Most distant object ever seen... maybe

The entire universe in blog form
May 25 2011 12:07 PM

Most distant object ever seen... maybe

Is this the most distant object ever seen?

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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[Click to deathfromtheskiesenate.]

That is GRB 090429B, a gamma-ray burst (or just GRB to those who want to sound nerdcool), the catastrophic and extremely violent detonation of a massive star. Think of it as a super-supernova, the death throes of a star that lived a short, hot, turbulent life. I wrote about them extensively in my book "Death from the Skies!", or you can get the details about how they form and why they're so awesome in an earlier post.

Its distance is estimated to be a whopping 13.14 billion light years. If this holds up, it may be the single most distant object ever seen by humans.

But is this really a record-breaker? And why aren't we sure? OK, this takes a wee bit o' explaining, but I think you'll like it. After all, it's an explosion so big it'll crush your mind into dust.

[UPDATE: Due to a typo in my math notes early on, I incorrectly said the distance to this burst was 13.4 billion light years. D'oh! I have corrected all the numbers below, and I apologize for the error.]



Boom! goes the dynamite

The important thing here is that they are so bright -- emitting more light in a few seconds than the Sun will over its entire lifetime -- that they can be seen for tremendous distances. In fact, they can be detected from clear across the Universe, which is where GRB 090429B comes in.

It was first seen on April 29, 2009 (hence the name 090429B -- it was actually the second GRB seen that day) by Swift, NASA's satellite specifically designed to detect GRBs and rapidly transmit their locations to telescopes on the ground. GRBs fade very quickly, in minutes or even seconds, so rapid response is critical. In this case, observations by ground-based telescopes quickly revealed this was an unusual burst. Within hours astronomers began to suspect it was vastly distant. Estimates started putting it at greater than 13 billion light years away, almost as far as an object can be in the distant Universe.



Far, far away

Frustratingly, clouds prevented the monster Gemini 8-meter telescope from getting a spectrum of the burst, which would have nailed down its distance. Without that, the distance can only be estimated. However, several factors indicate it really is at this extreme distance:

1) Using different filters, astronomers found that the burst was visible in the infrared, but not visible light. Why is this important? Because the Universe is expanding. OK, bear with me here; there are a few steps to this logic.

As light from very distant objects comes to us, it is working against that expansion. Light can't slow down -- it always travels at the speed of light (duh) -- but it does lose energy. That shifts the color of the light into the redder part of the spectrum. At huge distances, far ultraviolet light gets shifted all the way into the visible part of the spectrum.

Here's the fun part: as it happens, some gas in the Universe absorbs ultraviolet light on its way here, but lets visible light pass through. Now imagine a GRB really far away, so far that UV gets redshifted to the visible part of the spectrum, while the visible light from the GRB shifts into the infrared. Here on Earth, we'd see IR from the GRB (which started out life as visible light) but not anything in the visible (which started out as UV). That's what we see with GRB 090429b; IR but no visible -- that's what that picture above from Gemini is showing. That strongly implies that the light from the burst is coming from a long way off. Examining just which colors of light got through versus what got blocked allows astronomers to estimate the amount of redshift and therefore the distance to the GRB (I describe this technique in more detail here).

2) The host galaxy is invisible, even to Hubble. Gamma-ray bursts like this one come from the explosions of massive stars, which don't live long. They are born, live out their lives, and die violently in the span of a million or so years. These kinds of stars are formed in giant clouds of gas inside galaxies, and can be seen for quite some distance with powerful telescopes. After the GRB faded, Hubble was aimed at that spot of the sky and saw... nothing (as you can see -- or can't see, I suppose -- in the inset image; click to embiggen). That again implies a huge distance to the galaxy, so far that even its mighty light is faded away to nothing.

3) Less convincingly, but still important, is that the burst had a relatively faint afterglow in X-rays. Had it been at a less extreme distance, it would've been brighter. This is by no means solid evidence of great distance (some bursts are just fainter in X-rays) but it's consistent.

So we can't absolutely confirm it's at this vast distance, but it seems very likely (a calculation of the odds puts it at 99.3% certainty).



Broken record

And even if it truly is at this mind-numbing distance, it still may not be the record holder for most distant object: one galaxy has been observed that might be even farther away, but the thing is that hasn't been confirmed yet either. So it's a fair bet that GRB 090429b is the single most distant discrete object* ever seen.

In a funny coincidence, the light from this burst reached Earth less than a week after another burst which broke the previous record. GRB 090423 -- seen on April 23 2009 --blew away the previous record for most distant burst, with a distance of 13.04 billion light years (technically, that's not the distance now but tells us instead how long the light has been traveling) -- technically speaking it had a redshift of 8.25. But it only held that record for six days. GRB 090429b has a redshift of 9.4, giving it a distance of 13.14 billion light years -- 100 million light years farther away.

To give you some idea of how amazing this is, the host galaxy of the burst must be one of the first that formed after the Big Bang itself. It may have even still been in the process of initially forming when the burst lit off -- the Universe had only been around for about 600 million years when the burst occurred. Think of it this way: the most distant thing we can possibly see would be 13.7 billion light years away, since that's how old the universe is.

This burst is 96% of the way to that distance.

As our tech improves, and telescopes get bigger and more sensitive, we'll no doubt find bursts farther away than this one. But not by very much! The farther back we go, the less room there is in the early Universe for such an event to occur. At some point we'll reach that limit, and then these record-breaking events will come far less often. They'll all be as far as they possibly can be from us.

But that's no reason to be jaded. Each of these events is second only to the Big Bang as far as sheer violence and energy, and were so titanic they must have had a profound effect on the environment around them. I suspect that when we have a good catalog of the most distant bursts, we'll find out just how much of an effect they had on the early Universe, so record-breaking or not, they'll help us understand what happened when the Universe was very, very young.

As if GRBs weren't cool enough.

Image credits: GRB: Credit: Gemini Observatory / AURA / Levan, Tanvir, Cucchiara; Swift image: NASA / Swift / Stefan Immler; artwork: NASA /Swift /Cruz deWilde; brightness sequence: Gemini Observatory/AURA/Penn State/UC Berkeley/University of Warwick, UK; Hubble: Levan / Tanvir / Cucchiara for NASA/Hubble



* I say "discrete" because the background glow of the Big Bang, called the Cosmic Microwave Background, has been observed, and that's at a redshift of about 1000, at a distance of 13.7 billion light years. We literally cannot see any objects farther than this, because they didn't exist yet! In case your mind hasn't already been blown by this post.




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