Another record breaker: ultra-deep image reveals ultra-distant galaxy

The entire universe in blog form
Jan. 26 2011 11:03 AM

Another record breaker: ultra-deep image reveals ultra-distant galaxy

Astronomers have just announced they have discovered what may be the most distant galaxy ever seen, smashing the previous record holder. This galaxy is at a mind-crushing distance of 13.2 billion light years from Earth, making it not just the most distant galaxy but also the most distant extant object ever detected!

Here is the object in question:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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The small box shows the location of the galaxy, which is invisible by eye in the image. The zoomed region shows it in the infrared, where it glows more strongly.

[NOTE: Let me be clear up front and say that this is a candidate galaxy, since it hasn't been confirmed using other distance determination methods. However, having read the paper I think the astronomers did an excellent job showing this is very likely to be a galaxy 13.2 giga-light years away. From here on out I'll refer to it as if it's real, but to be fair bear in mind there is some small chance it may turn out not to be real.]

Named UDFj-39546284, the galaxy is seen as it was just 480 million years after the Universe itself formed! The previous record holder -- which was announced just last October -- was 13.1 billion light years away. This new galaxy beats that by 120 million light years, a substantial amount. Mind you, these galaxies formed not long after the Big Bang, which happened 13.73 billion years ago. We think the very first galaxies started forming 200 - 300 million years after the Bang; if that's correct then we won't see any galaxies more than about 13.5 billion light years away. Going from 13.1 to 13.2 billion light years represents a big jump closer to that ultimate limit!

The galaxy was found in the infrared Hubble Ultra Deep Field, or HUDF, an incredible observation where Hubble pointed at one patch of sky and stared at it for 173,000 seconds: 48 solid hours! After Hubble's Wide Field Camera 3 observed it, this supposedly blank patch of sky came alive with thousands upon thousands of distant galaxies, and in fact the last record-breaking galaxy was found in the image. The picture here shows the whole HUDF image, with the first picture at the top of this post outlined. Click it to see it in full size, and you'll start to get an appreciation of just how freaking tough these observations are. The sky is full of faint galaxies!

This new discovery was found using what's called the dropout technique. It works in a clever way: hot stars inside a galaxy can produce ultraviolet light that can ionize hydrogen, that is, remove the electron from a hydrogen atom. So if there is a cloud of hydrogen atoms between you and a galaxy filled with such hot stars, the UV light you see from that galaxy is absorbed handily by that gas, and you don't see the galaxy. However, visible light can pass through the gas, so if you use filters to observe the galaxy, you'll see it in the red filter, the green filter, the blue filter, but then pop! In the UV filter it's gone. The galaxy has dropped out of sight.

The thing is, because the Universe is expanding, light from a distant galaxy gets red shifted -- literally, the light we see from it has a longer wavelength, similar to the way sound from a receding car drops in pitch -- and the farther away a galaxy is the more its light gets shifted. If it's at the right distance, then the dropout happens in the blue filter (as well as the UV), because the UV light emitted from the galaxy has shifted to the blue. A more distant galaxy will have its UV light shifted into the green so the galaxy is now invisible in the UV, blue, and green filters, and so on.

This becomes a handy way to measure a galaxy's distance! All you need to do is observe it in a bunch of filters and see which one it disappears in. While it's a bit crude -- you can't get an exact distance, just a rough estimate -- it works well enough to find the most distant galaxies. Thousands of distant galaxies have been found this way, confirmed later using more sophisticated, accurate, and sensitive techniques.

And that's what these astronomers have done. The image above shows UDFj-39546284 in various filters of the Deep Field; the left image is visible light, and the next three in near-infrared filters. You can see the galaxy suddenly pop up in the H filter, at a wavelength of 1.6 microns (a little over twice the wavelength the eye can detect). They analyzed the light in that filter carefully, making sure the galaxy is indeed real and not some random fluctuation in the background noise, a nearby star, or a closer galaxy masquerading as a more distant one.

The fact that the galaxy is not seen in the shorter wavelength filters means all its light must be redshifted by a factor of about 11.3 (what those in the know refer to as z = 10.3*) , meaning the wavelength has been stretched by that much. Using models of the Universe's expansion and age, the astronomers could then determine its distance of 13.2 billion light years.

In fact, they found several other galaxies in the HUDF at large distances, though none as far as UDFj-39546284. These other galaxies are at redshifts of roughly z = 8, putting them a little over 13 billion light years away, which is still really, really far away.

Let me note that this research wasn't done just to break a record. There's real science here, and important science. The brightness of the galaxy reflects how many stars are forming there, so comparing galaxies at a z = 8 and 10.3 tells us how the Universe was changing over time when it was young. What was found was that the star formation rate increased rapidly between those two epochs. That's interesting! We know that if we start at the present and wind the clock backwards, we see stars forming more rapidly in the past than today. But if you go far enough backwards that trend reverses, and apparently at some point between 480 million and 600 million years after the Big Bang star formation rate really hit the gas. So to speak.

Not only that, but the number of galaxies seen at that whopping distance will tell us how galaxies formed, too. As more of these galaxies are detected -- especially with the upcoming James Webb Space Telescope, which should find them by the hundreds or more -- we'll start to get a grip on just what our Universe looked like when it was very young.

All of this is part of the ultimate goal of understanding the Universe itself: how it was formed, how it's aged, what's inside of it... and maybe even if the Big Bang itself was a singular event, or if there could be other Universes out there -- if we're part of a bigger metaverse. All those answers are out there, waiting to be found. And the deeper we look, the more answers we'll get... as well as more questions. But that's why this is so much fun!

Image credit: NASA, ESA, Garth Illingworth (University of California, Santa Cruz) and Rychard Bouwens (University of California, Santa Cruz and Leiden University) and the HUDF09 Team.



* That's not a typo; z refers to a mathematical term that is the actual redshift factor minus 1. It makes the math easier to use it that way.



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