glow of hot, young stars.
Crockett and his team selected NGC 4150 for their Hubble study because a ground-based spectroscopic analysis gave hints that the galaxy’s core was not a quiet place. The ground-based survey, called the Spectrographic Areal Unit for Research on Optical Nebulae (SAURON), revealed the presence of young stars and dynamic activity that was out of sync with the galaxy.
“In visible light, elliptical galaxies such as NGC 4150 look like normal elliptical galaxies,” said team member Sugata Kaviraj of the Imperial College London and the University of Oxford. “But the picture changes when we look in ultraviolet light. At least a third of all elliptical galaxies glow with the blue light of young stars.”
“Ellipticals are the perfect laboratory for studying minor mergers in ultraviolet light because they are dominated by old red stars, allowing astronomers to see the faint blue glow of young stars,” said Crockett.