Radio "giant". The origin of the largest object in the Universe has been uncovered

Be it a beauty contest or a track event, on the field or off the field, the competitive spirit urges us to be the best -the fastest, the strongest, the wisest- in every walk of life. Science is no exception, and the astronomical observatory of the Jagiellonian University in Krakow has discovered a new radio galaxy, which is the largest known object yet in the universe -a cosmic record, no less-.

The radio giant has been named J1420-0545: a bit of an anti-climax there, but then scientists often tend to prefer the prosaic. But it is not just the size of the radio galaxy that is noteworthy; its origin is even more puzzling because the galaxy was formed when the Universe was so dense that such a colossal structure had no business to be born. The radio galaxy is hosted by an elliptical galaxy, which, in optical light, can be observed only by means of the largest telescopes. At the centre of the host galaxy lies a black hole hundreds of millions times more massive than the Sun. The black hole attracts matter from the central region of the galaxy. Such a type of black hole is generally referred to as an active galactic nucleus. The black hole in the radio galaxy J1420-0545 uses some of the matter to increase its mass; at the same time, large chunks of it are thrown away in the form of two oppositely directed jets running at great speed. When such a jet hits the intergalactic medium, a shock wave is created, which acts as a barricade for the particles. Most of these particles rebound, forming extensive lobes (Figure 1). The particles in the lobes, moving along magnetic field lines, radiate radio waves that are captured on the Earth by means of huge telescopes.

Some radio structures are so small that they cannot leave the galaxy in which they were born; others, such as J1420-0545, extend far beyond their host galaxy, even as far as 15 million light years. In other words, it would take 15 million years for the light from one end of this source to reach the other end. What governs the size of the radio galaxy? Since a galaxy continues to grow as it ages, surely J1420-0545 must be the oldest radio galaxy? It is not, as it turns out: at 50 million years, J1420-0545 has come of age only recently compared to other radio galaxies, and we must look elsewhere for an explanation of its immense size.

It is a well-known fact that the black hole inside a host galaxy sends beams of particles only when there is enough gas around it to feed the beam. An active galactic nucleus is not active all the time but works in cycles. Therefore, it is possible that a galaxy forms lobes but then stops supplying them with new particles until a fresh cycle begins. In the older lobes, the particles radiate energy in the form of radio waves and thus slowly lose their energy and literally fade away as their signals become progressively weaker: the old lobes become less visible and new, bright, lobes emerge closer to the host galaxy. Because new lobes expand in less dense medium then the old ones do, they can grow into giant structures, a conjecture confirmed by observations. A dozen or so radio sources are known to have two pairs of radio lobes: an older, more distant, pair and a younger one closer to the centre -like a pair of siblings who cling to their parents-. However, only one pair of lobes is visible in J1420-0545, and we cannot say with certainty whether this radio galaxy is a product of a few cycles of galactic activity. Can something else account for the gigantic size of J1420-0545? Let us look into the past for an answer.

After the Big Bang, the Universe began expanding and the matter that formed the Universe began to get less and less dense. From the opposite perspective, because light travels with a finite velocity, the further the objects that we observe, the further into the past we look and the denser Universe we see. Thus, the intergalactic medium that surrounds distant objects is denser than that which surrounds nearby objects. A less dense medium offers less resistance to the particles in jets, allowing the particles to move faster and farther. Why then has such a huge object been found so far and not in our cosmic vicinity? The answer became clear when we measured the density of the intergalactic medium around this radio galaxy. J1420-0545 is located in a region less dense than the average. The Universe is not homogeneous, and most of the galaxies form large-scale structures in the Universe called walls. There are huge voids between such walls with very low-density intergalactic medium. And it is one such void that our giant is expanding in.

Before J1420-0545 dethroned it, 3C236 was the largest; as we expand the reach of our observations, more distant and more interesting objects may well be found yet. It took 40 years to topple 3C236; let us see how long J1420-0545 continues to rule.