6. Science Topic 3: Galaxies and Cosmology.

Chair: B. Leibundgut, ESO.

Chairman's Summary of the Panel Discussion

NB:  A terse but semi-verbatim text of the actual panel discussions, recorded by by Bob Mann, is available here.

Cosmology dealing with the most distant galaxies will see a tremendous boost over the coming decade from investigations in the near and far infrared from space. These observations will cover the rest-frame optical wavelength region which is the best studied so far. With such a busy decade coming up, it is difficult to forecast the questions and problems remaining in ten or 15 years.

In addition the surface brightness dimming will make the extended objects harder to detect which argues for a large aperture telescope. What needs to be investigated is how faint, extended objects will be detected by the extremely large telescopes with adaptive optics and very small pixel sizes.

The panel discussed several topics ranging from dark matter to gamma-ray bursters. In the end they were collected into four major themes which each can be addressed by different experiments. Most of these themes require observations which can not be made with facilities in the next decade (mostly because of the spatial resolution requirements).

This text provides a basis for discussion. It tried to capture the discussion, but is certainly incomplete in many aspects. The experts should pick up their area of interest and complete/replace the text where they feel appropriate. It is also thought as a basis for continuous discussion of the science cases.


- Detailed picture of how the present-day galaxies formed

- Emergence of Large Scale Structure

- First light/The beginnings of galaxies

- Evolution of cosmological parameters

1. How did present-day galaxies form?

The connection between the Lyman-break galaxies at z > 2.5 and the present-day large galaxies is not completely clear. The mass history of massive galaxies, like the Milky Way, has not been observed. A goal should be to trace the assembly paths of different galaxy types. The role of dark matter in this assembly should be tracked through the history of galaxies. A major, baryonic component is currently only know as absorbing objects, the quasar absorption lines. The connection of these absorbing clouds with the luminous galaxies should be understood.

There were four specific projects which could be tackled with an extremely large telescope and should be part of a census of the building blocks of galaxies.

Kinematics of high-z galaxies

The current telescopes do not have the collecting area nor the resolving power to make a kinematic study of high-z galaxies. These galaxies have typical sizes of 0.5" or smaller. The goal has to be to spatially resolve these galaxies and measure velocities of stars in them. This will yield masses of these objects. By measuring line profiles one can separate the different stellar populations and measure their kinematics.

Identification of quasar absorbers

These objects are only know by their absorbing nature, but have not been seen in their own light. It will be important to differentiate between star forming and non-star forming clouds and understand the nature which transforms such clouds. This can be done by detailed metalicity studies of the absorbers, but also through the identification with real galaxies. The general structure of the inter-galactic medium  (IGM) is not known. The chemical enrichment of the IGM should be traced for a better understanding of the formation history of stars.

These studies would profit from an extremely large telescope in two ways. With increased spatial resolution, i.e. smaller PSF, the line of sight can be probed much closer to the quasar than with natural seeing and the faint, small objects, if they have comparable sizes to the known galaxies, will gain from the increased light gathering power.

Beyond the brightest high-z galaxies

Currently we know only the brightest high-z galaxies. A census of the fainter galaxies is completely missing. Their relevance for the present-day galaxies is unclear.It is not clear how much of these galaxies can be detected with the telescopes of the next decade.

Abundances in faint galaxies

The star formation is imprinted in the chemical enrichment of galaxies. So far this has only been possible in quasar absorbers, but the faint early galaxies have not been measured.

Observational techniques

The following instrumental capabilities were identified to attack the questions formulated in this section:

- High-resolution imaging

-Integral field spectroscopy

- Low resolution spectroscopy of faint galaxies.

These observations would preferebly be done at rest-frame optical wavelengths.

2. Emergence of Large Scale Structure

The highest-redshift clusters currently known are at about z=1.3. When did these clusters form? How did a massive object like the Coma cluster form and how would it have looked like at z=3? There is very little information at z > 1 and only some more at z ~ 3. For an identification the fainter cluster members have to be measured as well, which is currently not possible. The growth of massive structures should be measured from the earliest phases. This involves the identification of clusters, the mass determinations, the correlation lengths for various galaxy populations and the morphology-density correlation.

Identification of the SZ detections

The Planck satellite will provide a large list of SZ decrements and the redshift distribution will have to be established. Spectroscopic redshifts of many of the faint objects will need an extremely large telescope.

Masses can further be detected by weak lensing surveys. The question here is whether a sufficient background (z » 5) exists.


The progenitors of massive clusters should be identifiable out to z = 3, possibly with quasars and radio galaxies as light houses and sign posts. The identification and spectroscopy of the fainter galaxies is required for this.

Correlation lengths

What galaxies are in clusters? The morphology-density correlation should be characterized in much more detailed than is currently possible. This requires both spatial resolution and spectroscopy to resolve the morphologies and the redshifts.

Higher-order clustering


Observational techniques

The extended nature of clusters and proto-clusters requires wide fields. The identification has to be done with deep imaging and the the redshift distribution must be measured through spectroscopy.

3. First light /Beginnings of galaxies

We know the universe is fully re-lonized by z »6. The first emergence of light into the opaque universe should be found. These will be the earliest accessible stellar signatures and will shed light onto the history of the first stars and the enrichment of the IGM.

Search for the first observable objects

There are currently no established ideas what the first objects might look like. Presumably they are star formation regions of parsec size. The signature could be either Ly-a emission or the Ly-break through the absorption of the photons in the IGM. If the formation redshift is around 10, then these objects would be accessible in the near-IR.

Another way of finding the first elements is in the fossil record of the IGM. The first star will leave their ashes and pollute the gas with metals.

Observational technique

These objects have to be found in blind searches. The ideal way to search will be with integral-field spectroscopy as the line widths nor the position of the Ly-break will be known. Another option is to do wide-field narrow-band imaging between the OH sky emission. This will be restricted to specific redshift ranges. The fossil record in the IGM could be identified by high-resolution, high S/N spectroscopy.

4. Evolution of the cosmological parameters

By 2010 the current knowledge of the cosmological parameters will be highly refined. A critical information missing is the change of some of these parameters to distinguish between models which have decaying particle fields and others with a cosmological constant. An important test is to measure the evolution these parameters to find whether there is a time dependence.

Another refinement would be the measurement of the cosmological parameter with one single distance indicator. Cepheids will be calibrated through geometric means by GAIA. Measuring Cepheids out to z=1.2 would allow to see the early decelaration of the universe due to matter and the detailed transition to an accelerated expansion.

In addition, this will provide the calibration of Type Ia Supernovae and their potential use out to redshift of 5.

Observational technique

Regular imaging at the diffraction limit for the faintest possible point sources.