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.