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Image sky
Select targets
Design plug-plates
Plug fibers
Observe!
Extract spectra
Subtract sky spec.
Fit redshift
Make 3-D map
Test physics!
David Schlegel Cosmology @ Beach, Jan 2014!1
Cosmology on the Beach: Experiment to Cosmology
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Image sky
Select targets
Design plug-plates
Plug fibers
Observe!
Extract spectra
Subtract sky spec.
Fit redshift
Make 3-D map
Test physics!
David Schlegel Cosmology @ Beach, Jan 2014!2
Monday: Fitting the universe, detectors + imaging maps !Tuesday: Sloan Digital Sky Survey (SDSS) !Thursday: DESI and future spectroscopic surveys
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Optical redshift surveys are easy !Each night: ~100s optical photons from each distant galaxy Detectable at ~100% efficiency Enough information for a redshift -> 3-D map
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The Dark Energy Spectroscopic Instrument (DESI) and Euclid (space) redshift surveys
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Edge of the observable Universe at 13.7 billion light years
DESI and Euclid
SDSS-III/BOSS has only mapped <1% of the observable Universe
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Effective volume ~ ∫ dV / (1 + 1/nP)
Shot noise at scale of interest
On the BAO scale of 100 Mpc, shot noise small if n > 1 per (10 h-1Mpc)3
Cosmological constraints scale as “effective volume”
Mapping large volumes ➔ large telescope field-of-view + multiplexing many objects
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What’s after SDSS?
Wide-field optical solution discovered 2009
!7
DESI on the Kitt Peak 4-m telescope
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Coffee break, March 6, 2009
Mike Sholl
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26 days after that coffee break
DESI
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!10
The challenge is finding telescopes that have an optical solution for big fields
• Faster speed M1 = More difficult • Larger M1 ➔ larger C1. C1 diameter > 1.25m prohibitively expensive
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Redshift survey science goals• Distance-redshift relation
— Measure distance scale to <0.3% between 0.0 < z < 1.1 — Measure distance scale to <0.3% between 1.1 < z < 1.9 — Measure the Hubble parameter to < 1% in the bin 1.9 < z < 3.7
• Gravitational growth growth — Constrain the growth factor at ~ a few percent level up to z=1.5
• Beyond Dark Energy — Constrain spectral index of primordial perturbations and its
running to < 0.4% — Measure the neutrino masses to < 0.017 eV
!11
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N. Padmanabhan 11/21/2013 Directors Review 5
4 million LRGs
23 million ELGs
0.6 million Ly-A QSOs +1.4 million QSOs
Four target classes spanning redshifts z=0 ➔ 3.5 Includes all the massive black holes in the Universe (LRGs + QSOs)
What is DESI?
!12
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LRG Targets
!13
Exp
ansi
on R
ate
Luminous Red Galaxies ~4 x 106 goal 0.4 < z < 1.0
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ELG Targets
!14
Exp
ansi
on R
ate
Emission Line Galaxies ~24 x 106 goal 0.7 < z < 1.6
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QSO Targets
!15
Exp
ansi
on R
ate
Quasar Tracers ~1.6 x 106 goal 0.9 < z < 2.2
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Ly-α Forest QSO Targets
!16
Exp
ansi
on R
ate
Quasar Ly-α Forest ~0.6 x 106 goal 2.2 < z < 3.5
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DESI on the Hubble Diagram
!17
Exp
ansi
on R
ate
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Redshift surveys distinguishing MG from DE
!18z
fσ8
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3.2° field-of-view corrector
5000 fiber actuators
Spectrographs
DESI at the Mayall Telescope
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3.2° field-of-view corrector
5000 fiber actuators
Spectrographs
DESI at the Mayall TelescopeWeight of 3 Ford Explorers
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3.2° field-of-view corrector
5000 fiber actuators
Spectrographs
DESI at the Mayall TelescopeWeight of 3 Ford Explorers
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���22
Focal plane
5000 robotic positioners on a 4-m telescope
DESI is the next big step in mapping the Universe
>15X more powerful than SDSS-III/BOSS
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BOSS limitation using hand-plugged “plates”
Not possible to plug >2 million galaxies
1000 fibers
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DESI using robotically-positioned fibers
Map of 50 million galaxies possible
12 mm
5000 fibers robotically move in 1 minute
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Prime Focus Corrector
!25
• Requirements – Field of view: 3.2˚ linear = 8 sq. deg – Wavelength band: 360 – 980 nm – Geometric, manufacturing, and
alignment blur <19 µm RMS radius, field-averaged
– Tilt of chief ray: <1˚
1000 kg of glass lenses
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!26
Prime Focus Corrector• Includes atmospheric dispersion corrector
(SDSS does not have one)
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C.Bebek
Focal Plane
!27
5000 robotic positioners Reconfigure in < 1 minute
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Lots of development on robotic fiber positioners
• Reconfigure all fibers in 1 min; 20 min exposures • Two design approaches in play
– Tilting spine – Eccentric axis
- central and eccentric motors - planar motion, fast, accurate
- stick-slip piezo spine - more coverage overlap, simple, elegant
!28
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Analysis problem of robotic fiber positioners
• Fiber assignment more “fun” than in SDSS • Details will depend upon final design
!29
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Fiber robots: Tilting Spine
FMOS focal plane at Subaru
!30
Fiber ferrule
Carbon fiber tube
Counterweight
Piezoceramic tube(actuator) Cup and ball
(pivot)
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Fiber robots: Eccentric Axis
Fiber heldhere
!31
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Optical fibers are wonderfully good• Much longer fiber than for BOSS (35 m, compare to 1.7 m) • Loses ~30% of light in the blue, but places spectrographs
in stable environment away from telescope
!32
~3 m ~33 m
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Many fibers ➔ many spectrographs ➔ many z’s
BOSS had 4 cameras DESI will have 30 cameras
!33
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!34
Redshift surveys target the easiest galaxies
DESI
4 million LRGs
23 million ELGs
1 million Ly-A QSOs +2.5 million QSOs
Mayall telescope available up to 100% of dark time 30 million targets 14,000 deg2 (1/3 of the sky)
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!35
Redshift surveys target the easiest galaxies
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!36
1. Luminous Red Galaxies (LRGs)These are the most massive galaxy halos LRGs have been the workhorse of BAO surveys (SDSS, BOSS) All LRG spectra look nearly identical to z~1
Entire spectrum used for redshift, dominant features are “4000 Angstrom break” and “Ca H+K lines” to z=1.2
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!37
2. Emission Line Galaxies (ELGs)At z~1, galaxies were forming many more stars, easy to see !ELGs unique signature of [O II] doublet, detectable from z=0 to z=1.7
[O II] doublet at 3726.032 + 3728.815 Ang DESI detects to z=1.6 at 9700 Ang
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3. QSOs as tracersQSO spectra are obvious even at very faint S/N BOSS survey easily identifies to g=22, DESI extends to r=23
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!39
4. Lyman-alpha forest from QSOsQSOs at z < 2.2 will be observed once → “tracer QSOs” QSOs at z > 2.2 will be observed 5X for high S/N for “Lyman-alpha forest”
DESI signal
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David Schlegel Cosmology @ Beach, Jan 2014!40
λ
λ λ
Raw image Spectrum + errorsClassify as galaxy z=0.3597
Spectrum → classification + redshift
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David Schlegel Cosmology @ Beach, Jan 2014!41
Hypothesis testing Given: Galaxy spectrum templates Compute: Χ2 of each template at each redshift
=
Compute-intensive: Search all possible z’s for all templates (very little code)
Spectrum → classification + redshift
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David Schlegel Cosmology @ Beach, Jan 2014!42
Spectrum → classification + redshift
Galaxy spectra well-described by linear combinations of a small set of “eigenspectra”
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David Schlegel Cosmology @ Beach, Jan 2014!43
Hypothesis testing Given: Galaxy spectrum templates Compute: Χ2 of each template at each redshift
Result: Select template with smallest Χ2
Spectrum → classification + redshift
Χ2
z→
If ΔΧ2 too small, then solution uncertain
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David Schlegel Cosmology @ Beach, Jan 2014
Strong lenses discovered spectroscopically,from rogue emission lines (Requires very good noise model at the pixel level)
!44
Spectrum → classification + redshiftAn interesting example of where this did not work...
10-σ outliers from single pixels
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!45
The largest spectroscopic survey for dark energyVolume of redshift surveys
SDSS ~2h-3Gpc3 BOSS ~6h-3Gpc3
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!46
4 million LRGs
23 million ELGs
3 million QSOs
The largest spectroscopic survey for dark energySDSS ~2h-3Gpc3 BOSS ~6h-3Gpc3 DESI 50h-3Gpc3
Dark energy turns on here
?
Volume of redshift surveys
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!47
4 million LRGs
23 million ELGs
3 million QSOs
These are the easiest 30 million objects to observe
Dark energy turns on here
?
Volume of redshift surveys
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!48
DESI distance-redshift relation (predicted for 2022)
— BAO geometric probe with 0.3-1% precision from z=0.5 -> 3 — 35 measurements with 1% precision
HST Key Project, 10% error
SDSS-II + 2dF + 6dFGS BAO
WiggleZ BAO
BOSS (in progress)
DESI
Planck
z=1000
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DESI compared to BOSS
!49
3X aperture"5X fibers"1.1X field-of-view"~2X resolution (red)
2 million BOSS-quality spectra"possible in one lunation
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!50
Euclid satellite redshift survey
Technique of slit-less spectroscopy (“objective prism”) pioneered by Edward Pickering in 1882 to classify stars !Used very little for the past ~70 years … never used for galaxy redshift surveys
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Cosmology @ Beach, Jan 2014
Real grism data from duPont telescope 2.5-m 0.64-0.75 micron (Nick Mostek)
emission line
!51
Disperse all the light for all objects in your field-of-viewEuclid satellite redshift survey
Complicated: Mixes spatial + spectroscopic info
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Euclid satellite redshift survey
Why do this crazy prism survey? 1. Almost no moving parts 2. Can observe the brightest H-alpha (H 3-2 transition) from z=1-2
DESI uses this signature from O+
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D. Schlegel, 10 Dec 2013
DESI + Euclid “on the curve” for redshift surveys
!53HST Ultra-Deep Field 10,000 galaxies / (11 arcmin2)
140 billion
1980 20611000
Year
log N(galaxies)
SDSS, 2009 929,000
CfA1, 1983 1840
2dF, 2003 221,414
CfA-2, 1998 18,000
LCRS, 1996 18,678
← Redshift surveys complete in 2061
2025 30 million
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David Schlegel Cosmology @ Beach, Jan 2014
3-D map of “objects” (galaxies, quasars) Velocities Mass
DESI, Euclid are not the end for redshift surveys
DESI + Euclid only 30 million of observable galaxies (0.02%)
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~Billions of objects in SDSS, DES, LSST
Shapes of distant galaxies ∝∑mass line-of-sight
... but S/N~0.01 per object
Planck, LSST are not the end for mapping mass
DESI + Euclid only 30 million of observable galaxies (0.02%)
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Possible to measure S/N~1 per object by predicting shapes of galaxies -- c.f. M. Morales 2006
Shapes of distant galaxies ∝∑mass line-of-sight
... but S/N~0.01 per object
Planck, LSST are not the end for mapping mass
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shear
Imaging only: Intrinsic shapes of galaxies unknown
Spectroscopy: Intrinsic shapes of galaxies mapped
shearBOSS BOSS
Courtesy Matt George
Why stop at a galaxy density map? Map the 3-D mass!Huff et al. arXiv/1311.1489 George et al. in prep.
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DESI @ Berkeley, 15 July 2013 !58�4 �3 �2 �1 0 1 2 3 4
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Imaging only: Intrinsic shapes of galaxies unknown
Spectroscopy: Intrinsic shapes of galaxies mapped
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line-of-sigh velocities
Why stop at a galaxy density map? Map the 3-D mass!Huff et al. arXiv/1311.1489 George et al. in prep.
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!59
Conclusions + lessons
DESI, Euclid redshift surveys will be 20X better than BOSS Possible to be ~5000X better by looking at fainter galaxies
LSST will map projected mass densities Possible to measure mass density in all volume elements
Cosmology projections based upon DESI, Euclid, LSST … What data would be most constraining? … More than x-power measures in projection? (c.f. Uros Seljak’s lecture)