20 March 2017

The many-tentacled galaxy that could drive a physics revolution


Galaxy NGC 4258 with its extra tentacles protruding from its middle
Many-armed and dangerous


Just beyond the Local Group of galaxies that surround our Milky Way, where our map of the cosmos once plunged off an edge and into the unknown, lurks a galactic sea monster.

Weirdo galaxy NGC 4258 has two extra tentacles reaching out from its middle – part of a system that could one day drive or quench a revolution in physics.

It was 1961 when astronomers discovered the two anomalous arms in NGC 4258, which is 25 million light years away. They defied explanation. Unlike the arms in other spiral galaxies, which wind around in the same plane like the coils of a nautilus shell, these protruded out of the disc of the galaxy.


That was weird enough. But it turns out that the extra arms were just one part of an intricate machine unspooling from the middle of the galaxy – a structure now teaching us about everything from black holes to the expansion of the universe.

A microwave turntable

Zoom in close to where the arms begin, and you will see a short line of blobs at the centre of the galaxy. Each of these is a cloud full of steaming hot water molecules emitting microwave beams out into space.

The blobs on the right are hurtling towards us at 1000 kilometres per second. At the left, they are receding from us just as fast. In the middle, they appear to stand still.

To an astronomer, that’s a recognisable pattern: the line of blobs is really the edge of a gaseous wheel about half a light year in diameter, orbiting around an axis nearly perpendicular to our line of sight.

“I don’t think anybody had ever seen anything like this in this much detail,” says Mark Reid at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts.

The orbiting wheel obeys centuries-old laws. Isaac Newton or Johannes Kepler would have no problem understanding it. Because of that, this structure can double as a scientific apparatus to measure more exotic phenomena.

A precision instrument

An orbiting disc must be circling something. In the mid-1990s, this galaxy’s central wheel helped convince astronomers that nearly every galaxy has a supermassive black hole weighing down its center.

For the blobs to be moving at 1000 kilometres per second on a racetrack less than a light year in diameter requires that they be yoked to the gravity of something about 40 million times heavier than the sun, crammed in a tiny area. Only a black hole fits the bill.

Once you assume there is a black hole there, the galaxy starts to make sense. The spinning disc is caused by material orbiting the black hole like water swirling down the drain. And those wayward tentacles could form when the disc shoots out jets of energetic particles, which then eject heated material above and below the galaxy.

These processes aren’t unique to NGC 4258. Other galaxies have central discs; some may even have faint tentacles of their own waving into space. But because NGC 4258 is relatively nearby and because we see it edge-on, they show up in exquisite detail.

As such, this galaxy could also hold the key to understanding the expansion of the universe, a process driven by normal particles, mysterious dark matter and an equally mysterious ingredient called dark energy that pushes the expansion to accelerate. Studying this expansion requires precise measurements of cosmic distances, which are difficult to come by.

Watching the blobs orbit around NGC 4258 can help. From observations, we can gauge the physical distance between the blobs and the black hole, then with trigonometry, comparing that distance with the angular separation between the objects in the sky lets you calculate how far away this galaxy is from us.

That last part is crucial to cosmologists. Knowing the distances to faraway galaxies accurately is the key part of clocking the expansion rate of the universe.

That makes this galaxy a crucial data point in an ongoing argument between astronomers and cosmologists about just how fast the expansion is happening – a conflict that, if unresolved, could force a revision of modern physics.

Not a bad resume for a sea-monster galaxy that sports extra tentacles.

More on these topics: