The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX)

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The Hobby-Eberly Telescope Dark Energy Experiment
(HETDEX)

• Karl Gebhardt, Gary Hill, Phillip MacQueen
• Eiichiro Komatsu, Niv Drory, Povilas Palunas
• McDonald Observatory Department of Astronomy,
  University of Texas
• Peter Schuecker, Ralf Bender, Uli Hopp, Claus
  Goessl, Ralf Koehler
• MPE and Uni-Sternwarte Munich
• Martin Roth, Andreas Kelz
• AIP, Potsdam

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Hobby-Ebery Telescope (9.2m)
HET Mt. Fowlkes west Texas
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Goals for HETDEX

• HETDEX measures redshifts for about 1 million
  LAEs from 2ltzlt4
• Wavelength coverage 340-550 nm at R800
• Baryonic oscillations determine H(z) and Da(z)
  to 1 and 1.4 in 3 redshift bins
• Constraints on constant w to about one percent
• Tightest constraints on evolving w at z0.4 (to
  a few percent)

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Ly-a emitters as tracers

• Properties of LAEs have been investigated through
  NB imaging
• Most work has focused on z 3 4, little is
  known at z 2
• Limiting flux densities few e-17 erg/cm2/s
• They are numerous
• A few per sq. arcmin per Dz1 at z3
• But significant cosmic variance between surveys
• 5000 10000 per sq. deg. Per Dz1 at z3
• Largest volume MUSYC survey still shows
  significant variance in 0.25 sq. degree areas
• Bias of 2 3 inferred
• Basic properties of LAEs would make them a good
  tracer if they could be detected with a large
  area integral field spectrograph units (IFUs)
• Has the advantage of avoiding targeting
  inefficiency

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VIRUS

• Visible IFU Replicable Unit Spectrograph
• Prototype of the industrial replication concept
• Massive replication of inexpensive unit
  spectrograph cuts costs and development time
• Each unit spectrograph
• Covers 0.22 sq. arcmin and 340-550 nm wavelength
  range, R850
• 246 fibers each 1 sq. arcsec on the sky
• 145 VIRUS would cover
• 30 sq. arcminutes per observation
• Detect 14 million independent resolution elements
  per exposure
• This grasp will be sufficient to obtain survey in
  110 nights
• Using Ly-a emitting galaxies as tracers, will
  measure the galaxy power spectrum to 1
• Prototype is in construction
• Delivery in April

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Layout of 145 IFUs w/ 1/9 fill
New HET wide field corrector FoV
0.22 sq. arcmin
(20 dia field)

• Layout with 1/9 fill factor is optimized for
  HETDEX
• IFU separation is smaller than non-linear scale
  size
• LAEs are very numerous so no need to fill-in
  want to maximize area (HETDEX is sampling
  variance limited)
• Well-defined window function
• Dithering of pointing centers removes aliasing

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VIRUS on HET

• 145 VIRUS units will be housed in two saddle
  bags on the HET frame
• Fiber feed allows offloading of the mass of the
  instruments to this location

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VIRUS on HET (detail)

• HET will be upgraded with a new wide field
  corrector with 22 arc-minute field of view
• Substantial upgrade 3.5 arc-minutes ? 22
  arc-minutes
• New tracker and control system

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Optical design of VIRUS module
Flat mirror

• Science driver requires coverage of 340-550 nm at
  R800
• Very few elements, simple to set up
• Image quality easily meets spec
• With dielectric mirror coatings (340-680 nm)
  expect 70 thorughput
• Complexity of internal focus camera

Spherical collimator mirror
VPH Grating 115 mm beam
f/1.33 Camera 2kx2k CCD
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VIRUS Prototype Unit Spectrograph

• Will be completed this summer
• Tests the design and performance of the
  instrument
• Refines the cost estimate for replicating VIRUS
• Will be used for a 50 night pilot survey of LAEs
  on the McDonald 2.7 m

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Lyman-a Emitters

• There are ever increasing number of observations
  on LAE
• Compilation of the recent data and GALFORM
  modeling by Delliou et al. (2005)
• Most recent data very consistent
• Theory and data matching well
• Not very reliable, but useful starting point to
  design surveys
• More accurate number counts will be obtained from
  VIRUS proto-type.

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Predicted Number Counts

• Sensitivity of VIRUS (5-s)
• 2e-17 erg/cm2/s at z2
• 1e-17 erg/cm2/s at z3
• 0.8e-17 erg/cm2/s at z4
• Detected LAEs approximately constant with
  redshift
• sensitivity tracks distance modulus
• predict 5 / sq. arcmin 18,000 / sq. deg. per
  Dz 1
• With Dz1 and 1/9 fill factor, expect 3,000
  LAEs/sq. degree
• 0.6 million in 200 sq. degrees
• Sufficient to constrain the position of the BO
  peaks to lt1
• HETDEX will require 1100 hours exposure or 110
  good dark nights
• Needs 3 Spring trimesters to complete (not a
  problem HET is OUR telescope!)

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Experimental Requirements

• A LAE DE survey reaching lt1 precision requires
• large volume to average over sample variance
• 200-500 sq. degrees and Dz 2
• this is 6-15 Gpc3 at z2-4
• surface density 3000 per sq. degree per Dz1 1
  M galaxies
• LAEs have 18,000 /sq. deg./Dz1 at line flux
  1e-17 erg/cm2/s
• only require a fill factor of 1/9 to have
  sufficient statistics
• so we can trade fill factor for total area
• lowest possible minimum redshift (bluest
  wavelength coverage)
• z 1.8 at l3400 A is a practical limit
• ties in well with high redshift limit of SNAP and
  other experiments
• These science requirements determined the basic
  specifications of VIRUS

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Status of HETDEX

• The prototype VIRUS unit is being built and will
  be on the McDonald 2.7m in Aug 2006, with 50
  night observing campaign
• Pilot run on Calar Alto in Dec saw 4 hrs data in
  8 nights, but we will go back
• Full VIRUS is in design phase with full funding
  expect completion 2008-2009
• HETDEX will then take 3 years, finishing
  2011-2012
• 30M project (including operation cost and data
  analysis) 15M has been funded.

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HETDEX Uncertainties
N/2
Current HETDEX design

• HETDEX is sampling variance limited thus, the
  exact number of objects does not matter too much.
• Volume is more essential.

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H(z), Da(z), and w(z)
Point The integral dependence of H on w allows
low-z constraints from high-z observations
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HETDEX Measures w(z) at z0.4
HETDEX
Popular parameterization is
It is important to choose the appropriate pivot
point to overcome degeneracies.
SNAP
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Beyond wa Non-parametric Estimate of w(z)
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From data to w(z)
HETDEX and SNAP (2x) for a cosmological constant
and Planck priors
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H(z) more powerful than Da(z)
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From data to w(z)
Two different evolving models (made-up)
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Non-linearity in BAO
E. Komatsu
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Modeling Non-linearity 3rd-order Perturbation
Theory
Suto Sasaki (1991) Makino, Sasaki Suto (1992)
Does this analytical formula fit N-body
simulations at lt1 level?
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Linear Theory
PM code, 2563 particles
Jeong Komatsu (to be submitted)
256/h Mpc box
(22 sims averaged)
3PT prediction
512/h Mpc box
(70 sims averaged)
PeacockDodds 96
Errorlt1 at zgt2!!
3PT is much better than PD96 even at z1
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Kaiser Effect 3PT

• Since we are measuring redshifts, the measured
  clustering length of galaxies in z-direction will
  be affected by peculiar velocity of galaxies.
• Also known as the redshift space distortion.
• Angular direction is not affected by this effect.
• The clustering length in z-direction appears
  shorter than actually is.

In the linear regime, Pdd(k)Pdq(k)Pqq(k), which
gives the original Kaiser formula in the linear
regime. (qvelocity divergence field)
z direction
angular direction
No peculiar motion
Peculiar motion
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Work in progress (2) Non-linear Bias

• The largest systematic error is the effect of
  galaxy bias on the shape of the power spectrum.
• It is easy to correct if the bias is linear
  however, it wont be linear when the underlying
  matter clustering is non-linear.
• How do we correct for it?

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Non-linear Bias 3rd-order Perturbation Theory
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Powerful Test of Systematics
Work in progress (3) Three-point Function
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Status and Plans

• VIRUS prototype is in construction
• Will be used for pilot survey to establish
  properties of LAEs this fall
• HET wide field upgrade is mostly funded
• Private fundraising for VIRUS is continuing
• 30M total funding goal with 15M in hand
• 2009 start for survey with funding
• 3 years to complete

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Why LAEs?

• Target-selection for efficient spectroscopy is a
  challenge in measuring DE with baryonic
  oscillations from ground-based observations
• LRGs selected photometrically work well to z0.8
• High bias tracer already used to detect B.O. in
  SDSS
• Higher redshifts require large area, deep IR
  photometry
• Probably cant press beyond z2
• Spectroscopic redshifts from absorption-line
  spectroscopy
• OII and Ha emitters can work to z2.5 with IR
  MOS
• But difficult to select photometrically with any
  certainty
• Lyman Break Galaxies work well for zgt2.5
• Photometric selection requires wide-field U-band
  photometry
• Only 25 show emission lines
• Ly-a emitters detectable for zgt1.7
• Numerous at achievable short-exposure detection
  limits
• Properties poorly understood (N(z) and bias)