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Dead galaxies
show signs of life

By James Lloyd
Posted: 01 June 2011

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Using the Hubble Space Telescope, astronomers at the University of Michigan have discovered star formation in old, ‘dead’ galaxies, providing new insights into galaxy evolution.

Alyson Ford and Joel Bregman spotted this stellar activity in four galaxies situated 40 million light years away from us. "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. "Finding this evidence for low levels of ongoing star formation in these ‘dead’ galaxies was quite surprising."

Messier 105, seen in the top, left corner in an image from the Sloan Digital Sky Survey. The outlined region in the centre of Messier 105 is expanded to reveal Hubble's unique view of the galaxy's inner region, which is further expanded to unveil several individual young stars and star clusters (denoted by dashed circles; top, right). Image: H. Alyson Ford and Joel N. Bregman (University of Michigan).

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 observed colours: elliptical galaxies tend to have a yellow-red hue, whereas typical spiral galaxies have a blue glow emanating from their young, hot stars.

In this study, the astronomers used the Wide Field Camera 3 aboard the Hubble Space Telescope to make detailed ultraviolet images of the galaxies, allowing them to pick out individual stars that were previously unresolved.

The breakthrough came when observing Messier 105, an elliptical galaxy in the constellation Leo. 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. "We were confused by some of the colours of objects in our images until we realised that they must be star clusters,” explains Ford. This star formation represents a continuous process, rather than a burst of activity, in which stars form at an average rate of one Sun every 10,000 years.

Jaymie Matthews, an astronomer at the University of British Columbia, Canada, who was not involved in this study, is excited about these findings. “We astronomers bought into the textbook stereotypes of galaxy classification,” says Matthews. “[This study] will force us to stop and think more carefully about the chemical and dynamical evolution of elliptical galaxies.”

So what mechanisms could be responsible for this mysterious star formation in elliptical galaxies? 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, Jaymie Matthews has come up with a novel analogy for galaxy classification: “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.”

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