Is this the most impossible galaxy?
Hoag’s Object is strange enough as it is, but astronomers can also see a galaxy within the galaxy within a galaxy
It's a galaxy within a galaxy within a galaxy: but how?
The universe is a gorgeous place, but some images have a greater power to stop you in your tracks than others. “Hoag’s Object is fascinating because of its aesthetic value; it’s just a beautiful thing,” Dr Noah Brosch tells All About Space about one particular intergalactic gem. Indeed, one glimpse at its perfect circular ring of blue stars around a golden central ball and there’s no denying that most people would find it hard to tear their eyes away.
The ring-shaped galaxy was discovered by American astronomer Arthur Hoag in 1950. He had been working at the US Naval Observatory and originally thought the object was an ejection of ionised gas from a red giant star in its last throes of life – a planetary nebula.
Rejecting his own hunch on the grounds that the ring’s light was not being emitted at the expected wavelengths, he later theorised that the whole thing may have simply been an Einstein ring – an optical illusion caused by gravitational lensing of the source’s light. After all, in the very early images of the galaxy, a blue ring was seen to encircle a yellow core, and the Einstein ring was a suitable explanation. As clearer images of Hoag’s Object came into view in the late 1980s, however, that idea was soon dismissed too.
What remains is a mystery that astronomers are keen to figure out. The pictorial evidence is of a galaxy within a galaxy – two stellar circles separated by a dark gap – but studies have shown that they are both at the same distance from Earth since they have an identical redshift. This
points to them being one galaxy rather than two. In actual fact, it’s a peculiar galaxy quite unlike the majority of others. It is an immense eyecatching structure of gigantic proportion.
In terms of specifics, the red, dead inner core is 17,000 light years across. The dark ring around it measures 50,000 light years and the overall object from one side to the other is 120,000 light years, making it slightly larger than the Milky Way. The outer bright ring of stars is 70,000 light years away from the central component. What is perhaps most stunning, though, is that if you look even closer you can see another ring-shaped galaxy in the gap, pointing to a galaxy within the galaxy within a galaxy. It is simply jaw-dropping.
This has certainly been the takeaway of an image that has been doing the rounds of late. Taken by the Hubble Space Telescope and processed by geophysicist Benoit Blanco, the billions of blue stars are clearly separated from
the dense inner sphere made up of reddish stars. The other ring galaxy is visible as a red circle within the empty space – it’s actually far away and not part of Hoag’s Object itself. But the fact that it lines up in such a way that telescopes can pick up on it is nevertheless startling and a real cosmic coincidence.
“I think it’s incredible that there’s a ring galaxy in the background of a ring galaxy and that’s why I choose this object for an advanced processing project,” Blanco tells us, having made use of an artificially intelligent denoising algorithm. “I think it illustrates how infinite our universe is – the fact that in this case, you can look at a very unusual object and find one at the back.” But how are these peculiar galaxies forming?
First things first. Hoag’s Object – which is 600 million light years away from Earth towards the constellation of Serpens – is not a one-off, or even a two-off, given that we can see another peeking from the back. “Tens or more ring galaxies are known, but because of inclination and distance, none are as impressive as Hoag’s,” explains Brosch, who works at the department of astronomy and astrophysics and the Wise Observatory at Tel Aviv University in Israel.
Even so, that only accounts for about 0.1 per cent of all known galaxies, and it doesn’t make such peculiar galaxies, which also include NGC 1291 and PGC 1000714, easier to fathom. There have even been suggestions that an intelligent species is behind the phenomena, but those are perhaps the easiest to discard. “I think that explanation is all wrong,” Blanco says. “When a very unusual event is observed in the universe, we think about aliens, but it’s just that the universe is infinite and everything that can happen does seem to happen.”
So what could account for the processes involved in Hoag’s Object’s formation? “The most probable explanation is that a spiral galaxy has been captured by a spherical galaxy, but it’s unique for such a collision to create a perfect ring,” Blanco says of an event which would have affected its gravitational pull. Such a thing is believed to have happened to other ring galaxies, resulting in new elliptical shapes, mergers or density waves. “It’s likely to have happened because of a random collision 3 billion years ago that was so perfectly done, a part of the spiral material fell into the spherical and the remaining spiral created a perfect ring,” he continues. Yet no one can say for certain.
Indeed, in 1985 Brosch suggested that Hoag’s Object was a result of instability in a barred spiral galaxy billions of years ago. He theorised that the bar would have rotated, forcing gas to be pushed to
the outskirts where a ring would not only form, but stars and planets would end up being created.
Brosch also hypothesised that any old and dead stars would sink into the nucleus as the bar became used up and frittered away. But other scientists argued that it didn’t account for why that central part of the galaxy is spheroidal rather than misshapen or elongated. With no evidence that the galaxy was barred to begin with, competing theories were worked upon.
In 1987 François Schweizer, W. Kent Ford Junior, Robert Jedrzejewski and Riccardo Giovanelli suggested Hoag’s Object had formed from a merger with a smaller, lightweight galaxy. The gravity of the heavier, elliptical galaxy tore the visitor in such a way that the gas settled into orbit. With the heavier galaxy forming the core, the gas lends sufficient fuel for the formation of stars that eventually produced a separate ring. In this sense it would be on the same lines as a polar-ring galaxy, which are thought to occur when a couple of galaxies gravitationally interact with one another, producing an outer rotating ring of gas and stars.
One problem is that there is so much gas and stellar mass in that ring that it would have to have come from a galaxy far greater than a dwarf, in which case the disruption to the original galaxy would have been so great that Hoag’s Object could not have formed in the way it has. There is also an issue of there being no sign of a second galaxy that could have collided, even with the use of the most sensitive of telescopes. To that end, Brosch revisited Hoag’s Object in 2011 with Ido Finkelman,
Alexei Moiseev and Ivan Katkov. Their paper said the elliptical core formed in the early universe, with the surrounding disc being created afterwards by “prolonged ‘cold’ accretion of primordial gas from the intergalactic medium”.
In many ways their study was a continuation of Schweizer’s theory. Kinematic data suggested Hoag’s Object is a normal disc galaxy with a lowdensity hydrogen iodide disc that accretes onto the spheroid, and because of its lack of density forms stars in the ring. It’s a theory that supposes gas floating in space is pulled in via gravity.
“My personal opinion is that the central ellipticallike object is old,” Brosch affirms. “The star-forming
“I think it’s incredible that there’s a ring galaxy in the background of a ring galaxy; that’s why I chose this object”
Benoit Blanco
ring is relatively young, and was formed by recent accretion of gas-rich material.
“This happened either by the tidal disruption of a dwarf galaxy followed by the ring formation, or by the direct accretion of gas and dust from intergalactic space with the inflowing material coming along a dark-matter filament that did not reach to the centre where the elliptical core is.”
But what is stopping the outer ring from being pulled into the core? Why is this massive ring with ongoing star formation leaving a great gap between the two parts? To help answer this, in 2018 Elena Yu Bannikova from the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine wrote a paper called ‘The structure and stability of orbits in Hoag-like ring systems’.
In it she sought to explain where the forces between the core and the ring are gravitationally balanced. Through a series of mathematical equations she found that this was on the inner section of the ring – an unstable equilibrium called the Lagrangian circle. Furthermore, she found the outermost stable circular orbit (OSCO) is around the central galaxy.
“The masses of the central galaxy and the ring around it are comparable with each other,” she tells
All About Space. “The gravitational forces from the central galaxy and from the ring are acting in different directions, so there is a region where the gravitational field of the central galaxy is much stronger than from the ring, and the stars can move here in a circular orbit. There is also a region where the gravitational force from the ring is much more, so the stars will move around the ring.
“But there is also an intersecting region where particles and stars cannot move in a circular orbit due to competition between the gravitational forces from the central galaxy and the ring. We can say that the particles in this region do not belong in terms of gravitational attraction to either the central galaxy or the ring. It was surprising for me.”
This accounts for why there is a gap in the matter distribution “where the circular motion is not possible due to the competition between the gravitational forces by the central mass and the ring” – that is, a great area where no stars or gas particles can orbit. It doesn’t explain how it got to that state in the first place, but it points to why the ring remains and perhaps how the ring grew.
So what of that other ring-like galaxy that can be spotted far away behind Hoag’s Object – the one catchily called SDSS J151713.93+213516.8 that has had a fair bit of recent media coverage since NASA made it its astronomy picture of the day on 27 November 2019? Unfortunately, although it can be clearly seen between the nucleus and the outer ring, it is too distant to get a proper close-up look, but the fact that it is there, in the same image as one so stark as Hoag, is simply delightful given their rarity. Both are likely to continue to fascinate astronomers for many years to come.