GSMT Science - Case Studies

1. GSMT Science - Case Studies Large Scale Structure and Cosmology Matthew Colless 4-5 December 2002 • A 3-D baryon map at high redshifts • The dark energy equation of state • Survey telescope design issues

2. A 3-D baryon map at z31. Science goals and method • Goal: A 3-D map of the baryon contents of the high-redshift universe using galaxies and the Ly forest as tracers. This map will provide a rich source of information on the large-scale structure of the dark matter and the baryons, the formation of galaxies, stars and metals, and the interplay between these various components. • Galaxies: A redshift survey down to densities equivalent to that of L* galaxies today (the missing link to present galaxy population). • IGM: Tomography from many QSO sight-lines, tracing LSS on scales 1 Mpc (the Ly  forest traces regions within 101 of mean density; HI optical depth goes monotonically with line-of-sight mass density). • LSS and galaxy formation: the relative distributions of the IGM and the galaxies give the mass distribution and biases, and together with the locations of metals, strongly constrain galaxy formation models.

3. A 3-D baryon map at z32a. Key measurements and baseline program (a) Galaxy redshfit survey • Redshift range: What is the science value of different z ranges? What luminosities and redshifts are accessible on a 30m with optical and/or NIR spectroscopy? • Optical spectroscopy over z2-3.5 - optical z’s in this range are ‘easy’; HI from Ly ; can pre-select by optical imaging. • Sky coverage: What area gives acceptable cosmic variance? • Smallest dimension 100Mpc 4 at z3  20. • 20 gives 4x108 Mpc3 over the range 2 z  3.5 or 1.2x108 Mpc3 in each z0.5 range (cf.2dF or SDSS). • Sample size: How many galaxies are required? • Ly-break galaxy density implies ~5 x 105 L  L* galaxies per z0.5 over 20, but z-completeness bias is an issue.

4. A 3-D baryon map at z32b. Key measurements and baseline program (b) IGM tomography • Background probes: How faint can background galaxies be for useful Ly  forest measurements with a 30m? • Extrapolating from 8-10m studies, down to at least R24. • Sampling: How densely must the IGM be sampled to map the 3-D distribution? What is the surface density of potential probes? • The density of suitable galaxies with z3 and R24 is 103-104/; the sampling scale is thus ~ 1 or ~ 0.5 Mpc. • Field of view and multiplex: • For these densities, a fiber MOS with a multiplex of ~500 would cover all usable background sources over a 20 FoV with sufficient spectral resolution and range.

5. A 3-D baryon map at z33. Current status and future progress • Galaxy surveys: Existing surveys at z3 contain ~103 galaxies over small areas. Future work with VIRMOS, DEEP, IMAX etc. may increase this to ~104 galaxies. • Main limitation is that redshifts can only be measured for strongly star-forming LL* galaxies at z3. • IGM tomography: In its infancy now, but a rapidly growing field. • Strongly limited by the low surface density of sufficiently bright background sources at high enough redshift. • Prognosis: • 8m telescopes are beginning this work, but cannot probe the galaxy luminosity function at L* and below, and cannot densely sample the IGM on small scales.

6. A 3-D baryon map at z34. Need for a 30m telescope The gains from a 30m telescope over a 10m telescope are… • For the galaxy redshift survey: • to probe fainter down the galaxy LF, reaching below L* (a factor of 3 in luminosity) • to increase galaxy sample size in a given volume (a factor of about 10 in number density - depends on LF) • For the tomography of the IGM: • to increase the surface density of background probes (a factor of 10-100 - depends on LF and SFR of z3 galaxies) • These estimated gains follow from the larger aperture, under the assumption of comparable field of view. • Most sources are resolved, so only limited gain from AO.

7. A 3-D baryon map at z35. Instrument requirements • Field of view: This is survey science, with duration  1/FoV. Can such projects hope to get 100’s of nights on a 30m telescope? • Strong requirement for the largest possible field of view. • Adaptive optics: Targets are resolved in seeing 0.3 arcsec. • Good natural seeing sufficient; AO is not a significant issue. • Multi-fiber spectrograph for IGM: a 500-fiber MOS can do almost all available R24 IGM probes over 20 FoV; with 1 nt/field could do 105 IGM probes over 20 in 230 nts. • Wide-field, moderate-multiplex, moderate-resln fiber MOS. • Multi-slit spectrograph for galaxies: low-resolution (R500) multislit MOS with 500 slits over 20 FoV; 2hrs gives 80% completeness to R26.5; 5x105 galaxies over 20 in 250 nts. • Wide-field, moderate-multiplex, low-resln multislit MOS.

8. Dark energy equation of state1. Science goals and method • Goal: To determine the evolving equation of state (EoS) of the dark energy. Is the dark energy a cosmological constant? If not, can we constrain the nature of the dark energy based on the evolution of its EoS with redshift - i.e. w(z). • Method: Use LSS measurements at z1 to constrain w(z), which affects both the geometry and the growth of LSS, and hence… • the galaxy power spectrum • the Alcock-Paczynski z-space distortions of LSS • the cluster mass function • The strongest effects are at intermediate redshifts (z0.5-1.5): when during this crucial 1/3 of the universe’s history does the change from DM to DE domination occur, and how quickly?

9. Dark energy equation of state2. Key measurements and baseline program • A massive redshift survey: to measure the small changes in the LSS statistics due to the ~10% variation that might be expected in the DE EoS over z0.5-1.5, a survey of ~106 galaxies is needed. • Simulations: required to estimate the sample size and areal coverage for determining the evolution of the galaxy power spectrum and the A-P/z-space distortions. Issues include: • selection of target sample to minimize bias effects; • how to account for non-linear evolution & peculiar velocities. • Cluster redshifts: The survey should cover one of the deep, wide-field S-Z cluster surveys, so that z’s are obtained for a mass-selected sample of clusters, giving the evolution of Nclus(m,z).

10. Dark energy equation of state3. Current status and future progress • Supernovae: Currently ground-based high-z SNe searches; to be followed by SNAP satellite - complementary to LSS constraints. • Galaxy surveys: Ongoing DEEP/VIRMOS z-surveys; next step may be surveys using large FoV/multiplex MOS on 8m’s (e.g. FMOS on Subaru, KAOS on Gemini). • Cluster surveys: X-ray & S-Z surveys proposed - need follow-up. • Attempting to measure the DE EoS is a high-risk enterprise, but... • this is fundamental physics of the highest importance; • showing w-1 would be valuable, and determining w-1 and esp. dw/dz0 would be a vital clue to the nature of the DE; • multiple methods are essential to overcome degeneracies and systematics inherent in all approaches.

11. Dark energy equation of state4. (No) need for a 30m telescope? Program is not particularly well-suited to a 30m with a small FoV… • EoS from LSS statistics from a z-survey of ~106 galaxies at z~1: • survey area of 102-103 favors wider field (~1 not 20); • survey depth of R~24 is accessible with 8m; • 8m with 0.5-1 MOS now beats future 30m with 20 MOS. • Follow-up redshifts for S-Z or X-ray cluster samples: • cluster density is 1-100/, depending on survey details; • cluster z requires only a few z’s for brightest members; • survey clusters will mostly have z’s in the range 0.2-1.2, accessible to 8m (tho’ rare high-z clusters need 30m); • wide-field MOS on 8m seems optimal for this task also.

12. Survey telescope design issues • Most LSS observations are surveys, meaning that the area of sky to be covered is much larger than the telescope FoV, so that project duration is the product of the field area and the time per field… • For sky-limited observations, a survey’s duration is reduced in direct proportion to the ‘A’ product of aperture (reduced time per field) and field area (reduced number of fields). • Other things being equal (notably, the density of fibers/slits), then the survey duration will be the same on two telescopes if the mirror-diameter x field-diameter product is the same… • A survey can be carried out in the same time on a 10m with a 0.5 (1) FoV and a 30m with a 10 (20) FoV. • Since 8-10m telescopes with 0.5 FoV already exist, and an 8m with a 1.5 FoV (and proportionally massive multiplex) is mooted… • It will be hard to find cases where a 30m with a 20 FoV is going to have a major competitive edge for survey science.