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

HETMt. Fowlkes west Texas

Hobby-Ebery Telescope (9.2m)

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)

Ly- emitters as tracersProperties 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 z=1 at z~3

But significant cosmic variance between surveys 5000 – 10000 per sq. deg. Per z=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

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- emitting galaxies as tracers, will measure the galaxy po

wer spectrum to 1%

Prototype is in construction Delivery in April

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 (H

ETDEX is sampling variance limited) Well-defined window function

Dithering of pointing centers removes aliasing

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

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

Optical design of VIRUS module

Spherical collimatormirror

VPH Grating115 mm beam

f/1.33Camera2kx2k 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 (340-6

80 nm) expect 70% thorughput Complexity of internal focus camera

Flat mirror

VIRUS Prototype Unit Spectrograph

Will be completed this summerTests the design and performance of the instrumentRefines the cost estimate for replicating VIRUSWill be used for a 50 night pilot survey of LAEs on the McDonald 2.7 m

Lyman-α EmittersThere are ever increasing number of observations on LAECompilation 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.

“Predicted” Number CountsSensitivity of VIRUS (5-) 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 z = 1

With z~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 O

UR telescope!)

Experimental RequirementsA LAE DE survey reaching <1% precision requires: large volume to average over sample variance

200-500 sq. degrees and z ~ 2 this is 6-15 Gpc3 at z~2-4

surface density ~3000 per sq. degree per z=1; ~1 M galaxies LAEs have 18,000 /sq. deg./z=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 3400 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

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.

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.

H(z), Da(z), and w(z)

dzz

zwzhzH

z

Xm

0

3

1

)(13exp)1()(

Point: The integral dependence of H on w allows low-z constraints from high-z observations

HETDEX Measures w(z) at z~0.4

HETDEX

SNAP

Popular parameterization is:

)()( 0 aawwaw pa

It is important to choose the appropriate pivot point to overcome degeneracies.

Beyond wa: Non-parametric Estimate of w(z)

dzz

zwzhzH

z

Xm

0

3

1

)(13exp)1()(

2

2

2

Minimize

dzwdS

SL

From data to w(z)

HETDEX and SNAP (2x) for a cosmological constant and Planck priors

H(z) more powerful than Da(z)

From data to w(z)

Two different evolving models (made-up)

Non-linearity in BAO

E. Komatsu

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?

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)

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, PPPwhich gives the original Kaiser formula in the linear regime. (=velocity divergence field)

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?

Non-linear Bias: 3rd-order Perturbation Theory

Powerful Test of Systematics

Work in progress (3):

Three-point Function

Status and PlansVIRUS prototype is in construction Will be used for pilot survey to establi

sh properties of LAEs this fall

HET wide field upgrade is mostly funded Private fundraising for VIRUS is conti

nuing

$30M total funding goal with $15M in hand

2009 start for survey with funding 3 years to complete

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 H 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- emitters detectable for z>1.7 Numerous at achievable short-exposure detection limits Properties poorly understood (N(z) and bias)