Twin galaxies in cosmic

arm wrestle
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: June 27, 2008

Astronomers at the Gemini Observatory have captured the early stages of gravitational interaction between two similar galaxies as they tug on each other’s spiral arms.

The galaxies – NGC 5427 and NGC 5426, which are collectively known as Arp 271 – are around 90 million light years away in the constellation of Virgo. Their once near-identical masses, structures and shapes have already begun to distort as the mutual wrench of gravity overpowers their outer shells in a galactic embrace that could take a hundred million years to complete.

Appearances can be deceiving; in the image above, NGC 5427, the ‘face-on’ galaxy, is farthest away, while the oblique galaxy NGC 5426 is nearest. Over time, NGC 5427 will revolve around its twin, bringing it into the foreground. Image: Gemini Observatory.

A funnel of gas bridges the two galaxies, allowing the twins to feast on shared gas and dust across the 60,000 light years that currently separates them. The colliding gas also appears to have sparked localised bursts of star formation, including dense and curiously knotted pockets of stellar nurseries in NGC 5426, and a frenetic rate of star formation in NGC 5427, much more than expected for a typical galaxy of this type.

Even more astounding is the unusually straight western (top) arm of NGC 5427. It is as if the strong tidal forces have brutally snapped the arm in two, causing it to bleed starlight into the black canvas of space. Like watching a slow motion replay of two skaters spinning on ice, NGC 5426 has grabbed its twin by the arm and has begun to spin it around in a manoeuvre that will take millions of years to execute.

Once thought to be a rare event, gravitational interactions between galaxies are now known to be rather common, as highlighted by the breath-taking gallery of interacting galaxy images released by Hubble in April to mark its 18th anniversary. Such cosmic tug-o-wars are especially common in densely populated galaxy clusters and are considered to play an important role in galaxy evolution. Indeed, our own Galaxy will ultimately meet its fate with the Andromeda Galaxy, and the events played out within the Arp 271 system could be a sneak preview of what is to come.