Euclid is a European Space Agency astronomical mission that is surveying the cosmos, mapping galaxies over an incredible 1/3^rd of the sky. It does this at very high resolution as well, about 0.1 arcseconds — the full Moon is 1800 arcseconds wide on the sky, so Euclid can see astonishingly small details. It also is phenomenally sensitive, able to see very faint objects. It needs to: its goal is to look at dim, distant galaxies to help us understand the nature of dark matter, the 3D structure of the Universe, and how the cosmic expansion is accelerating. Those are all big deals.

Because it’s looking at such a huge area of the sky, it sees objects closer to us as well, and sometimes catches a surprise. With NGC 6505, it really did: Euclid images of this elliptical galaxy show a clear and bright Einstein ring, the effect of strong gravitational lensing!

OK, I’ll explain that, but first, the spectacular image:

Whoa. NGC 6505 dominates the image, a huge elliptical galaxy with a bright core that fades away with distance. But look again: right in the center, circling the core, is a perfect circle of light.

That’s the ring. And it’s not really in NGC 6505 at all: it’s actually an entirely separate elliptical galaxy far, far in the background. This more distant galaxy’s light has been amplified and warped into a ring by the gravity of all the stars in the middle of NGC 6505, acting like a lens. We call this phenomenon a gravitational lens, in fact.

NGC 6505 is about 590 million light-years form Earth, but the galaxy being lensed is 4.4 billion light-years from us, over seven times more distant [link to journal paper].

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I’ve written about gravitational lensing many times in the newsletter, but the most detailed explanation I wrote was on The Old Blog™. In a nutshell, matter warps the fabric of spacetime, which we perceive as gravity. Light, as it moves through space, has to follow this bending of spacetime as it moves, like a car driving over a winding road. This has some weird effects. For example, a beam of light traveling toward us might pass “above” a massive object like a galaxy, so its trajectory is bent down, toward us. If the intervening galaxy weren’t there, that beam would miss us, but the gravity of the galaxy bends it toward us, so we see an image of the background object above the galaxy. But another beam going under the galaxy gets bent upward toward us, so we see another image under the galaxy. In this way, gravitational lensing can create multiple images of a single background object.

It can also create arcs of smeared out light, depending on the geometry. And, if conditions are exactly right — the background object is aligned smack dab with the center of the intervening galaxy — the light going in every direction around the core of that galaxy gets bent toward us, so we see a perfect circle of light around the galaxy! 

That’s what’s happening with NGC 6505. The distant galaxy, the core of NGC 6505, and Earth are on a straight line so we see that circle, called an Einstein ring because he was the smartypants who predicted gravitational lensing.

In that close-up above you can see the ring really is a circle. The four brighter clumps of light are the central part of the background galaxy, seen multiple times due to lensing. Four images, all of the same galaxy! Lensing is bizarre.

But useful. It can be used to map out characteristics of the central part of NGC 6505, because the strength and shape of the lens is determined by the mass in its core. Astronomers used the image to determine that the mass in NGC 6505 causing this ring (the combined mass of the stars and dark matter we see inside the ring) is equal to about 100 billion Suns. That’s a lot of stars!

All galaxies have a supermassive central black hole as well, but they find the one in NGC 6505 to have only (“only”, heh) 900 million times the Sun’s mass. That’s big, but a small fraction of the mass of stars creating the lens, so it doesn’t contribute much to the lensing.

What’s really cool about this are the implications for Euclid. NGC 6505 is pretty close as galaxies go; the overwhelming majority of galaxies Euclid will be looking at are much more distant. The odds of a galaxy like NGC 6505 having such a strong lens are about 1 in 2,000, so this was lucky! It also implies there are anywhere between 4 and 20 more yet to be found at that same distance or closer to us. Euclid is expected to find over 100,000 lenses in its complete survey! Euclid only launched in 2023 and still has a six-year mission to map the sky, so there’s lots of time left to find them.

I was surprised the ring hasn’t been seen before, to be honest. Any galaxy with an NGC in its name is bright and close to us, in astronomical terms. The New General Catalog was initially compiled in 1888! So these are the easiest galaxies to see with big telescopes. Big digging around I only found about 15 papers that mention NGC 6505 at all, so it’s not well studied. Had Hubble or JWST looked at it the ring would’ve been found. Euclid, though, has an advantage because it sees so much of the sky over time. Hubble and JWST have tiny fields of view, like looking at the sky through a drinking straw. You have to point Hubble and JWST at the galaxies you want to see, while Euclid just sweeps them up naturally, so it’s able to make more discoveries like this (despite a literally shaky start). It should increase the number of known nearby strong lenses by a factor of five!

Euclid is an amazing mission and has a big goal for astronomers, so I’m very excited to see what it will show us in the coming years. The science will be incredible as well.

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