/ 18 August 2014

Astrophysical jets in distant galaxy could teach us physics

Arvind Ramessur.
Arvind Ramessur.

In the dark and cold chasm of space, huge jets of charged particles erupt around a super-massive black hole. Black holes, whose gravitational pull is so strong that not even light can escape, are usually found in the hearts of galaxies, known as active galactic nuclei (AGNs). These AGNs feed on gas, dust and stars swirling around them and shoot out jets from their polar regions at close to the speed of light. 

Unlike man-made jets (water from a hosepipe, for example) or natural jets (think volcanic eruptions), astrophysical jets rarely have a solid nozzle and rigid walls to direct the charged particles that they expel. Some of these particles are so energised that, instead of spiralling inwards towards the black hole, they shoot away in the jet and escape. These celestial jets allow us to investigate the laws of physics in extreme environments that cannot be simulated on Earth, and possibly find areas where they do not hold.

AGN-jets range from a few million kilometres to trillions of kilometres long. When the jets are only a few trillion of kilometres long, they start to reveal differences in the jet structure and brightness. But the smallest jets, which are only a few million kilometres long and are relatively close to the black hole, play a crucial role in furthering our understanding of the laws of physics in space, and answering the question of how these jets are formed in the first place.

In radio astronomy, such small AGN-jets are usually observed with a technique known as very long baseline interferometry (VLBI), which uses a number of radio telescopes, often continents apart, to look at the same object. The data from the different antennas are combined to create a far higher resolution than any single instrument could achieve. This technique has allowed radio astronomers to observe celestial objects and phenomena in unprecedented detail. 

The 26m VLBI dish at Hartbeest-hoek Radio Astronomy Observatory (HartRAO) in Gauteng, a facility of the National Research Foundation, was the continent’s only radio telescope until the construction of South Africa’s prototypes for its Square Kilometre Array (SKA) bid. Used primarily as a science instrument, HartRAO is also helping to boost skills development across the continent and prepare for the SKA. This radio observatory has also participated in global networks such as the European-VLBI (EVN) and will also be pivotal in the African-VLBI (AVN), a collection of radio telescope spread across the continent. 

A team of astronomers from Canada, the United States and HartRAO used VLBI to study the nucleus of the spiral galaxy Messier 81 (also known as M81 or NGC 3031). Situated about 12-million light years from earth in the constellation Ursa Major, the spiral galaxy M81 is similar to our own Milky Way Galaxy. With its elegant and symmetrical spiral arms and disc of stars, M81 is the closest spiral galaxy to host a low power AGN. Not every galaxy harbours an active black hole; our own galaxy, the Milky Way, is an inactive galaxy, though it shows signs of past activity.

The result was striking: the researchers imaged the central region of M81 and found an AGN-jet, whose length was about 4000 times the distance between our Sun and Earth, protruding from the nucleus. This observation showed that the core of M81 is indeed an AGN,

and ruled out the alternate possibilities of a starburst or supernova. They found that the jet is not straight, but has a noticeable bend, like hedgehog spines. It also changes direction in its orientation over time just like a spinning top slowing down. The nucleus of M81 is more powerful than that of our own galaxy, but much weaker than young galaxies such as quasars even further away. 

At present, technology can only aid us to see so much. When you zoom in to the degree required to see these jets, millions or billions of light years away, the details becomes blurred and it is difficult to see exactly what is going on in those AGNs and how the jets are launched. This is why radio astronomers rely on computational models to constrain observational data. But this is difficult because most AGN-jets appear one-sided due to an effect known as Doppler boosting, whereby the jet is like a beam from a lighthouse: it appears bright to passengers on a ship when it is directed towards them, but dim or unseen when directed away. 

The ability to model jet behaviour accurately for small, asymmetrical and bent AGN-jets will become even more important when the South Africa’s 64-dish MeerKAT and the giant SKA radio telescopes are built. There is on-going research at HartRAO to develop a computational model to decipher such radio sources. It will use VLBI data accumulated from over more than 10 years of observations to model M81’s jet, which will also be applicable to other sources. By identifying AGNs and modelling their jets, we will be able to broaden our knowledge of AGN formation and their effect on the evolution of their host galaxies, and how the laws of physics work in this extreme environment.

Arvind Ramessur is a MSc candidate at the University of South Africa.