Crippled and gigantic, a tadpole-shaped galaxy
An enormous discovery prompts a rethink on the maximum size of disrupted star systems. Andrew Masterson reports.
A huge but profoundly damaged galaxy has been identified by a team of researchers from Israel, the US and Russia.
The galaxy, which bears a passing resemblance to a tadpole, is more than 300 million light years away from Earth. It is described by lead researcher Noah Brosch, of the Florence and George Wise Observatory at Tel Aviv University as “disrupted”, because it has clearly been subjected to massive outside forces.
Usually, disrupted galaxies are relatively small – a result of their stars being either incorporated into a nearby more massive galaxy, or being ejected en masse into space as a result of a titanic collision between two star systems.
The latest find, however, will very likely prompt a re-examination of the constraints for the survival of such systems.
“We have found a giant, exceptional relic of a disrupted galaxy,” says Brosch.
The structure, which is inside a cluster of smaller galaxies called Hickson's Compact Group 98, is enormous. At approximately one million light-years long from end to end, it is 10 times the size of the Milky Way.
“What makes this object extraordinary is that the tail alone is almost 500,000 light-years long,” says co-author Michael Rich of the University of California, Los Angeles.
“If it were at the distance of the Andromeda galaxy, which is about 2.5 million light years from Earth, it would reach a fifth of the way to our own Milky Way.”
Brosch and his colleagues suggest that the galactic tadpole formed as the result of a previous invisible star-filled dwarf galaxy being ripped apart by the gravitational force of two larger galaxies, and its components being dramatically redistributed.
“The extragalactic tadpole contains a system of two very close ‘normal’ disc galaxies, each about 40,000 light-years across,” says Brosch. “Together with other nearby galaxies, the galaxies form a compact group.”
That group, the scientists stay, is far from a settled system. All the members of Hickson's Compact Group 98 are expected to merge into a single giant galaxy in about one billion years – at which time, presumably, the extragalactic tadpole will turn into a cosmic frog.
The research is published in the Monthly Notices of the Royal Astronomical Society.