The bright core of the galaxy M87 is located in the lower left of each of these Hubble Space Telescope images taken from 1999 to 2006, while the knot called HST-1 is the bright blob at centre. The glowing material at far right is part of a stream of particles in the jet that speed up and glow in the ultraviolet (Image: NASA/ESA/J Madrid/McMaster University)

A knot in a jet of matter streaming out of a nearby galaxy has brightened mysteriously over a period of several years, newly released Hubble Space Telescope images reveal. The bizarre behaviour may overturn theories of what causes other galaxies – which are too far away to study in such detail – to fluctuate in brightness.

The giant galaxy M87, which lies 54 million light years away, sports an extensive jet that spews out from its centre. The jet is made up of matter that was shot outwards by magnetic fields after spiralling towards the supermassive black hole at the galaxy’s heart.

But the jet is not a smooth stream of matter – bright knots dot its length. One such knot, dubbed HST-1, has changed dramatically in the years since it was first spotted by the Hubble Space Telescope in 1999, at one point brightening by a factor of 90.


Astronomers regularly observe the knot, which lies about 214 light years from M87’s core, at a variety of wavelengths. Now, Juan Madrid at McMaster University in Canada has analysed seven years’ worth of near-ultraviolet measurements taken by Hubble. They dovetail with observations made by other telescopes, such as the Chandra X-ray Observatory, revealing a knot that grew to outshine M87’s bright core.

What could be causing this flaring? “We do not know for sure yet,” says Madrid, adding that there are two broad possibilities.

Flowing stream

One is that material flowing down the jet is ramming into a cloud of dust or gas at the location of HST-1 and glowing. The other argues that the magnetic field lines that keep the jet focused in a beam may get squeezed together.

Researchers say that with either explanation, the jet can be thought of as a stream of water – an analogy first suggested by Dan Harris at the Harvard-Smithsonian Center for Astrophysics. Sometimes the water runs into rocks and sometimes the stream is constricted, creating white water that researchers detect as bright knots.

The white water can churn even more furiously – and the knots glow more brightly – if the stream is swollen after a storm – that is, if more matter is flowing down the jet from the black hole.

Pinched together

If the jet’s magnetic field lines are getting squeezed together, that would release a lot of energy, mimicking the process that creates solar flares. “The magnetic field lines in the sun get separated and then get close together, and when they touch, there’s a big flare,” says Madrid.

But what would cause the magnetic field lines to squeeze together at that spot? One possibility – previously suggested by Chi Cheung at NASA’s Goddard Space Flight Center and his colleagues – is that the density of stars and gas within the galaxy changes significantly at that distance, thinning out beyond that point.

So after travelling outwards from the dense inner region of the galaxy, the jet “hits this discontinuity and it sort of recollimates; it sort of focuses the beam again,” Cheung told New Scientist.

More observations at a range of wavelengths may help tease apart exactly what is causing HST-1 to fluctuate in brightness. Within a year, for example, Cheung hopes that high-resolution observations taken by an array of radio telescopes will provide a test of his team’s idea that a change in the galaxy’s density at HST-1’s location is the culprit.

‘Rosetta stone’

The findings could be a “Rosetta stone” to decode the mysteries of more distant galaxies that also fluctuate in brightness, says Cheung. Many of these galaxies, called blazars, lie too far away for telescopes to resolve knots such as HST-1 in their jets – flares from any such knots would appear to be coming from the centres of the galaxies instead.

“If M87 were at a greater distance, then one would be thinking the flaring is happening due to the black hole and not due to the jet,” Madrid told New Scientist.

Previously, any changes in a blazar’s brightness have been attributed to a change in how fast matter is falling into the galaxy’s central black hole, he says.

But since HST-1 lies more than 200 light years away from M87’s black hole, that suggests that some blazar flares may also arise in jets, Madrid says.

Eric Perlman, an astrophysicist at the Florida Institute of Technology who studies M87’s jet, agrees: “This is telling us that some of these flares occur quite far out [from the black hole].”

Journal reference: Astronomical Journal (vol 137, p 3864)