Cosmic Origins Spectrograph - Science Goals

Science Goals

The Cosmic Origins Spectrograph is designed to enable the observation of faint, point-like UV targets at moderate spectral resolution, allowing COS to observe hot stars (OB stars, white dwarfs, cataclysmic variables and binary stars) in the Milky Way and to observe the absorption features in the spectra of active galactic nuclei. Observations are also planned of extended objects. Spectroscopy provides a wealth of information about distant astronomical objects that is unobtainable through imaging:

Spectroscopy lies at the heart of astrophysical inference. Our understanding of the origin and evolution of the cosmos critically depends on our ability to make quantitative measurements of physical parameters such as the total mass, distribution, motions, temperatures, and composition of matter in the Universe. Detailed information on all of these properties can be gleaned from high-quality spectroscopic data. For distant objects, some of these properties (e.g., motions and composition) can only be measured through spectroscopy.

Ultraviolet (UV) spectroscopy provides some of the most fundamental diagnostic data necessary for discerning the physical characteristics of planets, stars, galaxies, and interstellar and intergalactic matter. The UV offers access to spectral features that provide key diagnostic information that cannot be obtained at other wavelengths.

Obtaining absorption spectra of interstellar and intergalactic gas forms the basis of many of the COS science programs. These spectra will address questions such as how was the Cosmic Web formed, how much mass can be found in interstellar and intergalactic gas, and what is the composition, distribution and temperature of this gas. In general, COS will address questions such as:

• What is the large-scale structure of matter in the Universe?
• How did galaxies form out of the intergalactic medium?
• What types of galactic halos and outflowing winds do star-forming galaxies produce?
• How were the chemical elements for life created in massive stars and supernovae?
• How do stars and planetary systems form from dust grains in molecular clouds?
• What is the composition of planetary atmospheres and comets in our Solar System (and beyond)?

Some specific programs include the following:

Large-Scale Structure of Baryonic Matter: With its high FUV spectroscopic sensitivity, COS uniquely suited for exploring the Lyman-alpha forest. This is the ‘forest’ of absorption spectra seen in the spectra of distant galaxies and quasars caused by intergalactic gas clouds, which may contain the majority of baryonic matter in the universe. Because the most useful absorption lines for these observations are in the far ultraviolet and the sources are faint, a high sensitivity FUV spectrograph with wide wavelength coverage is needed to perform these observations. By determining the redshift and line width of the intervening absorbers, COS will be able to map out the temperature, density and composition of dark baryonic matter in the Cosmic Web.

Warm–hot intergalactic medium: Absorption line studies of highly ionized (hot) gas (O IV, N V, etc.) and broad Lyman-alpha will explore the ionization state and distribution of hot intergalactic gas.

Great Wall Structure: Background active galactic nuclei will be used to study intergalactic absorbers to estimate their transverse size and physical density and determine how the distribution of material correlates with nearby galaxy distributions in the CFA2 Great Wall.

He II Reionization: Highly redshifted ionized helium will be used study the reionization process at a redshift (z) of ≈ 3.