Hobby-Eberly Dark
Energy Experiment

The Big Bang did not create a perfectly smooth universe, and the slight over- and under-densities of the explosion are imprinted in the microwave background. We can see these Baryonic Acoustic Oscillations, and measure the characteristic separation between the high and low-density regions using experiments like the Wilkinson Microwave Anisotropy Probe and the Planck satellite.  After recombination, however, these fluctuations were frozen into the matter distribution of the universe, so the characteristic scale imprinted on the microwave background is still with us today.  In fact, it can be used as a standard ruler.  If we could measure the scale of these fluctuations over cosmic time --- observe them at different redshifts --- then we could trace the expansion history of the universe itself, via the angular diameter-based distances, D_A(z), and the Hubble parameter, H(z). Both these parameters depend critically on the amount of Dark Energy in existence at any epoch.

Well, we can trace history of the microwave fluctuations. By observing the large scale distribution of galaxies which form from the fluctuations, and measuring how the characteristic spatial separation between clusters and voids changes with redshift, we can determine the expansion history of the universe to great precision. All we need is to obtain redshifts for large numbers of galaxies at each epoch.

To measure the expansion history of the universe, HETDEX will need to measure the redshifts of close to 1 million galaxies between redshifts z = 1.9 and z = 3.5. To do this, the experiment will use the Hobby-Eberly Telescope and what will be the world's largest integral field spectrograph, an instrument known as the Visible Integral-Field Replicable Unit Spectrograph, or VIRUS.

VIRUS is actually a very simple instrument. With the plate scale of the HET, it is relatively simply to make a dense cluster of about 448 optical fibers, called an Integral Field Unit (IFU).  Each fiber has a diameter of 1.5 arcsec, and is brought down to a spectrograph which is optimized for the wavelength range between 3500 and 5500 A. This will generate a set of 448 spectra over a 50" x 50" area of sky. Most of these spectra will be of blank sky, but if an emission-line galaxy happens to fall onto one of the fibers, we will be able to obtain its redshift. More specifically, if a galaxy in the redshift range 1.9 < z < 3.5 has Lyα in emission, will we be able to see this emission.

Of course, 50" x 50" is not a very large patch of sky, and finding 1,000,000 galaxies in this way would be very time-consuming. But we can accelerate the process by using industrial replication techniques and cloning the instrument 78 times.  (Actually, in the final design, there are 78 IFUs and 156 spectrographs, with each spectrograph handling half of its IFU's fibers.)  This is a new concept in astronomy:  normally, more than half the cost of a major astronomical project is in the engineering design, and copies of instruments are rarely made. By contrast, the 78 VIRUS spectrograph modules make up less than 10% of the project's total cost.  In addition, because the design is modular, the full experiment, from concept, through data acquisition, through data reduction, can be debugged and optimized using data from a single prototype unit.  In fact, a survey with the prototype VIRUS spectrograph, VIRUS-P, was conducted for several years and to date has produced more than a dozen papers.

In total, the 78 VIRUS units will have 34,944 optical fibers. Consequently, 34,944 separate spectra will be produced during every exposure. This will make the HETDEX survey the largest spectroscopic program ever conducted by several orders of magnitude!  Of course, most of the VIRUS fibers will fall on blank sky, but a significant number will not.   In fact, we expect that the VIRUS spectrographs on the Hobby-Eberly Telescope will measure the positions and redshifts of ~ 10,000 galaxies every night, and close to one million 1.9 < z < 3.5 galaxies in 140 nights. This will revolutionize our knowledge of large scale structure at a time when the universe was only between ~20% and ~35% of its present size and between ~13% and ~25% its present age.

The data produced by the HETDEX survey will be sufficient to fix both the expansion history of the universe, H(z), and the angular diameter distance, D_A(z), to better than 1% over the redshift range 1.9 < z < 3.5, and thereby tell us the evolution of Dark Energy during this epoch. At the same time, the experiment will also allow us to measure the curvature of the universe to one part in a thousand, and produce a measure of the epoch's galaxy power spectrum to ~ 1.5%.

HETDEX science verification began in the early spring of 2017, and the HETDEX observations began in December 2017.  The baseline parameters of the survey are:

• Total Area of Survey: 434 deg² with a filling factor of 1 in 4.5 (over 3 years); extension to 650 deg² with a 5 year survey
• Spring Field:  42 x 7 degrees centered at 13^h +53° (along a great circle);  extension to 42 x 10 degrees with a 5 year survey
• Fall Field:  28 x 5 degrees centered at 1.5^h +0° (along the celestial equator); extension to 28 x 8 degrees with a 5 year survey
• VIRUS Field of View:  22 arcmin in diameter
• Wavelength Coverage: 3500 to 5500 Angstroms
• Spectral Resolution: 6.4 Angstroms (R = 800)
• Exposure Time per field: 1200 seconds (using 3 separate dithered exposures)
• Total Time of Survey: 1200 hours of Observing Time (140 nights over 3 years)
• Emission Line Sensitivity Limit at 5000 Angstroms: 3.5 x 10^-17 ergs/cm²/sec.
• Continuum Sensitivity Limit: S/N of 10 at R = 22 mag

The main survey will produce:

• a direct detection of Dark Energy at z = 2.5 (for a simple Λ model).
• a measurement of the curvature of the universe to about 10^-3, i.e., 10 times better than the current measurement.
• a small improvement in the present day equation of state, w₀
• a significant improvement in the measurement of any change in w with redshift.
• a measurement of H(z=2.8) to 0.9%.
• a measurement of angular diameter distance D_A(z=2.8) to 0.9%.
• a measurement of the galaxy power spectrum to 1.5% for structure growth.

These precisions can be improved with additional survey time.

With these sensitivities, the baseline 3-year survey will obtain spectra for

• 0.8 million Lyα emitting galaxies between 1.9 < z < 3.5
• 1.0 million [O II] emitting galaxies with z < 0.5
• 0.4 million other galaxies
• 0.25 million Milky Way stars
• 2000 galaxy clusters with Abell richnesses > 1
• Between 10,000 and 50,000 AGN at z < 3.5

 The spectrum of a typical z = 2.21 Lyα emitter discovered by VIRUS during HETDEX science verification is shown below, along with CFHT and Hubble Space Telescope imaging of the region.

Because the HETDEX survey is untargeted, it will obtain spectra for every object in its field of view, including foreground stars and galaxies.  Some of the science projects that HETDEX will address are

• The first detection of Dark Energy at z > 2
• A large improvement in the measurement of curvature in the universe
• The first detection of the cosmic web in emission
• The best measurement of the total neutrino mass to date
• The measurement of any non-Gaussian features in primeval density perturbations
• The best measurement of the luminosity function, equivalent width distribution, and bias of Lyα emitting galaxies as a function of galactic environment
• Constraints on Lyα radiative transfer via measurements of the fraction of star-forming galaxies with Lyα in emission, as a function of stellar mass and environment
• The detection of 25000 active galactic nuclei with no pre-selection biases, and the measurement of correlations between AGN and galaxies
• The measurement of the local (z < 0.4) star-formation rate density through the detection of 1 million [O II] emission-line galaxies
• The measurement of dark matter in nearby galaxies, via measurements of stellar velocity dispersions at large radii
• Metallicity and age estimates for stellar populations of nearby galaxies at large galactic radii
• The detection of intracluster stars in nearby groups and clusters using planetary nebulae as tracers
• The detection of low-metallicity stars in the Milky Way
• The measurement of Milky Way structure and stellar kinematics

Some of the projects led by Penn State astronomers include the analysis of emission-line galaxy luminosity functions, the analysis of the systematics of Lyα emission, the detection of galaxy cluster evolution via the kinematics of intracluster stars, the measurement of the relationship between emission-line galaxies and Active Galactic Nuclei, and the theoretical interpretation of the large scale structure signal in the context of cosmology.   In addition, Penn State astronomers are also leading the effort to conduct a deep, broadband imaging survey over the entire HETDEX survey field. This parallel survey is crucial for obtaining the maximum amount of science out of HETDEX.

A unique aspect of the VIRUS instrument is that the spectrograph can be used in parallel with the other instruments of the Hobby-Eberly Telescope.  For example, if an astronomer is using the telescope's new low-resolution IFU spectrograph to study the line emission from a quasar, VIRUS will simultaneously obtain ~ 35,000 spectra of the surrounding 300 square arcminutes.  The same is true if the telescope's High-Resolution Spectrograph is being used, although, since observations with the HRS are not dithered, the area between the fibers of each fiber bundle will not be covered.

HETDEX Imaging

One limitation of HETDEX is that it is a purely spectroscopic survey, and for some galaxies, only a single emission line will be detected.  For these galaxies, it can sometimes be difficult to tell whether the detected emission line is Lyα at high-redshift, or another emission line (typically the [O II] 3727 doublet) in the foreground.  To discriminate between these two possibilities, the HETDEX fields are also being imaged down to g=25.1 and r=25.1 with the Kitt Peak 4-m telescope's Mosaic camera, the CTIO 4-m telescope's Dark Energy Camera, and the Subaru telescope's Hyper Suprime Camera. By comparing the strength of the emission line seen by HETDEX to the strength of the galaxy's continuum as measured on broadband images, we can discriminate Lyα emitters from foreground sources with better than 98% confidence.

HETDEX-SHELA

In addition to having deep, g- and r-band imaging over the entire HETDEX field, a 28 deg² region of the SDSS Stripe 82 has been selected for an intense, multi-wavelength study.  In this HETDEX-SHELA field, HETDEX will obtain a spectrum of every arcsecond of sky (filling factor 1:1). To complement these data the project also has

• Deep optical ugriz images taken with the CTIO 4-m telescope's Dark Energy Camera.  These data reach 5 σ depths of u=26.3, g=25.8, r=25.6, i=25.0, and z=24.5.
• Deep K-band images with the KPNO 4-m telescope's NEWFIRM IR array.  This survey reaches a 5 σ depth of K = 22.8 (AB mag).
• Deep 3.6 μm and 4.5 μm images with Spitzer's IRAC detector.  These data from the Spitzer-HETDEX Exploratory Large Area (SHELA) survey reach down to m(AB) = 22.0 with a S/N = 5, or a 50% completeness limit of m(AB) = 22.6.
• Far-IR imaging with Herschel's SPIRE detector.  The images from this Herschel Stripe 82 Survey (HerS) are shallower than those of the well-matched set of images listed above, but they are still useful for finding very dusty galaxies in the distant universe.  The 5 σ depths of this survey are 64, 53, and 66 mJy at 250, 350, and 500 μm, respectively.

The combination of HETDEX spectra and rest-frame UV through rest-frame IR imaging allow us to measure the stellar masses, dark matter halo masses, star formation rate, and dust content of virtually all 1.9 < z < 3.5 galaxies will stellar masses greater than ~ 2 x 10¹⁰ solar masses and star formation rates above ~5 solar masses per year.  Most importantly, the volume of 1.9 < z < 3.5 space covered by HETDEX-SHELA is 0.5 Gpc³.  This dwarfs that of all other high-redshift surveys combined, and thus includes the entire range of galactic environments, from the very richest proto-clusters in the universe, to the lowest density voids.    This will open up the high-redshift universe in much the same way as the Sloan Digital Sky Survey revolutionized our view of the local universe.

Additional information can be found at the main HETDEX web page.