Tracking down the Universe's
By KEITH COOPER
Posted: 5 May, 2009
Dan Stark is an STFC fellow based at the Institute of Astronomy in Cambridge. He uses the twin Keck telescopes to study the Universe’s first galaxies, and also studies when and how spiral galaxies assembled and began rotating.
Do we have any idea how massive these first galaxies that you have observed are?
Could these be the nuclei of spiral galaxies like our own?
There are very massive galaxies in the Hubble Ultra Deep Field (image left), and there seems to be a big gap between these first, small galaxies, and those massive galaxies. Is the hierarchical model in jeopardy?
We did find a handful of massive galaxies at redshift 6, galaxies with 10^10 solar masses in stars already. There were some claims, in fact I was on a paper of a claimed object that was about as massive as the Milky Way, perhaps at redshift 6.5. This object is very controversial and would be by far the most massive object in the very early Universe. But there are people who are saying it is just as easily at redshift 2. So I would throw that object out for now and just concentrate on the 10^10 solar mass objects, and then you can ask the question as to whether this is trouble for the hierarchical model. From a dark matter perspective, I don’t think it is, the dark matter haloes that can host these galaxies are there, it is just a matter of whether you can rapidly form stars in these systems, and we have had to adjust our gas physics in models in order to form massive galaxies that we are seeing at redshift five to six, but also at redshift 2 to 3 we are seeing a lot of massive galaxies that not only are in place but they are completely dead, they are not forming stars anymore. So this has led to a bunch of people looking at various ways to shut off star-formation in galaxies, resorting to feedback in active galactic nuclei. The actual numbers of these systems don’t actually present a problem for the dark matter picture of hierarchical formation, but it has forced us to rethink how baryons are converted in to stars and haloes of various masses throughout time. Getting back to these really small systems, I think what we are seeing at redshift 7 to redshift 3 is some of the best evidence for the hierarchical picture, whereby we are seeing that galaxies are getting bigger, galaxies are getting brighter, more luminous. Work by Richard Bouwens at Santa Cruz has shown this quite convincingly now, and there was a lot of controversy originally as to whether galaxies were growing in number or they were getting brighter, and now the field seems to have settled on the answer that galaxies are getting more luminous, or alternatively as you look further and further back in time, the average luminosity in an average galaxy is getting smaller, so your typical galaxy is becoming less luminous, and likely smaller and likely less massive, as you would expect in this hierarchical picture. So I think that at these times there is quite good evidence for this hierarchical picture.
How difficult is it to verify their distance?
When the James Webb Space Telescope (JWST, image below) comes along there will be a spectrograph that extends into the mid-infrared so you will be able to do this much better. We attempted to do it with Spitzer actually to look for hydrogen-alpha but the spectrograph is not sensitive enough. The Lyman-break galaxies, this is the alternative search technique, it is equally troubling because you see a break, but galaxies have several breaks in their spectra and what you could be seeing is perhaps the 4,000 Angstrom break, Balmer break, perhaps what you’re seeing is a very dusty galaxy, these have red slopes so it drops out of your filter because it is red. So trying to distinguish between these two is tough and as you go to higher and higher redshifts you get fewer and fewer bandpasses before it redshifts out of the atmospheric window.
The other problem is that you could try to follow up and look for Lyman-alpha for these drop outs, but one) at redshift 3 we see one out of four Lyman-break galaxies show strong Lyman-alpha emission, and two) once you get into the time where you think the intergalactic medium is neutral, what you find is that the Lyman-alpha emission, if these galaxies are embedded in a H2 region, would get eaten away by all the neutral hydrogen, so you’re not going to be able to confirm these objects unless they are already in place within an H2 region. So this creates two significant obstacles to being able to really verify beyond doubt the redshifts of these objects, so our best bet is if you are studying Lyman-alpha, you want to see an asymmetric line profile which is a characteristic line profile that most Lyman-alpha emitters show, and this is challenging in its own regard. Additionally with lensing you get a nice bonus because the geometry of the multiple images that you see is dependent upon the redshift, so if your source is at redshift two it will have an entirely different geometry in its images, as it would if it were at redshift 7. So to some extent lensing is the best way to allow you to break the degeneracy if you can identify the images, and not all high-redshift galaxies are not multiply imaged.
But you can see it is a tremendous headache for us and we are trying hard to verify all our sources. So when we publish our paper we publish them as candidates basically. The most distant verified galaxy is at 6.96 by the Japanese team that has been spectroscopically confirmed, asymmetric line profile, that one is beyond belief! Then there is a lensed source at redshift 7.6 by Bradley, Bouwens and collaborators, and the thing with that source that allowed them to think that they had observed it was that they found two breaks: the Lyman break, and then the Blamer break, with Spitzer, so they detected the source with Spitzer, measured the mass; it is not multiply imaged so you are still worried a little bit about the redshift but the multiple breaks in the Spitzer detection allows them more confidence in the redshift. Those are the two highest redshifts that the community feels very confident in at this point. But with Wide Field Camera 3 in May (Hubble repair mission) we are going to re-observe these ultra deep fields and we’re going to get hundreds of these [objects], and concurrently with that we’re going to get these infrared multi-object spectrographs, so basically you can imagine a hundred objects in a field and you take you multi-object spectrograph and study that field and putting a slit on every single object, and if even just one out of ten shows Lyman alpha, you’re going to be able to confirm it. So we’re going to be able to start confirming some of these galaxies. So that’s very exciting and probably in the next year that is going to be the biggest step forward. In the last six months we have been at a little bit of a standstill since the redshift 7.6 object came out.
What is the most exciting aspect of you research?