Dead Galaxies Show Signs of Life

1 Jun

Q: What do Lazarus, Harold Bishop and flesh-eating zombies have in common?
A: They’ve all defied the Grim Reaper by coming back from the dead.

Lazarus was four days gone when Jesus told him to get up; Harold disappeared from a coastal path, only to return to Neighbours five years later as an amnesiac who thinks he’s called Ted; and no zombie film is complete without at least one shot of a corpse twitching to life.

In no particular order: a trendy zombie, a biblical miracle man, and an Australian soap star

All this is a convoluted way of saying that dead things can sometimes come alive again. And that’s exactly what happened in the world of astronomy this week. Using the Hubble Space Telescope, two scientists at the University of Michigan have discovered star formation in old, ‘dead’ galaxies.

Dr. Alyson Ford and Prof. Joel Bregman spotted this stellar activity in four galaxies situated 40 million light years away from us – galaxies which were previously assumed to be pushing up the space daisies.

“We have directly detected individual young stars and star clusters in galaxies that were thought to have ceased forming stars a very long time ago,” says Ford, who presents the results this week at a Canadian Astronomical Society meeting in London, Ontario.

Ford and Bregman studied elliptical galaxies, mostly composed of older, low-mass stars. Unlike spiral galaxies such as our own Milky Way, elliptical galaxies have no disc of cold, dense gas from which new stars can be regularly formed.

Star formation in these galaxies was thus considered to be non-existent. This is supported by their typical colours: elliptical galaxies tend to have a yellow-red hue, whereas spiral galaxies generally have a blue glow emanating from their young, hot stars (see image below).

Two types of galaxy: elliptical (Centaurus A, left) and spiral (Messier 101, right). Credit: Canada-France-Hawaii Telescope / NASA / ESA

In this study, the Wide Field Camera 3 aboard the Hubble Space Telescope was used to make detailed ultraviolet images of the galaxies, revealing individual stars that were previously hidden amidst a haze of galactic light.

The breakthrough came when observing Messier 105, an elliptical galaxy in the Leo constellation. Ford and Bregman noticed a handful of very bright, blue stars – the first hints of star formation in this galaxy. They also detected objects that weren’t blue enough to be single stars, identified as star clusters. This star formation is a continuous process, rather than a burst of activity, in which stars form at an average rate of one Sun every 10,000 years.

The outlined region in the centre of the Messier 105 galaxy (top left corner) is expanded to reveal Hubble's unique view of the galaxy's inner region, which harbours several young stars and star clusters (top right corner)

So what mechanisms could be responsible for this mysterious behaviour? Ford and Bregman have no answer yet, but hope that further work may provide some hints. “We’re hoping to make more observations of other nearby elliptical galaxies to gain further insight into the star formation process, including any clues that can help us determine the history of the gas that these new stars are forming from,” says Ford.

Meanwhile, Dr. Jaymie Matthews, an astronomer at the University of British Columbia, Canada, has a novel take on the discovery:

“Astronomers may have been guilty of ‘stereotyping’ galaxies when we classify them. Spiral galaxies: lots of gas, many stars being born; the galactic equivalent of Los Angeles. Elliptical galaxies: no gas, only old geezers with all the bright lights long ago extinguished; the galactic equivalent of a Perry Como Fan Club meeting in a Prius factory. But like humans and societies, if you look closely enough, the stereotypes fall apart as they are far too simplistic and unfair.”

The moral of the story, then? Never judge a galaxy by its colour.

Based on an article written for Astronomy Now.
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