If you were to ask me “What is the weirdest galaxy we know of?” I’d without doubt say Hoag’s Object.
A Quick Backstory
Hoag’s object was discovered by Arthur Hoag in 1950 and at the time our knowledge of the universe couldn’t give us a precise description of the galaxy or even if it was a galaxy! That’s understandable, given that Hoag’s object clearly doesn’t look like a spiral or elliptical.
You’ll be pleased to know our advancement in science has not brought us much closer to understanding Hoag’s object, but at least we know that it’s some sort of irregular Ring Galaxy and it’s visible baryonic matter makes it about half the size of the Andromeda galaxy. However it has only around 8 billion stars, meaning it has fewer than 1% of the stars Andromeda has.
So What is Hoag’s Object?
I think you can easily tell Hoag’s object has a very different structure to your average spiral or elliptical galaxy. Astronomers class it as a ring galaxy, and as far as we know Ring galaxies don’t form “naturally” in a sense. A galaxy will not grow into a ring unless there’s some interference, which astronomers nowadays believe is usually a collision.
In the centre we have a gigantic yellow ball called the nucleus where we will likely find the usual: a supermassive black hole, and hot dense gas and some old yellow stars. And on the outer ring we have a lot of blue, which comes from stars that are on the hotter and more massive side of the stellar spectrum.
These characteristics are what you’d see in an average galaxy, but what’s really interesting is the lack of stars right outside the nucleus. There could be a few stars of clusters within the gap, but that’s not confirmed. In fact, it’s so sparse you can see other galaxies behind it, it’s like a translucent galaxy in a way.
On top of that, on the outskirts of the galaxy there exists a ring of hydrogen gas, obtained using the Westerbork Synthesis Radio Telescope.
Actually if you taka a look in the top of the ring gap, you can see yet another ring galaxy behind Hoag’s object. (SDSS J151713.93+213516.8). That is absolutely hilarious since ring galaxies themselves are so so rare to come by in the first place!.
How Did Hoag’s Object Form?
Ring galaxies in general are not well-known for sure, but it’s likely some rings have been caused by a collision. A galaxy, typically a spiral, is violently pierced by another galaxy referred to as a “bullet”, through the centre. This destabilises the galaxy and causes outward rippling that reorganises the galaxy into a ring, clustering everything outside the bulge of the galaxy into a ring.
It’s a bit like a drop of water hitting a pond or something similar. The drop causes waves in the pond to form and move outward. The drop is like the bullet galaxy, and the wave is a cluster of stars.
As these galaxies are few and far between, no astronomer has observed such an occurrence (or at least saw it and wrote about it). It is understandable, as collisions between galaxies are super rare, and ring galaxies form mostly from a bullseye collision!
A ring galaxy that we’re pretty confident in saying was caused by a collision and suits our theory is in fact, our dearly beloved Cartwheel Galaxy and her 3 friends G1, G2, and the never imaged G3.
The Cartwheel galaxy (the larger galaxy in the image) was for sure involved in a head-on collision, causing a shock wave through the galaxy and separating (though not completely) the galaxy into a centre and two rings. The outer ring is blue like Hoag’s object, and there’s a coffee coloured inner ring which blends well with the nucleus. It is believed that it’s third and unimaged companion galaxy, G3, was the bullet that shook the Cartwheel galaxy, since it’s connected via a Hydrogen tail.
How Does this Connect to Hoag’s Object?
To put it simply: We have no clue!
There are no suitable objects in Hoag’s vicinity that could have collided, not even galaxy remnants or fragments. This could either mean a collision never happened or it happened so long ago, I’m talking billions of years ago, that any piece of evidence has long gone, and we know from radio data that no galaxy has been eaten by Hoag’s Object for at least a billion years.
That being said, Hoag’s object has some peculiarities. The centre of the galaxy moves unreasonably slower than the outer blue stars. Could it be that two galaxies collided billions of years ago and now after so much time they’ve merged into some sort of Franken-galaxy?
The truth is, we’ll likely never now what happened until we find more objects like it.
If you prefer, this post in in ✨video ✨format! That’s right, I now have a YouTube channel! I would love to see you there!
And thanks a bunch for amazing people like James Pailly for all the support towards my work! It means the world to me, and I wish I could give back more to you guys!
Have an awesome day!