Podcast: A Puzzling Difference
Written by Fraser Cain
Imagine looking at red houses, and sometimes you see a crow fly past. But every time you look at a blue house, there's always a crow flying right in front of the house. The crow and the house could be miles apart, so this must be impossible, right? Well, according to a new survey if you look at a quasar, you'll see a galaxy in front 25% of the time. But for gamma ray bursts, there's almost always an intervening galaxy. Even though they could be separated by billions of light years. Figure that out. Dr. Jason X. Prochaska, from the University of California, Santa Cruz speaks to me about the strange results they've found, and what could be the cause.
Listen to the interview: A Puzzling Difference (7.8 MB)
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Fraser Cain: Okay, to give people some background, what's the difference between a gamma ray burst and a quasar? I guess they're pretty different.
Dr. Jason X. Prochaska: Yes, maybe I'll start with the similarities. They're both very interesting objects for the study of cosmology because they're extremely bright objects. Another similarity is that we believe they're both related to black holes, but after that, there's a big difference between the two types of objects. Quasars are believed to be supermassive black holes - so black holes, but extremely massive, in some cases as massive as a galaxy. Accreting gas onto the black hole heats up and the light that we see is the quasar. Because they're supermassive, they can accrete lots and lots of gas, and as a result, can shine very brightly which can be seen from very large distances.
A gamma ray burst, at least, which this paper is based on - there are two types - is the result of a massive star, a single star, but pretty massive, on the order of 10-50 times as massive as our Sun, arrives with the death of a star. At the end of its natural lifespan. Upon its death, it creates a black hole, and some fraction of these stars, we believe create gamma ray bursts.
Fraser: And you did a survey of quasars and gamma ray bursts, and what did you find?
Dr. Prochaska: I put a student on a project with quasars first. There's a public database called the Sloan Digital Sky Survey, and it's surveyed a large fraction of the northern sky. And they've taken a spectra of probably close to a million objects, mainly a galaxy survey at the heart of it. In addition to studying galaxies, they've also studied quasars. They've taken spectroscopy of about 60,000 quasars now, and they've released that data publicly to anyone on the planet who wants it. More or less, we trolled through that database, searching for signatures of galaxies that lie between us and the quasars. So if you have a quasar at a very large distance, as they tend to lie, there's a chance that there's a pretty large galaxy between us and that quasar. The galaxy reveals itself by the absorption lines on the quasar. So you analyze the spectrum of the quasar, you see these features associated with the quasar that are very distinctive, but you may see the absence of light in this case. The fingerprint of the galaxy itself that coincidently lies between us and the quasar. That kind of science is something that I've been doing for the last 12 years now. I've had my student look through these 50,000 quasars in the Sloan survey, and count up how often we have a galaxy lying between us and the quasar. That's the nuts-and-bolts first step, and there's a lot of science that can come out of such a search for these galaxies.
Fraser: So you might not be able to see visually if there's a galaxy there, but you can detect it.
Dr. Prochaska: That's right. Our own Milky Way is full of stars and gas and dust. As far as the baryons are concerned, the protons and neutrons. The main three phases that the baryons reside in the Milky Way are stars, which you see quite readily, gas, which is more or less invisible, but does emit at 21 cm - a well known technique used to map out gas in our galaxy with radio telescopes. But gas can also absorb light. It does emit at 21 cm wavelengths, but it also does absorb at specific frequencies. It'll absorb light from a background object. And so pretty much all galaxies have not only stars but the gas those stars are forming out of, and one can detect the galaxy, the signature of that galaxy by studying the gas. And that's the technique we use for the quasars, and it's the same technique we use for gamma ray bursts.
Fraser: Right, and what did you find with the gamma ray bursts?
Dr. Prochaska: Actually, one important point that I left out in comparing quasars to gamma ray bursts is that they're very bright. As with their name, they emit lots of gamma rays, but a good fraction of them - certainly more than half - also emit radiation in the ultraviolet, the X-ray, optical light, even radio light, and are very bright in those frequencies. And so we can see them across the Universe in the ultraviolet or optical frequencies, and use them to study the gas that lies between us and the gamma ray burst. What's different in the quasars, for the moment at least, is that there are many fewer gamma ray bursts that have been discovered. It requires a space satellite to detect these phenomena, a fair amount of technology that hasn't existed in a great level until recently. So the number of these things that have been detected still numbers in the 1000s, but only 1-200 that we can study in great detail. That's what we've been doing, taking even a subset of those 100 or so, acquired the spectrum of the gamma ray burst, and again searched for the signature of galaxies that lie between us and the burst, again through the gas. The nutshell result is that while we have a small sample of gamma ray bursts, a significantly significant overabundance of more galaxies towards gamma ray bursts then there are towards quasars.
Fraser: How many more?
Dr. Prochaska: The number now is 4, that's been well measured, I'd say the error is 1, so 4 plus or minus 1. What is significant is that it's an enhancement. The enhancement may one day turn out to be 3 or maybe 1.5, but the enhancement over the quasar is very sound.
Fraser: For some reason there are more galaxies in between us and the distant gamma ray bursts than there are between us and quasars. How's that possible? They're so far apart.
Dr. Prochaska: Right, and that's the thing to emphasize first is that in priori, we have no expectation that the galaxies that we randomly towards quasars or gamma ray bursts has anything to do with that background light source. Again, we find a quasar at a great distance from us, the galaxy's also at a distance from us, but also, at the same time, a very large distance from the quasar. So much so that you wouldn't expect any association; no gravitational association, no electromagnetic, no physical association between the galaxy that we're identifying and the quasar. And the same is true for the gamma ray burst experiment. The gamma ray bursts are at a great distance from us, we see galaxies towards it - they're at a great distance from us, but also at a great distance from the gamma ray burst. And again, we have no a priori expectations of any physical relationship between that galaxy and the gamma ray burst that lies behind it. Certainly on the surface it's quite stunning, the test is pretty straightforward. Our immediate reaction is, okay, what's going on?
There are three biases, or explanations - in astronomy we'd call them selection biases. And the three key explanations, the obvious explanations, that could give you this result are first: dust. Galaxies, as I said, have matter in three phases: in stars, gas and dust. Most galaxies, or probably all galaxies have dust within them. And the key aspect of dust is that it extinguishes the background source. So you sprinkle some dust between you and the quasar, and you're going to make it fainter. These galaxies all have dust in them, and you could imagine actually missing quasars, when you do this survey across the whole sky. Galaxies that have a lot of dust in them will obscure the quasar, and you'll never look at it. It'll never get counted into your sample. But gamma ray bursts, which are detected with a very different approach, using gamma rays, would not be as sensitive to this dust - you would still potentially detect the gamma ray burst and count it in your sample. So you'd end up with an overcounting of objects in the gamma ray sample, with an absence of quasars due to the dust. The reason why we don't think that's the answer is that we have a good sense of how much dust is the galaxies, and it's not enough to remove enough quasars from the sample to make up for the difference by a factor of 4.
So that's explanation number 1. Number 2 would be that our a priori assumption, that the gas has nothing to do with the gamma ray burst or the quasar is wrong. I've said that this gas is at a great distance from us, and from the quasar and from the gamma ray burst. Probably the most difficult problem in astronomy is actually gauging distance. I'm not really measuring the distance of the gas, I'm measuring the redshift of the gas, and that gives me an estimate of the distance, under the assumption that the redshift is due to the expansion of the Universe. Really redshift is just a velocity. So I'm measuring the velocity of the gas, I'm measuring the velocity of the gamma ray burst. I know the two are different, that I know with absolute scientific fact. I'm assuming that the difference in the velocities are due to the expansion of the Universe and hence the distance between the objects. But it is possible that the gamma ray bursts have actually spat out this gas during the explosion, say, at very high velocities so that it has a different velocity than the gamma ray burst itself, and that's the reason for the difference in redshift, and hence causing me to say they have difference distances. So, in a nutshell, the explanation for number 2 is that the gamma ray bursts are ejecting gas at very high velocities and we're measuring that gas and calling it a galaxy, when in fact it's just gas being ejected from the gamma ray bursts. That's still a viable option at the moment. The counterargument to it, and it's a solid one is that in many cases, we've identified not only the gas, but also stars from the galaxy that must be hosting that gas. So not only would the gas have to be ejected, but a galaxy would have to be ejected by the gamma ray burst, and that starts to stretch the imagination.
So that leads to door number 3, which is gravitational lensing. Galaxies, anything with mass, do have an effect by making objects behind them visually brighter than they actually are. We think we have galaxies here, we know that we have a mass concentration, so it's quite possible that they are impacting the brightness of the object behind them, and making gamma ray bursts much brighter than they would otherwise be. The main reason that we're seeing the gamma ray bursts is because we have a galaxy there. We need the galaxy there in order to see the gamma ray burst. And that's a selection effect where if we didn't have a galaxy, we wouldn't see it, and that leads to an overabundance of quasars, where the quasars are maybe bright enough without the galaxies. And gravitational lensing, as you can probably tell, isn't something I've worked on directly, but the experts in the field tell me that's not a likely explanation, or the dominant explanation of the result.
Fraser: So you're kind of running out of ideas.
Dr. Prochaska: Yes, we've certainly ran through the three obvious ones, the ones that anyone would come up with, and yet have pretty strong counter arguments to those. Another group came up with yet another 4th idea, that I think was quite clever, that quasars have a difference size than gamma ray bursts. It's a little bit subtle to how that could make a big difference, but they said, perhaps that is the explanation, yet we and others have come up with really strong counter arguments against door number 4 at this point. The 4 decent ideas that have been proposed have failings to them.
Fraser: So what's next then? I assume you'll look for more data.
Dr. Prochaska: Certainly I want to rule out that the gas is associated with the gamma ray bursts, that's it's being shot out of the gamma ray bursts. I'd really like to prove that one certainly not true, and the way to do that is to identify the actual galaxy and stars that are associated with the gas. So people in our team and other teams are going back and looking for the galaxy that's actually holding the gas. If we didn't find galaxies, I think that would hold more credence to the idea that the gas was ejected by the gamma ray burst. So there's certainly work to be done in studying the galaxies associated. In the same lines, we can infer how much mass is in the galaxies and better test the gravitational lensing hypothesis, as well as learn how much dust is in galaxies to test the dust hypothesis. Even while I'm down playing them, and I think it certainly behooves us to learn as much about the galaxies towards the gamma ray bursts to see if there's something funny going on, or any other properties that could explain the result. The other obvious thing to do, and this will be done, is just to wait for more gamma ray bursts to come along and repeat that experiment on more sight lines. And so currently there is this NASA Swift space telescope in operation, where we'll get 10s maybe even 100s more gamma ray bursts that we can repeat this experiment on, and very soundly figure out how statistically significant it is.
Fraser: Is there some kind of idea that's completely out there that you think might be possible?
Dr. Prochaska: I'm sure there are going to be papers written along those lines. It's not going to be my favorite option for the moment. But, I'm a scientist, I'm a realist. We've brought the message that there's this peculiar finding, and we looked very hard at how we did the study, we did apples to apples to the best of our ability, and I think we did a fair job of that. That's kind of step 1. Step 2, as an observer, I feel like I should be able to explain the result once we have it. As I said, we came up with the three ideas, and unfortunately, I don't think any of those have stuck at the moment. If I can kill all ideas, and if the result holds up well with the next 50 gamma ray bursts, at that point you do have to go back down to your initial assumptions; one of them is cosmology as we know it. I'm saying I'm anywhere close to that, but give me two years and if things don't change from what we see, yeah, I think you have to go all the way back to step 0 in your line of assumptions about the Universe.
Filed under: Astronomy, Gravitational Lensing, Podcasts
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