The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX)

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

3. Goals for HETDEX • HETDEX measures redshifts for about 1 million LAEs from 2<z<4 • Wavelength coverage: 340-550 nm at R~800 • 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 z=0.4 (to a few percent)

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 New HET wide field corrector FoV • 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 0.22 sq. arcmin (20’ dia field)

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 Flat mirror Spherical collimator mirror VPH Grating 115 mm beam • 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 f/1.33 Camera 2kx2k CCD

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-α 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 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.

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 Popular parameterization is: It is important to choose the appropriate pivot point to overcome degeneracies. SNAP

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. 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) Peacock&Dodds 96 Error<1% at z>2!! 3PT is much better than PD96 even at z=1

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. In the linear regime, Pdd(k)=Pdq(k)=Pqq(k), which gives the original Kaiser formula in the linear regime. (q=velocity divergence field) z direction angular direction No peculiar motion Peculiar motion

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)