Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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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
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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A 3-D baryon map at z3
1. 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 largescale 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.
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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A 3-D baryon map at z3
2a. 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.
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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A 3-D baryon map at z3
2b. 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.
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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A 3-D baryon map at z3
3. 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.
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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A 3-D baryon map at z3
4. 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 10100 - 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.
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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A 3-D baryon map at z3
5. 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.
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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Dark energy equation of state
1. 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?
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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Dark energy equation of state
2. 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).
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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Dark energy equation of state
3. 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.
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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Dark energy equation of state
4. (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.
Cosmology & Large Scale Structure Case, Matthew Colless, GSMT SWG, 4-5 Dec 2002
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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.