COS Science Goals and Requirements

1. COS Science Goals and Requirements Dr. Jon A. Morse CASA, University of Colorado COS SRR 20 January 1999

2. COS Science Goals • Large-scale structure, the IGM, and origin of the elements. • Formation, evolution, and ages of galaxies. • Stellar and planetary origins and the cold interstellar medium. “Spectroscopy lies at the heart of astrophysical inference.”

3. Wavelength (Å) COS will study large-scale structure and the IGM • Visualization concept from Schiminovich & Martin • Numerical simulation from Cen & Ostriker (1998) • Songaila et al. (1995) Keck spectrum adapted by Lindler & Heap QSO Absorption Lines trace the “Cosmic Web”

4. 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 a (l304 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)

5. 1. Large-scale structure, the IGM, and the origin of the elements • The Lyman a 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)

6. 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

7. 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 a and metal absorption systems. • Observing faint sources is necessary to obtain a sufficient number of sight-lines to map Ly a 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)

8. 2. Formation, evolution, and ages of galaxies • From Leitherer (1997) • 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 • N132D • in the LMC • From Morse et al. (1996)

9. 2. Formation, evolution, and ages of galaxies • Refine ages of globular clusters • constrain helium mixing in Horizontal-Branch stars that affects luminosity and lifetime estimates of H-B phase and relation to main sequence turn-off • measure abundances, Lbol, Teff,vsini’s in Horizontal-Branch stars • use tracers like Al to estimate mixing • H-B stars in globular clusters are easily studied in the UV • From Heap (1997) • Image from http://fondue.gsfc.nasa.gov/UIT/Astro2/

10. 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)

11. 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 • 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 • 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

12. COS Design Reference Mission • 2/3 of time spent using NUV channel • 2/3 of data come from FUV channel

13. COS Science Requirements • The COS Science Goals require: • point-source spectroscopy at UV wavelengths • medium spectral resolution (R > 20,000) • highest possible throughput • broad wavelength coverage in one exposure • These goals are met using a combination of: • Rowland circle spectrograph design (FUV) with only 1 reflection • high-efficiency 1st-order holographic gratings • large-format, solar-blind detectors • HST’s capabilities • large collecting area • UV coatings • excellent pointing stability • superb image quality (after aberration correction)

14. Spectral Resolution • 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 Lya 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. • 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 a, C IV, N V, and O VI. • Galactic ISM programs require S/N > 100 to detect weak lines. Signal-to-Noise

15. 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 ~87% slit throughput.

16. COS FUV Spectroscopic Modes Nominal Wavelength Resolving Power Grating Wavelength Range (R = l/Dl) b Coverage a per Exposure G130M 1150 - 1450 Å 300 Å 20,000 - 24,000 G160M 1405 - 1775 Å 375 Å 20,000 - 24,000 G140L 1230 - 2050 Å > 820 Å 2500 - 3500 a Nominal Wavelength Coverage is the expected usable spectral range delivered by each grating mode. The G140L grating disperses the 100 - 1100 Å region onto one FUV detector segment and 1230 - 2400 Å onto the other. The sensitivity to wavelengths longer than 2050 Å or shorter than 1150 Å will be very low. b The lower values of the Resolving Power shown are delivered at the shortest wavelengths covered, and the higher values at longer wavelengths. The resolution increases roughly linearly between the short and long wavelengths covered by each grating mode.

17. COS NUV Spectroscopic Modes Nominal Wavelength Resolving Power Grating Wavelength Range (R = l/Dl) b Coverage a per Exposure G190M 1700 - 2400 Å 3 x 45 Å 20,000 - 27,000 G260M 2400 - 3200 Å 3 x 55 Å 20,000 - 27,000 G230L 1700 - 3200 Å 1000 Å 850 - 1600 G130MB 1150 - 1800 Å 3 x 30 Å 20,000 - 30,000 a Nominal Wavelength Coverage is the expected usable spectral range delivered by each grating mode, in three non-contiguous strips for the medium-resolution modes. The G230L grating disperses the 1st-order spectrum between 1700 - 3200 Å along the middle strip on the NUV detector. G230L also disperses the 400 - 1400 Å region onto one of the outer spectral strips and the 3400 - 4400 Å region onto the other. The shorter wavelengths will be blocked by an order separation filter and the longer will not register because the detector is solar blind. The G230L 2nd-order spectrum between 1700 - 2200 Å may be detectable along the long wavelength strip. b The lower values of the Resolving Power shown are delivered at the shortest wavelengths covered, and the higher values at longer wavelengths. The resolution increases roughly linearly between the short and long wavelengths covered by each grating mode.

18. Sensitivity Limits • 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.

19. Sensitivity Limits • Goal is obtain moderate resolution spectra with S/N = 10 per spectral resel of ~1.5e-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. • 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 ~87% 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 ~ 5000. 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.