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SYFY WIRE Bad Astronomy

A distant galaxy pushes Hubble beyond its limits

By Phil Plait
The “true” shape of the lensed galaxy (right, inset), reconstructed by mapping out the structure of the galaxy cluster and determining how it distorted the light of SGAS 1110 on its way to Earth. Credit: NASA, ESA, and T. Johnson (University of Michigan)

I’ve written about gravitational lensing a lot the past few months, but that’s because it’s so dang cool. It distorts the shapes of distant galaxies into incredible patterns, it can slide the apparent positions of stars around, it can make horseshoes out of spirals, and it can brighten distant objects so much that we can spot an individual star at the staggering distance of nine billion light-years from Earth!

And here’s a new one: It has also allowed astronomers to push Hubble well beyond its internal limits, giving it the ability to see clumps of star forming gas with 10 times better resolution than it could on its own.

This is pretty amazing.

Briefly, gravitational lensing is when the gravity of a massive object — a star, a black hole, a galaxy — bends space around it, causing light passing by to curve, like a car following the curve of a road. Einstein first proposed this, saying that matter bends space and we perceive this bending as gravity. So we call this gravitational lensing; the object bending space is the lens, and the object whose light is distorted the lensed object.

The first experimental proof of this was done in a 1919 solar eclipse, when it was shown that a star (the lensed object) very near the Sun (the lens) at the time of the eclipse had its position distorted; the Sun’s gravity bent the path of that star’s light on its way to Earth by a measurable amount (it was done during an eclipse because otherwise the Sun’s fierce light would have swamped the star’s far more feeble glow).

Nowadays we’ve seen this effect countless times. It’s a little odd; not only can a lensed object’s position in the sky be moved around a little by this effect, but it can also be distorted, stretched out like taffy, even wrapped around the lens like a ring (we call those Einstein rings, in fact), and made far brighter by the lens than it would be otherwise.

The galaxy cluster SDSS J1110+6459 can be seen here in the as hundreds of galaxies held together by their mutual gravity. The blue arc of the lensed galaxy SGAS J111020.0+645950.8 can be seen on the left.

And that brings us to the object SGAS J111020.0+645950.8 (let’s call it SGAS 1110 for short). It’s a galaxy far, far away: 11 billion light-years from Earth, in fact. That puts it nearly clear across the observable Universe! It’s smaller than the Milky Way, about 25,000 light-years across, with only a couple of billion stars in it (the Milky Way has more like 200 billion stars). Normally, a galaxy like that would be very faint and difficult to see … but I bet you can guess where this is going.

Yup. Between us and SGAS 1110 is a massive object, or more accurately a collection of massive objects: a galaxy cluster about 6 billion light-years from us. Called SDSS J1110+6459, it’s a huge group of galaxies, with a combined mass of several hundred trillion times the mass of the Sun: The equivalent of hundreds of Milky Way galaxies. It’s a big cluster.

Big enough to radically distort the light from SGAS 1110 on its way to us. Due to the quirks of gravitational lensing, the more distant galaxy’s light was 1) magnified by a factor of 30, making it appear far brighter than it would have otherwise; 2) spread out substantially, making it look bigger; 3) copied multiple times, making three separate images of it; and 4) aligned such that the three projections of the galaxy make a long, thin arc.

That’s amazing enough, but here’s the fun part: This arc was found by astronomers looking for strong examples of gravitational lensing, in the hopes that they could learn more about these distant galaxies by capitalizing on the lens making them brighter and bigger. The idea was that they could use sophisticated software to invert the process, and create an image of what the lensed galaxy would look like without the distortion.

The “true” shape of the lensed galaxy (right, inset), reconstructed by mapping out the structure of the galaxy cluster and determining how it distorted the light of SGAS 1110 on its way to Earth. Credit: NASA, ESA, and T. Johnson (University of Michigan)

So once the arc was found in a survey, they pointed Hubble at it to get much sharper images. They remapped the galaxy to see what it looked like sans lensing, and what they found was a big surprise: They could see two dozen bright clumps of light in the galaxy. These are gas clouds collapsing to form stars, something, to be fair, we see in many galaxies.

But in this case things are different. These clumps are so small that without the lensing effect they would have been too small for Hubble to see; the galaxy would have looked smooth. But the magnification made them visible to Hubble, and their sizes measurable. And this is where the astronomers got a shock: The clumps of gas were far smaller than expected, only about 200 light-years across.

Why is this surprising? Up until now, both theory and observations supported the idea that when the Universe was young, star-forming regions were generally about 3000 light-years across. This is the size where the gravity of the gas can overcome its internal pressure and collapse to form stars. But these new observations show that’s wrong: Much smaller clouds can collapse, only a tenth the size as previously supposed (and maybe even smaller; the Hubble image indicates a background glow in the galaxy that may very well be even smaller gas clouds forming stars, and they’re so small they blur together to form that smooth-looking glow).

That’s a big deal. In the time since the Universe began, the rate at which stars are born has changed a lot. It peaked about 10 billion years ago, and by 9 billion years ago fully half the stars we see today were born (our Sun is a relative latecomer, forming just 4.56 billion years ago). We see SGAS 1110 as it was 11 billion years ago, when the star formation rate was ramping up (we see it forming stars over 8 times faster than the Milky Way is now).

Mind you, there’s likely nothing terribly special about this galaxy; it just happens that we can see it due to a quirk of geometry, so we can tentatively assume it’s typical for galaxies at that time. And what it’s showing us is that typical galaxies 11 billion years ago were forming stars in much smaller gas clouds than we thought they could. Today, in the local Universe (which we see as it is pretty much “now” compared to far more distant galaxies), stars are born in clouds that can be as small as a light-year or two across, with the bulk of star formation happening in clouds dozens to a couple hundred light-years in size. It was thought that wasn’t possible in the early Universe, but now we see it not only is possible, it was happening.

And now we need to figure out why.

And that’s the whole point to this new research. The astronomers are looking to gravitational lenses to literally help magnify what was going on back when the Universe was shiny and new, so that we can better understand its behavior and make sure that the physics we’re using to study it accurately reflects what was going on.

In this case, it looks like it’ll take some rethinking. But ask any astronomer, any scientist, what they think when something like this happens, and they’ll almost all tell you the same thing: That’s where the fun is.

Another puzzle to solve! That’s one of the things science is all about.