12 | NewScientist | 5 June 2010

THIS WEEK

Linda Geddes

–Target acquired–

Stem cells to seek and destroy cancer

Bubble trouble at the centre of the galaxyIS THE Milky Way blowing giant

bubbles? A pair of gamma ray

bubbles, shaped like an hourglass,

seem to be spewing from the black

hole we think lies at the centre of our

galaxy. That is according to the latest

maps from the Fermi Gamma-ray

Space Telescope. Its large area

telescope has been scanning the

whole sky every three hours since

June 2008.

The source of the bubbles is a

mystery but it seems unlikely that

dark matter is responsible. This was

what Douglas Finkbeiner of the

Harvard-Smithsonian Center for

Astrophysics in Cambridge,

Massachusetts, first suspected when

he looked at the maps with his

colleagues last year (arxiv.org/

abs/0910.4583 .

But a new analysis with more

Fermi data suggests that the gamma

radiation traces out a pair of distinct

bubbles that span some 65,000 light

years from end to end – towering

above the 2000-light-year-thick disc

of the galaxy. Such a well-defined

shape is inconsistent with dark

matter, which you would expect to be

smoothly distributed and produce a

diffuse glow, from gamma rays

produced as dark matter particles

meet and annihilate each other. “We

are pretty sure the majority of

emissions are not from dark matter,”

says Finkbeiner’s student Meng Su.

Instead, they think the bubbles

may have been blown out by the

explosion of short-lived, massive

stars born in a burst of new star

formation about 10 million years ago.

Alternatively, the bubbles may have

been forged 100,000 years ago by

high-speed jets of matter created

when roughly 100 suns’ worth of

material fell into the black hole at the

centre of our galaxy. The team

presented its analysis last week at

the American Astronomical Society

meeting in Miami, Florida.

Fermi team members have also

found more gamma radiation than

expected in the region but say it’s too

soon to tell whether it forms an

hourglass shape or what its source

may be. Rachel Courtland

“The bubbles may have formed when 100 suns’ worth of stuff fell into our galaxy’s giant black hole”

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