Un objet dans le ciel défie tout ce que l'on connait des galaxies !

In 1950, the American astronomer Arthur Hoag was looking in the sky of the planetary nebulae, large gas clouds in expansion associated with the death of a star. During one of his nights of observation, he had the surprise of making a very strange discovery. For the first time, he identified a perfect circular object, like traced to the compass, something we usually never saw in the sky. He drew the attention of the community of astronomers on the matter and offered his first speculations. Maybe a nearby planetary nebula or a much farther galaxy. Unless it's a gravitational lens effect. From that moment, the body adopted its name. It became the object of Hoag. Very studied during the following decades, he delivered some of his mysteries, while others are, on the contrary, still so thick. We know today what the object of Hoag is, but we can't say how it was formed. Today, we're going to take the time to discover one of the most fascinating mysteries in the sky and the story as strange as it could tell. Observed in visible light, like here by Hubble, the object of Hoag appears like a bright yellow ball surrounded by a blue ring. Between the two, the void. No bars, arms or even gas. The two regions that form it are completely separated, almost as if they were two different objects. Today, we know its distance. It is located 600 million light-years from the Sun. So it is clearly not part of the Milky Way. It is not a planetary nebula. The object of Hoag is an entire galaxy. We can put it in the category of ring galaxies, even if it is distinguished by certain aspects of it. Ring galaxies indeed have this same general structure, a ring that surrounds the galactic center. We know a few tens of thousands, and they are all quite different from each other. They have irregular structures and they shelter gas in the part that separates their center from their rings. By chance, there is a ring galaxy that evolves in the eye of the object of Hoag, between the galactic center and the ring. This already strongly distinguishes this class of objects from classic galaxies. We are quite far from the shape of the Milky Way with its long spiral arms. It's not a matter of size. Some ring galaxies and even the object of Hoag are about the same size as the Milky Way. So what drives them to adopt such a particular shape? There are several theories here. The most accepted is that ring galaxies are the result of a collision between a large flat galaxy, like the Milky Way, and a smaller galaxy. When the small galaxy crosses the big one, its gravity will attract billions of stars as well as gas clouds. This will create a compression wave, what astronomers call a density wave in the big galaxy. This will push stars outwards and create a new generation of young stars by concentrating the gas. This is how we would end up with a kind of ring of young stars that surrounds a center of older stars. This is also why the majority of ring galaxies would have an irregular shape. They would have been deformed by the collision. And that's what's wrong when we try to adapt this scenario to the object of Hoag. This one does have young stars in its ring and older ones in its center. Exactly what we would expect to see after a collision between a galaxy and its density wave. Its absolutely perfect circular shape however leaves room for doubt. After a collision, we should see the remains of the perturbation, at least a small asymmetry that would point to the violence of the event. We would also expect to see the ring extend outwards under the effect of the wave. But there is none. Measurements have shown that the ring of the object of Hoag is perfectly stable. It's as if it had been there, completely circular, for billions of years. In addition, the impacting galaxy should also be visible. There is nothing around the object of Hoag, no companion galaxy, not the slightest trace of a nearby passage. If there was a collision, then the culprit flew away without leaving a trace, which is impossible. Finally, the gulf between the center and the ring is perhaps the most disturbing. In a classic ring galaxy, this region is full of dust and gas. The galaxy of the wheel of Chariot is a typical example. In the object of Hoag, we have the impression of a surgical cut between the two regions, as if they had nothing to do with each other. Another possible mechanism shows a gravitational evolution from barred spiral galaxies. Barred spiral galaxies, like here NGC 1300, have a central bar from which the arms extend. It is probably a temporary structure, once again due to the internal density waves to which the galaxy is subjected. In some cases, it would end up looking like a classic spiral galaxy. But in other cases, the bar would act as a kind of front, concentrating matter on the external part and in the center of the galaxy. The bar itself could end up dissolving, and this is how some ring galaxies could be born. Galaxies like NGC 1291 have probably been subjected to this phenomenon. Although it is a ring galaxy, we can still distinguish the bar in the latter. Again, this explanation could work for some aspects of the object of Hoag. The transition from barred galaxy to ring galaxy is supposed to create a ring of young stars, as our previous explanation. This would be due to the fact that the bar concentrates matter, notably gas clouds, towards the external part, thus giving birth to a new generation of stars. We could thus find the general properties of the object of Hoag by imagining that its bar has dissipated to give way to a perfect ring. But again, this explanation also faces difficulties. In the ring galaxies that have experienced this process, like NGC 1291, we can still see traces of the bar, a slightly stretched center, and residues of spiral arms extending in or around the ring. These are particularly visible on a galaxy like NGC 7742. The latter is a kind of hybrid between a ring galaxy and a spiral galaxy. It has a very yellow center, a bluish ring, and spiral arms on the outside. Unfortunately, it does not look at all like what we observe for the object of Hoag. The latter does not present the residues of a bar in its center, nor the slightest fossil of a spiral arm. It is as if the galaxy had always been a ring. If the object of Hoag once had a bar, it completely erased its trace. This would be a unique case in the catalog of ring galaxies. A third possible explanation puts at stake an accretion system. The external disk could have slowly formed from intergalactic matter, which would have found a stable orbit. The object of Hoag would have slowly accumulated hydrogen from the intergalactic environment to form a new generation of stars in its ring, at a good distance from its center. It is a scenario that does not require any major disturbance, and which can therefore be attributed to the very geometric appearance of the object. This would also explain why the heart and the ring appear chemically different, one being rather yellow and ancient, and the other blue and recent. To work, this explanation requires, however, that the accretion has been roughly balanced in all directions, and that the heart itself is very stable and round without major irregularities. This is what would allow the gravitational capture of intergalactic gas without forming arms. Finally, we can find articles that defend slightly more exotic hypotheses. My favorite is that of an American physicist who sees the possibility of a type 3 civilization, which would have domped and reorganized its entire galaxy. This would explain why it appears so imperturbable and perfect. A megastructure at a scale simply unimaginable for humanity, extending over more than 100,000 light years. This does not tell us why an intelligent species would specifically choose this galaxy design, but why not? We can at least keep the idea in the back of our minds. If we ever catch a strange infrared signature in the ditch that separates the center from the ring, it could really become interesting. The last clue that has been explored for some time is to consider the object as two galaxies, which would be superimposed by the chance of an alignment. Only deviations to the red have confirmed that the two parts of the object have the same distance. It is one and the same object. For the moment, there is no perfect explanation for the object. A collision that leaves no trace, an old bar completely erased, an accretion of improbable gas or a civilization in unimaginable power. Most likely, what gave this appearance to the object has happened a long time ago. This ring has not formed recently, without which we would have a much firmer clue to trace its origin. Galaxies with rings as a whole are quite rare objects. They represent 0.1% of the known galaxies. It is therefore possible that the object of Auge is the only one to have such a perfect ring. In any case, for the moment, it has not been found an equivalent. The strangest mystery of the object of Auge may remain its central void. Between the heart and the ring, it extends over 70,000 light years. A huge distance where there seems to be absolutely nothing. Not a single star or even the smallest cloud of gas. It is a unique configuration that must give rise to strange observations. What does the starry sky of a civilization that would live in the central core look like? And what would another civilization see evolving on the periphery? Despite this amazing structure, the object of Auge remains an entire galaxy, with billions of stars. It is therefore very likely to see life emerge. A more detailed study of the two components of the object may one day be able to reveal its secret. The small variations in the ring or the precise shape of the center may be detailed by future observatories. We will then finally be able to unfold the unique scenario that allowed the creation of this galactic eye. This is the end of this space file. I invite you to share your questions and remarks in the comments. We will answer them in a future video. You can consult the description to find our sources and social networks. As for us, we will meet again next week for a new DNDE. And until then, citizens, take care!