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    1 : 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
    2 : HET Mt. Fowlkes west Texas Hobby-Ebery Telescope (9.2m)
    3 : Goals for HETDEX HETDEX measures redshifts for about 1 million LAEs from 2
    4 : 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 Dz=1 at z~3 But significant cosmic variance between surveys 5000 – 10000 per sq. deg. Per Dz=1 at z~3 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
    5 : 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, R=850 246 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
    6 : Layout of 145 IFUs w/ 1/9 fill (20’ dia field) New HET wide field corrector FoV 0.22 sq. arcmin 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
    7 : 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
    8 : 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
    9 : Optical design of VIRUS module Spherical collimator mirror VPH Grating 115 mm beam f/1.33 Camera 2kx2k CCD Science driver requires coverage of 340-550 nm at R~800 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 Flat mirror
    10 : 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
    11 : 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.
    12 : “Predicted” Number Counts Sensitivity of VIRUS (5-s) 2e-17 erg/cm2/s at z=2 1e-17 erg/cm2/s at z=3 0.8e-17 erg/cm2/s at z=4 Detected # LAEs approximately constant with redshift sensitivity tracks distance modulus predict ~5 / sq. arcmin = 18,000 / sq. deg. per Dz = 1 With Dz~1 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 <1% HETDEX will require ~1100 hours exposure or ~110 good dark nights Needs 3 Spring trimesters to complete (not a problem: HET is OUR telescope!)
    13 : Experimental Requirements A LAE DE survey reaching <1% precision requires: large volume to average over sample variance 200-500 sq. degrees and Dz ~ 2 this is 6-15 Gpc3 at z~2-4 surface density ~3000 per sq. degree per Dz=1; ~1 M galaxies LAEs have 18,000 /sq. deg./Dz=1 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
    14 : 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.
    15 : HETDEX Uncertainties Current HETDEX design N/2 HETDEX is sampling variance limited; thus, the exact number of objects does not matter too much. Volume is more essential.
    16 : H(z), Da(z), and w(z) Point: The integral dependence of H on w allows low-z constraints from high-z observations
    17 : HETDEX Measures w(z) at z~0.4 HETDEX SNAP Popular parameterization is: It is important to choose the appropriate pivot point to overcome degeneracies.
    18 : Beyond wa: Non-parametric Estimate of w(z)
    19 : From data to w(z) HETDEX and SNAP (2x) for a cosmological constant and Planck priors
    20 : H(z) more powerful than Da(z)
    21 : From data to w(z) Two different evolving models (made-up)
    22 : Non-linearity in BAO E. Komatsu
    23 : Modeling Non-linearity: 3rd-order Perturbation Theory Suto & Sasaki (1991); Makino, Sasaki & Suto (1992) Does this analytical formula fit N-body simulations at <1% level?
    24 : PM code, 2563 particles 256/h Mpc box 512/h Mpc box (70 sims averaged) (22 sims averaged) 3PT prediction Peacock&Dodds 96 Linear Theory Error<1% at z>2!! 3PT is much better than PD96 even at z=1 Jeong & Komatsu (to be submitted)
    25 : 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. z direction angular direction No peculiar motion Peculiar motion In the linear regime, Pdd(k)=Pdq(k)=Pqq(k), which gives the original Kaiser formula in the linear regime. (q=velocity divergence field)
    26 : 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 won’t be linear when the underlying matter clustering is non-linear. How do we correct for it?
    27 : Non-linear Bias: 3rd-order Perturbation Theory
    28 : Powerful Test of Systematics Work in progress (3): Three-point Function
    29 : 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
    30 : 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 z~0.8 High bias tracer already used to detect B.O. in SDSS Higher redshifts require large area, deep IR photometry Probably can’t press beyond z~2 Spectroscopic redshifts from absorption-line spectroscopy [OII] and Ha emitters can work to z~2.5 with IR MOS But difficult to select photometrically with any certainty Lyman Break Galaxies work well for z>2.5 Photometric selection requires wide-field U-band photometry Only ~25% show emission lines Ly-a emitters detectable for z>1.7 Numerous at achievable short-exposure detection limits Properties poorly understood (N(z) and bias)

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