COS Science Goals and Requirements

Cosmic Origins SpectrographSystem Requirements Review

Cosmic Origins SpectrographHubble Space Telescope

Jon MorseJanuary 20, 1999

COS Science Goals and Requirements

Dr. Jon A. Morse

CASA, University of Colorado

COS SRR20 January 1999




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.”




QSO Absorption Linestrace the “Cosmic Web”

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




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





• 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)




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)




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/




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




Scientif ic Program Aperture G130M G160M G140L G190M # G260M # G230L Targets FUV exp NUV exp Total exp cts/s/resel FUV cts NUV cts Total cts # orbits cts/orbit kBytes/orbit

HeII Gunn-Peterson PSA 45000 0 0 0 0 0 0 0 5 225000 0 225000 0.003 2700000 0 2700000 83 32530.1 130

D/H PSA 20000 20000 0 20000 1 20000 1 0 10 400000 400000 800000 0.1 1.6E+08 6E+07 2.2E+08 296 743243 2972

Ly-alpha forest PSA 20000 20000 0 20000 3 20000 3 0 20 800000 2400000 3200000 0.08 2.6E+08 2.9E+08 5.4E+08 1185 459072 1836

Hot Gas in Halos PSA 20000 20000 0 20000 3 20000 3 0 20 800000 2400000 3200000 0.08 2.6E+08 2.9E+08 5.4E+08 1185 459072 1836

Local Starbursts PSA 25000 25000 0 25000 2 25000 1 0 10 500000 750000 1250000 0.02 4E+07 2.3E+07 6.3E+07 462 135281 541

Young SNRs PSA 0 0 15000 0 0 0 0 15000 3 45000 45000 90000 0.03 54000 67500 121500 33 3681.82 14

HB stars in GCs PSA 20000 20000 0 20000 1 20000 2 0 5 200000 300000 500000 0.1 8E+07 4.5E+07 1.3E+08 185 675676 2702

Cold ISM PSA 10000 10000 0 10000 2 10000 1 0 6 120000 180000 300000 0.05 2.4E+07 1.4E+07 3.8E+07 111 337838 1351

UV extinction PSA 0 0 8000 0 0 0 0 8000 10 80000 80000 160000 0.1 3200000 1200000 4400000 59 74576.3 298

UV extinction: STDs BOA 0 0 100 0 0 0 0 100 5 500 500 1000 10 2000000 750000 2750000 5 550000 2200

Planet occultations PSA 0 0 3000 0 0 0 0 3000 5 15000 15000 30000 5 3E+07 1.1E+07 4.1E+07 11 3750000 15000

Pluto and Triton PSA 20000 20000 0 20000 2 20000 2 0 2 80000 160000 240000 0.01 320000 240000 560000 88 6363.64 25

QSO Legacy Proj PSA 33000 33000 0 33000 3 33000 3 0 50 3300000 9900000 13200000 0.03 4E+08 4.5E+08 8.4E+08 2444 344313 1377

Local Group PNe PSA 12500 12500 0 12500 3 12500 2 0 30 750000 1875000 2625000 0.01 3000000 2812500 5812500 972 5979.94 23

Variability in AGN PSA 2500 2500 0 0 0 0 0 0 250 1250000 0 1250000 0.1 5E+08 0 5E+08 462 1082251 4329

UV upturn in E gals PSA 40000 40000 0 0 0 0 0 0 10 800000 0 800000 0.01 3.2E+07 0 3.2E+07 296 108108 432

UV lines of YSOs PSA 5000 5000 0 5000 1 5000 1 0 10 100000 100000 200000 0.1 4E+07 1.5E+07 5.5E+07 74 743243 2972

SN1987A impact PSA 3000 3000 0 3000 3 3000 4 0 1 6000 21000 27000 1 2400000 3150000 5550000 10 555000 2220

Variability in CVs I PSA 0 0 9000 0 0 0 0 0 5 45000 0 45000 5 9E+07 0 9E+07 16 5625000 22500

Variability in CVs II PSA 10000 0 0 10000 1 10000 1 0 30 300000 600000 900000 0.2 2.4E+08 1.8E+08 4.2E+08 333 1261261 5045

Hibernating novae PSA 0 0 0 0 0 0 0 10000 3 0 30000 30000 0.01 0 45000 45000 11 4090.91 16

Abs in young SNRs PSA 2000 2000 0 2000 1 2000 1 0 4 16000 16000 32000 5 3.2E+08 1.2E+08 4.4E+08 11 4E+07 160000

Diffuse clouds PSA 5000 5000 0 5000 1 5000 1 0 5 50000 50000 100000 2 4E+08 1.5E+08 5.5E+08 37 1.5E+07 59459

Peculiar stars PSA 5000 5000 0 5000 1 5000 1 0 10 100000 100000 200000 2 8E+08 3E+08 1.1E+09 74 1.5E+07 59459

LMC/SMC/MW ISM PSA 10000 10000 0 10000 1 10000 1 0 6 120000 120000 240000 1 4.8E+08 1.8E+08 6.6E+08 88 7500000 30000

LMC/SMC Fe group PSA 5000 5000 0 5000 2 0 0 0 6 60000 60000 120000 2 4.8E+08 1.8E+08 6.6E+08 44 1.5E+07 60000

M31 Fe group PSA 20000 20000 0 20000 2 0 0 0 3 120000 120000 240000 0.02 9600000 3600000 1.3E+07 88 150000 600

MW Halo (parallel) PSA 75000 75000 0 0 0 0 0 0 1 150000 0 150000 0.005 3000000 0 3000000 55 54545.5 218

Totals: 5053000 4528000 185500 9734000 9052000 170500 525 1E+07 19722500 30155000 1.216357 4.7E+09 2.3E+09 7E+09 8718 3906816 15626.964

sum sum sum sum sum sum sum sum sum sum average sum sum sum sum average average

14571.4 12607.1 1253.6 8767.86 7875 1289.3 13306.8 25156.25 38463.01 0.1 2018

average average average average average average av per prg av per prg av per prg median median

percent usage 0.1676 0.1502 0.006 0.3228 0.3002 0.006 0.346 0.654 0.6681 0.3319

COS Design Reference Mission

• 2/3 of time spent using NUV channel• 2/3 of data come from FUV channel




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)




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.

• 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 ~87% slit throughput.




COS FUV Spectroscopic Modes

Nominal Wavelength Resolving Power

Grating Wavelength Range (R = 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.




COS NUV Spectroscopic Modes

Nominal Wavelength Resolving Power

Grating Wavelength Range (R = 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.




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.




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.

Documents

COS Science Goals and Requirements