The Horseshoe Galaxy

Horseshoe galaxies and hand grenades

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Recently, I wrote about a collaboration between amateur and professional astronomers: Its purpose is to use the expertise many amateur astronomers have in image processing to create better and more dynamic images of planets to support planetary science and spacecraft.

The results were spectacular. But they were using a 1-meter telescope; small by pro standards these days, but still pretty big, and located on a distant mountaintop, as well. What happens when you try something like this with a sub-1-meter ‘scope?

Well, let me introduce you to the work of Rolf Wahl Olsen, a Dane who now lives in New Zealand. Armed with a 32-centimeter telescope he built himself, he takes images that are of superior quality. But he’s not content to just take pretty pictures: He goes after unusual objects, ones that are very difficult to capture, and, to be honest, ones I would’ve thought would be well beyond the capability of small telescopes. But, again and again, Rolf flips my expectations around.

His latest work is crushing my brain. Using his home-built ‘scope, he managed to get an image of a galaxy billions of light years away. And not just any galaxy, but the Horseshoe Galaxy: one of the finest examples of gravitational lensing found, and one so small and faint — and bizarre — it was only discovered in 2007!

The Horseshoe Galaxy

The Horseshoe Galaxy imaged using a 32 cm telescope (left and upper right), with a total exposure time of 9.5 hours versus Hubble in 3 hours (lower right). Credit: Rolf Wahl Olsen & ESA/Hubble & NASA


This’ll take a moment to explain…

The wide-angle image is lovely, with red and blue stars dazzling. You can also see quite a few fuzzy, distant background galaxies, huge cities of stars like our Milky Way but reduced by distance to faint smears.

But if you look carefully, you can see one (outlined) that’s different than the rest. Even zoomed in (upper right) the Horseshoe Galaxy at first doesn’t look like much; just a red star-like object with a faint, discolored halo around it. An image taken by Hubble (bottom right) shows it more clearly, though it’s still odd-looking, like a blue galaxy smeared around a red one.

Because that’s exactly what it is. Or really, what it looks like. What you are seeing here is an amazing collision of geometry, relativity, gravity, and luck.

I’ve had many occasions lately to write about gravitational lensing. This is when the light traveling through space from a distant background source is bent by the gravity of some other object. Einstein, himself, came up with the idea that what we perceive as gravity is really a warping of space. It’s similar to the effect of putting something heavy like a bowling ball on a mattress. The mattress bends, and if you roll a marble past it, the path of the marble will curve.

It light passes a massive object like a galaxy, the path it takes will bend. Anything that bends light is called a lens, so we call this a gravitational lens, and they can cause all kinds of weird effects, including magnifying the brightness of the object as well as distorting its shape. It depends on the mass of the lensing object, how it’s distributed, and the precise alignment of the background source, the lens, and the observer (us).

This video shows gravitational lensing by a galaxy, but the principle is the same. Credit: ESA/Hubble, L. Calçada


If the Earth, the lensing object, and the more distant source are in a perfect line, the lensed source will appear like a circle, a ring, around the lensing object. That’s because light initially sent out by the background galaxy in a direction not exactly toward us can get bent enough to head straight at us. So, light sent slightly to the left, or right, or above, or below (in terms of what you see in a photo, I mean) by the galaxy will all get bent toward us, and what you wind up seeing is a ring. In fact, we call them Einstein Rings.

That’s what you’re seeing with the Horseshoe galaxy! Now, get this: The blue galaxy, itself, is a staggering 11 billion light years away, more than three-quarters of the way to the edge of the observable Universe! That means we’re seeing it as it was 11 billion years ago, when it was young. At that time, it was actively making stars, so it appears quite blue due to the massive, hot, and very luminous blue stars that were being born.

The light from that galaxy traveled a long way to get here. But, on the way to us, it got distorted: A very massive galaxy, called LRG 3-757, was in the way. This is a massive galaxy, perhaps ten times as massive as our own Milky Way, about 4.7 billion light years away (so, less than half the distance to the Horseshoe) and possesses a strong gravitational field. It bent the light from the more distant galaxy, and we see that as a nearly complete circle of blue light around the red galaxy. It goes about 300° around, making it one of the more complete Einstein Rings ever found.

Horseshoe galaxy

The Horseshoe Galaxy imaged by the Hubble Space Telescope in 2010. This image is comprised of three filtered observations made by Hubble, for a total exposure time of about 3 hours, and approximates a “natural color” view. Credit: ESA/Hubble & NASA


Mind you, the Horseshoe galaxy is probably a lovely spiral galaxy, but we see it as a circle due to this distortion. It was discovered in a survey by astronomers looking for gravitational lenses, and follow-up observations determined its nature. These are important objects, because they can tell us a lot about the mass and distribution of matter in the lensing galaxy, which would otherwise be very difficult to determine.

And that’s why it’s staggering that Rolf was able to detect it! These galaxies are tremendously far away, very faint, and so close together that being able to separate them is a feat unto itself. Of course, the Hubble image is clearer and brighter, but it has a lot of advantages: it uses a mirror over seven times wider than Rolf’s ‘scope, and it’s up in space above our soupy atmosphere.

But the fact that Rolf could capture this at all is remarkable! He took a total of 9.5 hours of exposures to create it (the Hubble image is about three hours total). The camera he used, the QSI 683wsg, costs about $4000, and is very high quality. I’m not sure how much the Wide Field Camera 3 on Hubble cost, but the Hubble camera I worked on, called STIS, was around $100 million. So, that also may give Hubble something of an unfair advantage.

Still, they compare pretty well, I think, given the different circumstances. Close enough, at least. And you know what they say about Horseshoe galaxies and hand grenades.