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

Cosmic Origins SpectrographHubble Space Telescope

Jon A. MorseUniversity of Colorado

Cosmic Origins Spectrograph

Science Goals and

Observing Strategy



• 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 interstellarmedium of the Milky Way• Supernovae, supernova remnants and the origin of the elements• Young Stellar Objects and theformation of stars and planets• Planetary atmospheres in the Solar System

Quasar Absorption Linestrace the “Cosmic Web” ofmaterial between the galaxies



• STIS G140M spectrum (resolution ~ 19 km/s) from Penton et al. (2001) showingseveral 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-estimatesof the true H I column density.



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)



• The distribution of the frequency, dN/dz, of strong and weak Lyman absorbers with redshift (from Shull et al. 2001). Strong absorbers (log NHI > 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 NHI 13) over the redshift range z = 0.1 - 1.6.

HST-FOSresults

Ground-basedresults

HST-COSto come




• 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 continuousand line-blanketed IGM (Fardal et al. 1998)



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




• 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)



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)

N132Din the LMC

• From Morse et al. (1996)



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)



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



• COS has 2 channels to provide low and mediumresolution UV spectroscopy– FUV: 1150-1775Å, NUV: 1700-3200Å

• FUV gratings: G130M, G160M, G140L

• NUV gratings: G185M, G225M, G285M, G230L

– All M gratings have a spectral resolution requirement of R 20,000

NUV MAMADetector

(STIS spare)

CalibrationPlatform

FUV XDLDetector

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 GHRSbench



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.



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.



• 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 ~ 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.



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.



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.

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