The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX): Description and Early Pilot Survey Results

TL;DR

The paper outlines the HETDEX program, which targets high-precision cosmology by mapping the expansion history at $1.9<z<3.5$ through Ly$\alpha$ emitters and BAO measurements, aiming for a direct $3\sigma$ detection of dark energy and a curvature precision of $\sim0.1$–$0.2\%$ with a survey of $\sim9\ \mathrm{Gpc^3}$ and $\sim10^{-4}\ \mathrm{Mpc^{-3}}$ tracers over $\sim420\ \mathrm{deg^2}$. It details the HET Wide Field Upgrade and the VIRUS instrument, a replicated array of 150 fiber-fed IFUs delivering broad spectral coverage ($350$–$550$ nm) with high multiplexing to obtain roughly $3.36\times10^4$ spectra per exposure and ~12 million resolution elements. The pilot survey with VIRUS-P on the McDonald 2.7 m validates the methodology, achieving $5\sigma$ sensitivity of $5\times10^{-17}\ \mathrm{erg\ cm^{-2}\ s^{-1}}$ and identifying LAEs and contaminants across multiple fields, while indicating Ly$\alpha$ luminosity function evolution consistent with expectations. Together, these efforts demonstrate the feasibility of massive IFU-based surveys for constraining high-redshift dark energy and cosmological curvature, with significant implications for future large-scale structure studies.

Abstract

The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) will outfit the 10 m HET with a new wide field and an array of 150 integral-field spectrographs to survey a 420 sq. deg. area in the north Galactic cap. Each fiber-coupled unit spectrograph will cover 350-550 nm, simultaneously. This instrument, called VIRUS, will produce ~34,000 spectra per exposure, and will open up the emission-line universe to large surveys for the first time. The survey will detect 0.8 million Lyman-alpha emitting (LAE) galaxies with 1.9<z<3.5 and more than a million [OII] emitting galaxies with z<0.5. The 3-D map of LAE galaxies in 9 cubic Gpc volume will be used to measure the expansion history at this early epoch using baryonic acoustic oscillations and the shape of the power spectrum. The aim of HETDEX is to provide a direct detection of dark energy at z~3. The measurement will constrain the evolution of dark energy and will also provide 0.1%-level accuracy on the curvature of the Universe, ten times better than current. The prototype of the VIRUS unit spectrograph (VIRUS-P) is a powerful instrument in its own right. Used on the McDonald 2.7 m, it covers the largest area of any integral field spectrograph, and reaches wavelengths down to 340 nm. VIRUS-P is being used for a pilot survey to better measure the properties of LAE galaxies in support of HETDEX. We report initial results from this survey.

Figures (1)

• Figure 1: Example spectrum of a z=3.415 LAE in the COSMOS field from HETDEX pilot survey data obtained on the McDonald 2.7 m with VIRUS-P. The LAE is seen in four separate dithered fiber positions (shown on the right), and combined for a strong detection.