The Hobby-Eberly Telescope D ark Energy Experiment (HETDE X)

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The Hobby-Eberly Telescope D ark Energy Experiment (HETDE X)

Karl Gebhardt, Gary Hill, Phillip MacQueen Eiichiro Komatsu, Niv Drory, Povilas Palunas

McDonald Observatory & Department of Astronom y, University of Texas

Peter Schuecker, Ralf Bender, Uli Hopp, Claus Goe ssl, Ralf Koehler

MPE and Uni-Sternwarte Munich Martin Roth, Andreas Kelz

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HET

Mt. Fowlkes west Texas

Hobby-Ebery Telescope (9.

2m)

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Goals for HETDEX

• HETDEX measures redshifts for about 1 mill ion 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 per cent

• Tightest constraints on evolving w at z=0.4 ( to a few percent)

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Ly- 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 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. deg ree 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, 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

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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 (HETD EX 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

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 (340-6 80 nm) expect 70% thorughput

 Complexity of internal focus camera

Flat mirror

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VIRUS Prototype Unit Spectrograph

Will be completed this s ummer

Tests the design and p erformance of the instr ument

Refines the cost estima te for replicating VIRUS Will be used for a 50 ni ght pilot survey of LAEs on the McDonald 2.7 m

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Lyman-α Emitters

There are ever increasing num ber of observations on LAE

Compilation of the recent data and GALFORM modeling by D elliou et al. (2005)

 Most recent data very consistent

 “Theory” and data matching well

Not very reliable, but useful st arting point to design surveys

 More accurate number counts will be obtained from VIRUS pr oto-type.

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“Predicted” Number Counts

Sensitivity 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. degr ee

 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 goo d dark nights

 Needs 3 Spring trimesters to complete (not a problem: HET is O UR telescope!)

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Experimental Requirements

A 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 specifi cations 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, wit h 50 night observing campaign

• Pilot run on Calar Alto in Dec saw 4 hrs data i n 8 nights, but we will go back

• Full VIRUS is in design phase; with full fundi ng expect completion 2008-2009

• HETDEX will then take 3 years, finishing 2011 -2012

• $30M project (including operation cost and d ata analysis): $15M has been funded.

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HETDEX Uncertainties

Current HETDEX design N/2

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

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

 ^dz

z z z w

h z H

z X

m ^ ^^  ^_





0 3

1 ) ( 3 1

exp )

1 ( )

(

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 z~0.4

HETDEX

SNAP

Popular parameterization is:

) (

)

(a w₀ w a a

w   _a _p 

It is important to choose the appr opriate pivot point to overcome de generacies.

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Beyond w

_a

: Non-parametric Estima te of w(z)

 ^dz

z z z w

h z H

z X

m ^ ^^  ^_





0 3

1 ) ( 3 1

exp )

1 ( )

(









2 2

2

Minimize

w

S d

S

L  

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From data to w(z)

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H(z) more powerful than Da(z)

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From data to w(z)

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Non-linearity in BAO

E. Komatsu

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Modeling Non-linearity:

3

^rd

-order Perturbation Theory

Suto & Sasaki (1991); Makino, Sasaki & Suto (1992)

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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)

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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.

z direction

In the linear regime, P_P_P_which gives the original Kaiser formula in the linear regime. (=velocity divergence fiel d)

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Work in progress (2): Non-linea r Bias

The largest systematic error is the effect of galaxy bi as on the shape of the power spectrum.

It is easy to correct if the bias is linear; however, it w on’t be linear when the underlying matter clustering i s non-linear.

How do we correct for it?

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Non-linear Bias:

3

^rd

-order Perturbation Theo

ry

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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 establi sh properties of LAEs this fall

HET wide field upgrade is mostly f unded

 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

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Why LAEs?

Target-selection for efficient spectroscopy is a challenge in measu ring 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)