Dan Stark:

Tracking down the Universe's

first galaxies

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?
Measuring their mass is difficult. What we’re doing is measuring primarily the rest-frame ultraviolet region of these galaxies. So we’re at redshifts of say 9 to 10, we’re primarily sensitive to the rest-frame UV which is predominantly related to the current generation of stars forming. So that tells you what the current star-formation rate is, what it doesn’t tell you is the amount of old stars, which you need to know to know the mass. You can get some estimates of the mass by assuming a rough age, so the galaxies that we found at redshift 10 have star-formation rates of 0.1 solar masses per year, and if you assume they have been forming stars for a few hundred million years, you could say that these are anywhere from, say, 10^7, 10^8 solar mass objects, so very, very low mass. Now if you go slightly down in redshift to say 6 or 7, for example Abell 2218 at redshift 6.7, this is low enough redshift that Spitzer can detect its rest frame optical emission, and from this we can get some constraint on the age and the mass of the system, and that galaxy is about 10^8 solar masses, or maybe 10^9 solar masses depending on your assumptions. Again, much less massive than the other objects we’ve been seeing with conventional surveys.

Could these be the nuclei of spiral galaxies like our own?
That is what has been suggested for a while, that the star-forming galaxies that have been observed, all the way from redshift 3 to say, 7, you are forming the bulge, basically, because they are very compact what we are seeing. Their clustering seems to make sense with them perhaps being bulges. But you need to measure the kinematics and see whether they are rotating or not. Recently there has been a host of work done both by us measuring the lensed-systems and also Reinhard Genzhel and Natasha Forster-Schreiber have been working on this as well. What these groups have been finding is perhaps a model whereby these star-forming galaxies at redshift 3 to 4 are assembling a bulge, but there is also some sort of rotating disc outside and you are getting fragmentation within the disc of giant clumps of star-forming gas, and these clumps are they rotate in this gaseous disc undergo dynamical friction and they wind up moving towards the core, so what you end up getting is this core of stuff with relatively older populations, some residual star formation, and clumps on the outside that are active star-forming regions. So in order to actually make these claims you need to resolve these star-forming galaxies, and this is really the next step with adaptive optics and gravitational lensing. We’re able to resolve these galaxies, study their resolved stellar populations, look at where the mass is relative to where the star-formation is, and study how the kinematics work. I think this is going to help us build up a picture and to answer more precisely whether these objects, these Lyman-break galaxies, are bulges in formation or young discs, or perhaps both. There’s a theory paper coming out every week on this topic at this point.

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?
It is tremendously difficult. We’ve been agonising over this for years now, and this is one of the major problems of the field right now, trying to confirm these early systems. The problem is twofold: one, these objects are tremendously faint, they are at the limits of the survey, so you are always worried that they are real or noise, and the main problem after that is trying to figure out whether they are at high redshift or whether they are interloping galaxies, and main problem is that if we are doing an emission line search for Lyman alpha emitters, we have, ideally, an object’s red shift that you believe is one in which you’ve identified multiple emission lines, so at redshift two you can identify galaxies at Lyman-alpha emission line, but then we can go look at the O II, OIII emission lines, we can look at hydrogen-alpha, we can look at absorption lines in the continuum spectra. But the problem at redshift 10 is that you have Lyman alpha, say in the near infrared, the next bright nebula emission line is already redshifted out beyond the K-band, beyond 2.4 microns, so you can’t detect it from the ground, the atmosphere is too bright, so this means that you can’t confirm it based off the presence of additional emission lines – yet.

      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?
It really is a missing chapter. We have this snapshot of the Universe from the cosmic microwave background at redhsift 1,100, and then we have this picture of the Universe studying galaxies and quasars at redshift 6.5, and this window between 380,000 years after the big bang and 900 million years after the big bang is wide open, we have no clue as to what happens, and hidden within that period are the keys to understanding the formation of the first stars, the formation of the first black holes, galaxies, the reionisation of hydrogen, so there is a tremendous amount of information to find in this final missing chapter. So it is pushing the final frontier and trying to fill in the gaps in cosmic history, and just this notion that you can take a telescope today and catch the first galaxy as it turns on is truly remarkable, so within my lifetime with JWST, with 30-40 metre class ground-based telescopes, it is fully conceivable that we will be able to do this, and I think it is a tremendously exciting era to be studying the first galaxies, first stars, so I’m happy to be in the field right now, so that’s why I’m in the game I guess.