1. The Hobby-Eberly Telescope Dark Energy Experiment
Caryl Gronwall, Robin Ciardullo (PSU),
Karl Gebhardt, Gary Hill, Eiichiro Komatsu (UT),
Niv Drory (UNAM), & the HETDEX Team

2. HETDEX Consortium University of Texas: MPE/USM: Texas A&M: AIP:
Josh Adams
Guillermo Blanc
John Booth
Mark Cornell
Taylor Chonis
Karl Gebhardt (PS)
Jenny Greene
Gary Hill (PI)
Eiichiro Komatsu
Hanshin Lee
Phillip MacQueen
Jeremy Murphy
Steve Odewahn
Marc Rafal (PM)
Richard Savage
Matthew Shetrone
Masatoshi Shoji
Sarah Tuttle
Brian Vattiat
MPE/USM:
Ralf Bender
Maximillian Fabricius
Frank Grupp
Ulrich Hopp
Martin Landriau
Ariel Sanchez
Jan Snigula
Jochen Weller
Houri Ziaeepour
Texas A&M:
Richard Allen
Darren DePoy
Steven Finkelstein
Jennifer Marshall
Casey Papovich
Travis Prochaska
Nicolas Suntzeff
Vy Tran
Lifan Wang
AIP:
Svend Bauer
Roelof de Jong
Roger Haynes
Andreas Kelz
Volker Mueller
William Rambold
Martin Roth
Mathias Steinmetz
Christian Tapken
Lutz Wisotzki
Others:
Carlos Allende-Prieto (IAC)
Viviana Aquaviva (Rutgers)
Niv Drory (UNAM)
Eric Gawiser (Rutgers)
Lei Hao (SHAO)
Donghui Jeong (Caltech)
Jens Niemeyer (IAG)
Povilas Palunas (LCO)
Penn State University:
Robin Ciardullo
Caryl Gronwall
Ana Matkovic
Larry Ramsey
Don Schneider 2

3. The HETDEX Project
HETDEX is a spectroscopic survey to measure the evolution of Dark Energy via the Power Spectrum of Lya Emitters between 1.9 < z < 3.5.
The instrument for this project is VIRUS, the Visible Integral-field Replicable Unit Spectrographs.
HETDEX is a 3-year Stage III DE experiment, which will begin in Fall It is extendable to 5 years (Stage IV).
Total budget = $37 Million including HET upgrade. Fully funded via private donations, institutions and the NSF.

4. The Instrument: VIRUS
VIRUS is an integral field spectrograph, consisting of ″ diameter fibers feeding an R ~ 750 grating covering 3500 Å < l < 5500 Å. This instrument is then cloned IFUs feed 150 spectrographs to generate 33,600 spectra per observation!
100”
15.6’

5. VIRUS field layout
IFUs layout have a 1/4 fill factor over 22’ diameter field
The central gap feeds other instruments (LRS, MRS, HRS)
In addition to HETDEX, VIRUS will be observing in parallel with other programs.
100”
15.6’
LRS feed
VIRUS IFUs

6. The HETDEX Survey
The HETDEX survey will consist of 20 min exposure time per field, divided into 3 dithers. The (5s) survey limit is about R ~ 22 mag, or Fl ~ 3.5 × ergs cm-2 s-1.
10s 5s 3s

7. The Survey Region
HETDEX will survey ~ 300 deg2 with a filling factor of 1 in 5. The primary field is in the north, to take advantage of the HET tracking.
There will also be a fall equatorial field deg2 of which will be surveyed by Spitzer as part of the SHELA (Spitzer-HETDEX Large Area) survey.

8. The Survey Region
HETDEX will survey ~ 420 deg2 with a filling factor of 1 in 7. The primary field is in the north, to take advantage of the HET’s slow track.
A equatorial fall field will also be observed (likely with a higher fill-factor). Spitzer is surveying 28 deg2 of the fall region.

9. The Spectra
HETDEX is a blind spectroscopic survey. Most of the fibers will fall on blank sky. But some will fall on Lya emitters (and other galaxies and stars).
A 3 year pilot survey with the prototype IFU spectrograph (VIRUS-P) was completed last year, delivering proof-of-concept, and a database for the development of the software pipeline (CURE).
(The latest software release is CURE: Treatment – 2).
Adams et al. (2011)

10. J
How Many Spectra???
HETDEX
HETDEX will obtain spectra for > 2 million objects. Because the targets are not pre-selected, the density of survey observations is off-scale!
HETDEX
objects
Baldry et al. 2010

11. The Need for Imaging
Many objects will be single emission-line detections. Are they Lya Emitters or foreground [O II] l3727 emitters?
Lya emitters at 1.9 < z < 3.5 generally have much larger equivalent width than [O II] emitters at z < (Note that w = w0 (1 + z), so the difference is even larger due to redshift boosting.)

12. The Equivalent Width Cut
Spectroscopic follow-up of z = 3.1 Lya emitters in the ECDF-S, (all selected to have EWobs > 80 Å via narrow-band photometry) shows that
The brightest object (by far) is an [O II] emitter at z = 0.337
Half of the next 14 brightest objects (and 3 others) are x-ray sources (AGN)
All remaining objects are confirmed Lya emitters at z = 3.1

13. Measuring Equivalent Width
HETDEX will classify an object as a Lya emitter if it has a rest-frame equivalent width greater than w0 > 20 Å. So
Since the limiting line-flux of HETDEX is known, this defines the required depth for the continuum detection:
This is far below the limit of VIRUS. Comparison images are needed to help discriminate [O II] from Lya.

14. Goals and Timeline
The HETDEX survey will begin next fall and take 3 years (1400 hours) to complete. It will
Detect Dark Energy at z > 2
Measure curvature to 10-3
Measure H(z=2.8) to 0.9%
Measure DA(z=2.8) to 0.9%
Slightly improve w0
Greatly improve in w(z)
Provide a database for a host of other programs
 3s DE detection
5s DE detection 
DETF FoM figure-of-merit of (x 5 improvement over current experiments.)

15. Expected Database
When completed, the HETDEX database will contain spectra for
4.2 × 108 spectra + additional spectra from parallel-mode observations
0.75 million Lya emitting galaxies between 1.9 < z < 3.5
1 million [O II] l3727 emitting galaxies with z < 0.5
0.4 million other galaxies
0.3 million Milky Way stars down to R ~ 22
2000 rich galaxy clusters (Abell class R > 1)
~104 Quasars (z < 3.5) AGN
A bunch of asteroids, supernovae, intragroup PN, etc.
Numbers increase by substantially with parallel observations!

16. … and many other projects!
Additional Science
In addition to the measurement of Dark Energy at z > 2, HETDEX will
Obtain a measure of non-Gaussianity as good as Planck
Generate the best measurement of the total neutrino mass
Map out the cosmic web in emission
Measure the efficiency of star-formation as a function of environment at z < 0.5 and 1.9 < z < 3.5
Measure the AGN-galaxy correlation function
Map out nearby galaxy clusters
Probe stellar populations at large galactocentric radii
Identify extremely low metallicity stars
… and many other projects!

17. Summary
HETDEX will generate huge numbers of spectra in a 300 deg2 region of the northern sky. The experiment will:
measure the evolution of Dark Energy to z ~ 3 by measuring the redshifts of 0.75 million Lya emitters with 1.9 < z < 3.5
obtain blue (< 5500 Å) spectra for >1 million other objects
HETDEX starts next year, but obtaining complementary imaging is a challenge!