Science Interviews

Interview

Sun, 1st Nov 2009

The most distant object ever discovered

Professor Nial Tanvir, University of Leicester

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Chris - Now also in the news this week, two international teams of astronomers have described what we can only describe as the most distant object ever discovered.  What they've seen is the gamma ray burst of the star that died when the universe was just 640 million years old.  That’s less than 5% of its present age.  The universe would’ve been only about 9% of its present size and that means the light from that star has been going across space, coming towards us for over 13 billion years.  And one of the people who manage to see was Professor Nial Tanvir who’s at the University of Leicester and is with us now.  Hello, Nial.

Nial -   Yes, good evening.

Chris -   Welcome to the Naked Scientists.  So, tell us first of all, how did you make this observation?

Nial -   So, these gamma ray bursts are actually quite tricky to observe because theirSwift Satellite main characteristic is a short flash of gamma rays and of course, the gamma rays don't penetrate the earth’s atmosphere.  So, we have to observe them initially with satellites and the current sort of work-horse satellite doing this stuff is one called Swift and that’s the satellite that the UK is partially – built part of the satellite so has an interest in.  So, Swift finds about, on average, two gamma ray bursts every week and it reports the positions on the sky of these events down to the ground within a matter of seconds or minutes so that observers on the ground like myself can make follow up observations using a variety of different large ground-based telescopes.  And it’s by analyzing the information that we get from those ground-based telescopes that we ascertain things like the distance and in this case of course found that it was a record breaker.

Chris -   What actually causes the gamma ray bursts in the first place?

Nial -   That’s a good question.  It’s something that is still enshrouded in a certain amount of mystery, but we think actually, there are a number of different kinds of objects in the universe that can give rise to these sorts of flashes gamma rays.  But in particular, the one that seems to be most frequently are seen are related to the collapse of a very massive star.  So, we have a star that’s maybe 20, 30, 40 times the mass of the sun.  At the end of its life, ceases to create energy in its core and no longer supports  nuclear reactions.  And so, the star just collapses on the under gravity there’s no pressure, radiation pressure to keep it up.  Now, that’s a reasonably common and fairly well understood process but it seems that in certain rate situations, instead of just producing a normal supernova which is what we would expect normally to happen, such a star can also produce an extremely energetic and highly relativistic jet of material that pierces its way after the star.  And if you happen to be sort of lying and looking along the line of site, down the barrel of this jet as it were then you see this phenomenon of the gamma ray burst.

Chris -   Why is it that we’ve not seen one that’s this old before?  Because presumably, the universe has had stars forming and ploughing themselves to pieces and producing gamma ray burst like these for a long time.

Nial -   Yeah.  The problem really boils down to just rarity.  It seems that it’s only very exceptional stars that explode in this way.  And therefore, even in the whole universe, as I say, Swift sees a good chunk of the universe in a sense as it scan off the sky every day, and yet, it still only detects in the whole universe a couple a week.  So really, it just boils down to the fact that we needed to wait a long time until we were lucky enough to spot one at this sort of distance.  If we carry on observing, we may getting more sensitive satellites and scan even more of the sky then the hope is in the future that we’ll find more at this sort of age.

Chris -   What can you learn from the fact or what can you infer from the fact that there was a star burning 600 million years after the Big Bang when the universe was created?  What’s written into that gamma ray burst in terms of the signature and the chemistry that can inform you of the structure of the early universe around that time?

Nial -   Right, okay.  Well, I should say that in this particular instance, gamma ray burst and what – the initial flashes is very bright in the gamma rays, but then the object will then sort of track through the ground-based telescopes is a sort of fading ember.  It’s what we called the afterglow of the burst.  And in principle, the afterglow can give you a great deal of information about the local chemistry and the conditions at the time and in the vicinity of the burst.  And so, that’s very important for example if we find that there is a lot or a certain amount of elements heavier than hydrogen and helium.  So for instance, oxygen, carbon, iron, all the things that we’re familiar with.  If we find that those are present at this time, we know those could’ve only been cooked in the senses of stars.  And so, it gives a very important clue, not just to what’s happening there and then, but what must have happened at even earlier times to produce those heavier elements.

Chris -   Are they there?

Nial -   So that’s the principle.  But on the other hand, the problem is that with the GRB afterglows, they come in a sort of great range of different brightness.  And really, if we’re going to find that sort of information from a gamma ray burst afterglow, it needs to be a particularly bright afterglow and unfortunately, this one wasn’t a particularly bright one.  I mean, it was fairly bright, but it wasn’t bright enough.  I mean, we didn’t really – at the end of the day, manage.  If we had been very lucky with the kind of data that we’d got, then it’s possible we would’ve done it.  But with these things that they go from random times and so, you have to live with whatever telescopes can make the observations at that time and we’ve made the first observations in fact, using telescopes in Hawaii which we sort of triggered from the UK.  But conditions weren’t great in Hawaii that night, so you see the problem.  We obtained enough data to prove that we got this record breaker, but we unfortunately didn’t manage to get good enough data to sort of take it to the next step to do those sort of, you know, refined pieces of analysis.

Chris -   I see.  And just to finish off, Nial.  Can you tell us?  What does this tell us about the structure of the universe at that time, the fact that there was this big star burning at that time?  How does this inform our understanding of the early universe?

Nial -   Well, it tells us that there’s at least one star and of course, one who seems that there were more and we hope in the future, by building up statistics of these things, we’ll be able to really sort of measure the rates of star formation in the universe, even at those very early times.  The other thing it does is it pinpoints the position on the sky presumably, it’s galaxy that hosted this stars.  So, stars forming galaxies and we’d like to know, not only about the properties of stars at this time, but also the properties of galaxies.  And so, the galaxy is going to be far, far fainter than the gamma ray burst because gamma ray bursts is stupendously bright and galaxies only have, you know, a few hundred million stars in them or something like that.  So they're much fainter.  But we can now know the position and knowing the distance, we can go away with all the other facilities that we have without disposal like the Hubble Space telescope, and look really hard for the host galaxy.  And so, that’s certainly something that we haven’t yet achieved, but we hope to do next year is to really search very hard and see if we can find this host galaxy and therefore, for the first time, learn something about the properties of the galaxies which existed at this really early era.

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