COSMOS at the Institute for Astronomy

These pages are the repository of the the contribution of the Institute for Astronomy at the University of Hawaii to the COSMOS ACS Legacy Survey.

What is COSMOS ?

COSMOS is a Hubble Space Telescope (HST) Treasury Program aimed at studying the interplay between Large Scale Structure of the Universe (LSS) and the formation and evolution of galaxies, using the Advanced Camera for Surveys (ACS). We are also gathering data on the COSMOS field with other ground and space based telescopes at various wavelengths to study in detail various aspects of galaxy evolution. The project is lead by Dr. Nicholas Scoville (Caltech), and the COSMOS Team consist in nearly forty scientists from various institutions in the US -including the IfA- Europe and Japan.

The four main characteristics of the COSMOS survey are:

• Deep ACS imaging
• A large field (2 square degree)
• A wealth of multi-wavelength data
• A quick access to the data for the scientific community.

Deep ACS imaging

ACS is a superb instrument to conduct imaging surveys for the study of galaxy evolution and Large Scale Structure. It offers a very good sensitivity, allowing for detecting faint and distant galaxies. A "typical" galaxy observed at a redshift of 2 has roughly a magnitude in the I band (814 nm) of 26. Our ACS observations will reach a depth of I=27 at 10 σ.

An other very important advantage of ACS imaging for the COSMOS project is its spatial resolution. HST imaging is diffraction limited while ground based imaging suffer from seeing, the effect of atmospheric turbulence. Typical ground based images in the optical achieve 0.7 arc second of resolution, while the ACS provides 50 milli-arcseconds image quality. With such an improvement, it is possible to measure accurately the shape of galaxies. This allows to:

• map the distribution of mass in our field using the technique of weak lensing.
• quantify precisely the morphological evolution of galaxies.

A large field

In order to measure the impact of large scale structure (LSS) on galaxy evolution, a large field is called for. For example, the Virgo cluster of galaxies, the nearby rich cluster of galaxies, encompasses a mass of 10¹⁴ Solar Masses in a sphere of 10 Mpc of diameter. The largest structures identified so far, the "bubble" and "voids" that form the LLS have sizes reaching 40 Mpc. Therefore, if one observe a given field to study the LSS, such a field has to be big enough to contain voids and bubbles.

A 2 square degree field fit this bill. Indeed, 1.4 degree on the sky correspond to:

• 50 Mpc at z=0.5
• 88 Mpc at z=1
• 137 Mpc at z=2
• 170 Mpc at z=3

The following figure illustrates this point. The two gray scale images represent the distribution of dark matter (gray background) and galaxies (black dots, the size indicating the brightness of the galaxies) at z=1 and 2 from simulations of the Virgo Consortium. The LSS is clearly visible with its clusters of galaxies, its filaments and voids. In between the two maps are the field sizes of the famous Hubble Deep Field North and South, as well as the two GOODS fields.

A wealth of multi-wavelength data

To complement our ACS observations, the COSMOS Team is gathering data with other means and at other wavelength to allow us to reach our scientific goals. This include for example:

• Deep ground based optical imaging in other bands to compute photometric redshifts.
• A spectroscopic survey to measure the redshifts of galaxies in the COSMOS field and map the 3 dimensional structure, using European Southern Observatory Telescopes.
• X-ray observations using XMM-Newton, to search for clusters of galaxies and active galaxies

The IfA astronomers involved in COSMOS are especially involved in the gathering of ground based imaging data and space based infrared data.

The status of our gathering is maintained on a separate page.

A quick access to the data for the scientific community

The COSMOS project is a HST Treasury Program. Calibrated ACS and ancillary data will be made available to the community roughly 1 year they have been taken.