Chris Lintott’s Universe

At any conference there’s one talk that changes the way you think about something, or crystalizes thoughts that you’ve had anyway. In the last few months I’d been thinking carefully about the answer to the question ‘but what happened before the Big Bang’, and a talk by Cosmic Variance blogger Sean Carroll crystalized some of those thoughts. He was clear that he was on the edge of speculation at times, but you can read the short version of my thoughts at the BBC website.

I’ll write more about my thoughts here over the weekend, so watch this space.

Update : Woo! Number 1 most emailed article…

I’ve spent most of the day running around having meetings so there’s been little blogging from me. I will write about the talk I’ve just come out of by Sean Carroll of Cosmic Variance, but it’s going to take me a while to digest my thoughts. In the meantime, I should remind you all you can catch up with all the news from here at Astronomy Cast Live.

Imagine being in our solar system, standing just where the Earth is now, roughly four and a half billion years ago. Around you would be the detritus of star formation, left over material forming a protoplanetary disk from which the planets are coalescing. Understanding just how this disk of dust and gas became the eight planets of the Solar System is one of the fundamental questions of astronomy, and in the penultimate plenary talk today David Wilner from the Harvard-Smithsonian Centre for Astronomy took us through the latest results from this fast moving field.

There are just two major problems with looking for these disks. The first is that most of the disk is cold, dark and not shining, and the second is that they are very small and thus difficult to image. For a nearby example, you’d be lucky if your disk appeared to have a size of a 1/4000th of a degree.

Battling manfully against these challenges, astronomers have had extraordinary success to understand these systems. To get around the first, we can look for comparatively rare chemicals such as hydrogen cyanide (HCN) or Carbon Monoxide (CO) and use these to trace the structure of the disk.

Through these and other methods, we can now measure the mass of a typical disk, and find that 1/100th of the mass of the Sun is typical; that’s a good number because you have enough stuff to form a Jupiter and a few other planets besides, but it does illustrate how small a proportion of the whole we’re talking about. We can also work out a lifetime for the disk. About half of the material disappears in the first 3 million years, and most of the rest in another 2 million; the blink of an eye in astronomical terms.

The next stage is to try and see these things; to do that, we string together light from many separate telescopes, producing images which are as good as telescopes many times larger than we could possibly produce. Unfortunately, perhaps, the disks tend to look just like blobs, but at least we are seeing the places where planets are forming for the first time.

So how do these disks form stars? Well, we have to first get dust grains to stick together. As the clumps get larger, then their gravity can attract further material to keep them growing all the time. If there’s enough material, then gas will be attracted too and you can form a nice new Jupiter. Remarkably, we can actually see evidence for each of these processes in the details of our observations of protoplanetary disks.

One nightmare researchers like the speaker have is their inability to prove that the disks they’re seeing are the same as those which go on to produce planets. The best evidence that they’re getting it right is the presence of large gaps in some of the studied disks; just as small moons of Saturn can create gaps in the rings, so small, newly formed planets still embedded in a protoplanetary disk might produce the gaps we see.

In some cases it’s even possible to make this connection directly. TW Hyd is a star which is known to have a disk, with an observed hole. Earlier this year, a large planet (about 10 times Jupiter’s mass) was detected orbiting the star, and while this planet isn’t the one creating the observed hole at least there’s a system with both a disrupted disk and a planet.

Our early Solar System, then, will have gaps in the disk where Earth and the rest are forming. For now, we can only see such things in detail in our mind’s eye, or in the distorted world of computer simulations. That might chance soon though, with the advent of the ALMA array high up in the Chilean Atacama desert. ALMA’s high resolution images might reveal not only the gaps in the disks, but also material streaming into those gaps to form smaller dust disks around the nascent planets. In viewing these secondary disks, we will be seeing not the birth of planets, but the birth of systems of moons like those around Jupiter. Now that’s a view worth waiting four and half billion years to get.

I wrote yesterday about the somewhat surprising link that’s been discovered between the tightness of a galaxy’s spiral arms (the angle at which they uncurl) and the size of the black hole that lurks in their centre. I managed to catch up with Marc Seigar, and the interview is now up on Youtube

The Great Observatories Origins Deep Survey (GOODS) : An Observational Legacy for Studying Galaxy Evolution
Prof Marc Dickinson

The following was written during the final plenary talk of the first day at the American Astronomical Society Meeting in St Louis. I was going to post as we went along, but the wireless connection in the meeting room was very flaky (probably just as well – it means the audience are paying attention to the speaker!) I’m posting it in a lump near the end of the talk. Images to follow.

In the introduction to this talk, we were told that the hallmark of modern astronomical research is the survey, and it’s certainly true that astronomers have learnt to make use of projects which carefully chart sections of the sky. The speaker began by reminding us that it’s more than a decade since the Hubble Deep Field – as he said, every time you get a new telescope the temptation is to push it to its limits. After 150 orbits staring at the same field, it turned out Hubble was excellent at seeing the distant Universe.

The data were released, and then most other major observatories all observed the same field, producing hundreds of papers to understand this region of the sky. Not bad for a patch just 2.5 arcminutes square (an arcminute is a sixtieth of a degree). But the question is, with such a small area how can we be sure that we have a fair census? What if that patch turned out to be unusual in some way? Even if we’ve got lucky and picked the right region, then rare objects will be missed entirely.

GOODS is the solution to this problem; using Chandra in the x-ray, Spitzer in the infrared and Hubble’s ACS camera (not available at the time of the original HDF), they set out to cover two regions, each thirty times larger than the original Hubble Deep Field. The aim was to disentangle the evolution of normal galaxies in the first third of the Universe’s evolution, taking a census of black hole growth and activity, understanding how and when star formation takes place and so on.

Each telescope had a different role to play; Spitzer, for example, in the mid Infrared allowed the team to weight the galaxies. The total stellar mass in a galaxy turns out to be very sensitive to the brightness in this band (although you have to worry about the evolution of the stars, we’ve got pretty good at doing that). As before, other observatories have chipped in, with GALEX providing the Ultraviolet, for example, and the SCUBA camera on the JCMT providing a view of the cold early Universe in the sub-mm region of the spectrum.

Astronomers are greedy, though, and as well as imaging we demand data. The first step in understanding an object is to work out how far away it is, and for objects as far as those in GOODS that means measuring their redshift. Lines in their spectrum will be shifted due to the expansion of the Universe; in all more than 5000 GOODS objects have had their distance measured. That’s not a huge number compared to something like the Sloan Digital Sky Survey, but the objects are much further away (so more telescope time is required per object to get a decent spectrum).

The results were far too numerous to go into here, but there are some nice highlights. For example, we can show that galaxies were, on average, smaller in the past, just as you’d expect if the systems we see around us today were assembled by mergers of smaller galaxies. Arguments are raging about the star formation history of the Universe; we know our Universe is past its peak, forming ten times as many stars about 6 billion years ago as it does today, but the GOODS data suggest that looking further back the rate drops once more.

One of the reasons this is controversial is that most of the energy emitted by the newly formed stars is absorbed and then reradiated by dust. This process makes the galaxies bright in the infrared, and so Spitzer can help here. Prof Dickinson went so far as to call the early Universe (before z=0.7 if you understand and care about redshifts) ‘the age of obscurity’.

As well as changing sizes and star formation rates, the population of galaxies has changed too. In the first third of the Universe’s evolution, the average massive galaxy was what is called a ULIRG – an Ultra-Luminous InfraRed Galaxy. Spin forward to today and you’ll find that in the present day the typical massive galaxy is an elliptical – old, red and dead, devoid of star formation and about as far from a ULIRG as it’s possible to be while still being a massive galaxy.

Disentangling everything that might contribute to the light we receive from a galaxy is hard work, to say the least. The team looked at a set of galaxies which had an excess of light in the mid-infrared – the massive galaxies described in the previous paragraph. It’s tempting to assume that the infrared is due entirely to star formation, but by looking with Chandra they detected x-rays from hidden Active Galactic Nuclei. In other words, these galaxies are not just forming stars, but half of all galaxies had black holes at their centre which were in the act of consuming large amounts of material. As Prof Dickinson said, it seems that around 4 billion years after the Big Bang was an important time in a galaxy’s life.

Perhaps one of the most surprising results is the presence of another population of galaxies at this time. There seem to be a set of galaxies which aren’t doing very much at all – they’ve already formed their stars and are already quietly and passively enjoying the galactic equivalent of late middle age. One mystery is that there are smaller for their weights than we’d expect – and it’s hard to imagine how they might ‘puff up’ to see the galaxies that we see today.

Looking further back, the team managed to detected light emitted from galaxies when the Universe was not much more than a billion years old. Even at this time, there’s evidence for a fairly mature stellar population, so substantial numbers of stars must have been formed before the epoch of the earliest galaxies astronomers have seen to date. They have some candidates from this early epoch, but it’ll have to wait for the next generation space telescopes to confirm these detections, so don’t hold your breath.

As if all of that wasn’t enough, the team realised that by going back to the same parts of the sky every 40 or so days, they stood a great chance of discovering distant supernovae. Of those they discovered, almost 50 are a particular type of exploding star – supernovae type 1a. These explosions seem to contain a clue to their true luminosity, and so by comparing how bright they appear with how bright they actually are we can try and measure the acceleration of the Universe.

At this stage I’m being to feel a bit breathless after all the work the GOODS team have done. Prof Dickinson is finishing his talk by asking ‘are we done yet’? The answer, perhaps not surprisingly, is an emphatic no. One of the major problems is tht the measured star formation rate should tie up with the measurements of the total number and mass of stars –and they don’t. They also know there must be more black holes hiding, because they see energetic x-rays with no obvious source. Black holes hiding behind dust are the obvious candidates.

What we really need is a new telescope, working in the far infrared. ESA’s Herschel space telescope – larger than any other telescope ever to fly into space - is due for launch early next year is designed to solve this problem, and will take a long early look at the GOODS fields. I’m planning to head straight from AAS to go and visit Herschel which is undergoing final tests, so it’s great to hear that people are already anticipating the data it will provide.

I’m sitting at the back of the second press conference of the day, desperately swapping between laptops to run the Astronomy Cast Live feed, catch up with press releases from the speakers and to write this.

Currently, we’re listening to the announcement of the smallest extrasolar planet to date, a 3 Earth Mass planet orbiting a brown dwarf. 3 Earth masses is small enough to all but convince me that we must be talking about a rocky or an icy planet, not a gaseous system, and it orbits its host at about the distance Venus is from the Sun. Of course, as its parent star is much fainter and smaller than the Sun, it will be freezing and there’s no hope of liquid water on the surface.

Phil, whose head is visible in the live feed at times, has all the details.

One issue that came up in the questions, and which has intrigued me for a while, is whether there is any real division between large planets and small brown dwarfs. This is something the International Astronomical Union are going to have to wrestle with as when they demoted Pluto, the declined to define what a planet is beyond our solar system.

The best supported theory of brown dwarf formation claims that these small stars are just the detritus left over from the formation of larger stars. If this is correct then there should be a theoretical minimum mass of about 5-10 times the mass of Jupiter, but as smaller and smaller brown dwarfs are found this limit is being approached.

In one of the other papers presented at the exoplanet session, Michael Liu of the University of Hawai’i talked about weighing brown dwarfs by watching binary pairs orbit each other. This isn’t easy - you need adaptive optics to give you a resolution equivalent to being able to read a magazine from a mile away, you need pointing accuracy equivalent to hitting the bullseye on a dart board in Hong Kong from an oche here in St Louis, and, perhaps most difficult of all, you need the funding and the motivation to study the slow-moving stars for years on end.

Having done all that for a pair of pairs, the results are just about perfect; our theoretical predictions are good, but not that good. The masses are supposed to depend on the energy output of the star, and on its temperature. One star was cooler than expected, and the other warmer. While this isn’t a big deal - no theories are going to be overturned, and no Nobel prizes won - it’s this kind of detailed work that’s essential in making sure our observations and understanding are as good as they can be.

The masses of the stars were well above the theoretical limit; the smallest were about 30 times Jupiter’s mass. More careful work with this technique and study of other pairs might bring us closer to the limit, though.

One of the most intriguing of the morning press releases is now being described by Marc Seigar from the University of Arkansas, who has been trying to weigh the supermassive black holes that lurk at the centre of galaxies. Ideally, you’d do this by measuring the speed of the gas rotating around it, but that’s hard for distant galaxies.

Prof Seigar’s suggestion is to look not directly at the central gas, but - if the galaxy you’re interested in is a spiral - then you just look at the arms. His team found that the looser the arms, the smaller the black hole - and you could potentially see these features out to billions of light-years away. Compare Andromeda with the other major member (along with the Milky Way) of the local group, M33, the Triangulum Galaxy.

Andromeda has just about the tightest arms of any spiral galaxy, and a large black hole, weighing in at 180 million times the mass of the Sun. Triangulum has loose arms, and if it has a black hole at all it’s smaller than 1500 solar masses.

I have to confess I was sceptical about this release when I saw it, but the data shown during the press conference looks fairly convincing. It’s especially exciting for me as Galaxy Zoo 2 will collect information about the tightness of spiral arms among many other things, and it’s a fascinating thought to think that we might be probing the behaviour of the central black hole at the same time.

What is causing this link? It’s fair to say that they don’t know, but are blaming differences in the distribution of dark matter.

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