Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado Cosmic Origins Spectrograph Science Goals and Observing Strategy

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado Visualization concept from Schiminovich & Martin Numerical simulation from Cen & Ostriker (1998) STIS spectrum from Penton et al. (2001) COS will study: Large-scale structure by tracing Hydrogen Lyman  absorptions Formation of galaxies Chemical evolution of galaxies and the intergalactic medium Hot stars and the interstellar medium of the Milky Way Supernovae, supernova remnants and the origin of the elements Young Stellar Objects and the formation of stars and planets Planetary atmospheres in the Solar System Quasar Absorption Lines trace the “Cosmic Web” of material between the galaxies

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado STIS G140M spectrum (resolution ~ 19 km/s) from Penton et al. (2001) showing several low-redshift intergalactic Lyman  absorbers along the sight-line to QSO TON-S180, including a pair of Ly  pairs. Significantly lower resolution would mistake the “pair of Ly  pairs” as just two broad components, leading to over-estimates of the gas temperature and under-estimates of the true H I column density.

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado 1. Large-scale structure, the IGM, and the origin of the elements The Lyman  Forest –conduct baryon census of the IGM –derive space density, column density distribution, Doppler widths, and two-point correlation functions –test association with galaxies and consistency with models of large-scale structure formation and evolution –tomographic mapping of cloud sizes and structure, requiring multiple nearby QSO sight-lines From Stocke (1997)

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado The distribution of the frequency, dN/dz, of strong and weak Lyman  absorbers with redshift (from Shull et al. 2001). Strong absorbers (log N HI > 14) were studied in the UV with FOS at low spectral resolution, but virtually nothing is known about the distribution of the far more numerous weak absorbers at far-UV and near-UV wavelengths. High signal-to-noise COS UV spectra are needed to determine the distribution of weak absorbers (log N HI  13) over the redshift range z = HST-FOS results Ground-based results HST-COS to come

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado 1. Large-scale structure, the IGM, and the origin of the elements He II Gunn-Peterson effect –trace the epoch of reionization via redshifted He II Ly  ( 304 Å ) absorption in low-density IGM –determine whether He II absorption is discrete or continuous –allows estimates of “ionization correction” in order to count baryons in the IGM –allows estimate of flux and spectral shape of background ionizing radiation from quasars and starbursts He II opacities predicted for continuous and line-blanketed IGM (Fardal et al. 1998)

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado 1. Large-scale structure, the IGM, and origin of the elements Origin of the elements –measure the primordial D/H to test Big Bang nucleosynthesis –track evolution of D/H with redshift and metallicity –track star formation rate and heavy element abundances with redshift

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado 1. Large-scale structure, the IGM, and the origin of the elements All observing programs to study the IGM require numerous QSO sight-lines to find rare damped Ly  and metal absorption systems. Observing faint sources is necessary to obtain a sufficient number of sight-lines to map Ly  cloud space density distribution and geometry. COS sensitivity allows access to many more QSO sight-lines than previously possible with moderate resolution UV spectrographs. From Penton & Shull (1998)

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado 2. Formation, evolution, and ages of galaxies QSO sight-lines will probe hot gas associated with galaxy halos –measure abundances in halos and energy content of gaseous outflows –study interface between halos and IGM –connect abundances to star formation rates and feedback to galaxies Origin of young stellar systems and the heavy elements –local counterparts to high-z star forming galaxies –the violent ISM of starburst galaxies –nucleosynthesis in ejecta-dominated supernova remnants From Leitherer (1997) N132D in the LMC From Morse et al. (1996)

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado 3. Stellar and planetary origins and the cold interstellar medium Cold gas in the interstellar medium –physics and chemistry of translucent clouds –measure gas-phase atomic and molecular abundances in the regime where gas is predominantly molecular, dust grains accrete icy mantles, and the first steps in the condensation process, ultimately leading to star formation –determine ubiquity of PAHs in the ISM and molecular clouds –measure UV extinction toward highly reddened stars From Snow (1997)

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado 3. Stellar and planetary origins and the cold interstellar medium Planetary science in our Solar System –occultation studies of planetary, satellite, and cometary atmospheres –COS provides access to numerous background OB stars –probe atmospheric pressure, temperature, density profiles to nano-bar levels –determine abundances of volatile gases and albedos of Pluto and Triton 4. Other potential COS observing programs AGN monitoring campaigns UV upturn in elliptical galaxies UV monitoring of distant supernovae observations of SN1987A as it impacts circumstellar rings stellar winds and UV properties of LMC/SMC massive stars monitoring of CVs and other high- energy accretion systems SEDs of YSOs; diagnostics of heated accretion columns chromospheres of cool stars planetary aurorae and cometary comae detection of faint UV emission in ISM shocks

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado COS has 2 channels to provide low and medium resolution UV spectroscopy –FUV: Å, NUV: Å FUV gratings: G130M, G160M, G140L NUV gratings: G185M, G225M, G285M, G230L –All M gratings have a spectral resolution requirement of R  20,000 NUV MAMA Detector (STIS spare) Calibration Platform FUV XDL Detector OSM2: G185M, G225M, G285M, G230L, TA1 OSM1: G130M, G160M, G140L, NCM1 Aperture Mechanism: Primary Science Aperture, Bright Object Aperture Optical bench (not shown): re-use of GHRS bench

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado Spectral Resolution Most extragalactic/IGM programs require S/N > 10 per spectral resolution element, and ideally S/N = 20  30 is needed for accurate abundance measurements using redshifted lines of, e.g., Ly , C IV, N V, and O VI. Many Galactic ISM programs require S/N > 100 to detect weak lines. Signal-to-Noise Moderate spectral resolution of R > 20,000 (= 15 km/s) is required to resolve D I on wings of H I features (4-5 resels separation), measure Doppler widths of Ly  clouds, and detect weak absorption features from continuum. “Survey mode” of R = ~1000  3500 available for characterization of spectral energy distributions, UV extinction curves, and detection of the very faintest UV sources.

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado Wavelength Accuracy Extragalactic moderate resolution programs generally require absolute wavelength accuracy of ~ +/- 1 resel ( = +/- 15 km/s), with relative accuracy of 1/3 resel rms across the spectrum. Some programs that require higher accuracy can use “tricks” to obtain needed calibration  e.g., using known wavelengths of ISM lines along sight-line. Target Acquisition COS is a “slitless” spectrograph, so precision of target acquisition (placement of target relative to calibration aperture) is largest uncertainty for determining the absolute wavelength scale. Goal is to center targets routinely in science apertures to precision of +/- 0.1 arcsec (= +/- 10 km/s). Throughput is relatively insensitive to centering due to large size of science apertures; centering of +/- 0.3 arcsec necessary for >98% slit throughput.

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado The Primary Science Aperture (PSA) is a 2.5- arcsecond field stop located on the HST focal surface near the point of maximum encircled energy. This aperture transmits ~98% of the light from a well-centered aberrated stellar image delivered by the HST OTA. The PSA is expected to be used for most COS observations. The Bright Object Aperture (BOA) also is a 2.5- arcsecond diameter field stop and contains a neutral density (ND2) filter that permits observation of bright targets. Because COS is a slitless spectrograph, the spectral resolution depends on the nature of the target. The medium-dispersion gratings deliver resolutions R > 20,000 for unresolved sources (intrinsic size 0.1” FWHM). However, for an extended source, for example, ~0.5” in diameter, the spectral resolution is degraded to R ~ Though not optimized for extended objects, COS can be used to detect faint, diffuse sources with degraded spectral resolution. It is also important to note that the science apertures are NOT re-imaged by the spectrograph (as with STIS); the apertures are slightly out of focus and do not project sharp edges on the detectors.

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado Sensitivity Estimates COS is designed to break the “1e-14 flux barrier” for moderate resolution UV spectroscopy, enabling order of magnitude increases in accessible UV targets for a broad range of science programs.

Cosmic Origins Spectrograph Hubble Space Telescope Jon A. Morse University of Colorado Sensitivity Limits Goal is to obtain moderate resolution spectra with S/N = 10 per spectral resel of ~1e-15 flux sources in 10,000 seconds through the Primary Science Aperture. Bright Object Aperture (with a factor of ~100 attenuation) provides access to bright sources  e.g., calibration sources  and increases overlap with STIS flux range to serve as back-up.