From the Intergalactic to the Interstellar Scales -- EQUALS: a High-resolution Legacy Survey of Gas in the Distant Universe Using ESPRESSO

Trystyn Berg, Valentina D'Odorico, Elisa Boera, Giorgio Calderone, Rodrigo Cuellar, Guido Cupani, Stefano Cristiani, Simona Di Stefano, Andrea Grazian, Francesco Guarneri, Vid Iršič, Sebastian Lopez, Dinko Milaković, Pasquier Noterdaeme, Luca Pasquini, Matteo Viel, Louise Welsh

First listed 2025-12-05 | Last updated 2025-12-05

Abstract

Understanding how the Universe evolved from diffuse primordial gas into the rich cosmic web we observe today is one of the great challenges of modern astrophysics. Quasar absorption lines - the imprints left by intervening gas on the light from distant quasars - provide key diagnostics of many aspects of this investigation, ranging from fundamental physics to cosmology and galaxy formation. The unprecedented combination of extremely precise wavelength calibration, high spectral resolution and high sensitivity of the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) has finally enabled observations that will further constrain both state-of-the-art cosmological simulations of galaxy evolution and theoretical stellar nucleosynthetic yields. In this article, we present the ESPRESSO Quasar Absorption Line Survey (EQUALS), an ESO Large Programme, designed to tackle several outstanding questions from constraining the properties of dark matter at the smallest scales probed by the Lyman-alpha forest to determining the temperature of the intergalactic medium at z ~ 4 and precisely quantifying the chemical contributions of stellar populations in the early Universe. EQUALS will provide a legacy sample of deep spectra to showcase ESPRESSO capabilities to the quasar absorption line community whilst providing epoch measurements for the key science goals of upcoming spectroscopic instrumentation on the next generations of telescopes.

Short digest

EQUALS is an ESPRESSO Large Programme delivering R ≈ 140,000 spectra for 23 bright z ≈ 4 QUBRICS quasars, reaching ~20 per 1 km s−1 pixel in the Lyα forest and ~40 redward via hour-long, Fabry–Pérot–calibrated blocks. The survey is tuned to the small-scale Lyα forest (k > 0.1 s km−1), promising a ~20× tighter IGM temperature measurement at z ~ 4 and, consequently, 5–15× stronger constraints on dark-matter particle mass. The resolution enables weak-line work (e.g., Mg II) to separate thermal from turbulent broadening, and isotopic C II shifts (~3 km s−1) in ~8 high-N(H I) systems to probe early stellar yields. Data products will be released via CANFAR and the ESO Archive as a legacy benchmark for future epoch measurements.

Key figures to inspect

• Figure 1 (SNR vs rest-frame wavelength): Verify the survey depth—~20 per 1 km s−1 pixel in the Lyα forest and ~40 redward—and how SNR changes blueward/redward of Lyα, confirming sensitivity to weak metal lines and small-scale Lyα power.
• Figure 2 (Si II 1526 and Fe II 1608 toward J1621-0042): Compare ESPRESSO to UVES to see additional narrow components resolved at R ~ 140k; note how this separation improves constraints on thermal vs turbulent broadening and component structure.
• Figure 3 (Lyα forest flux power sensitivity): Inspect the extension to higher k and the forecasted gains—~20× smaller IGM-T uncertainties and 5–15× tighter dark-matter mass limits—relative to previous high-resolution samples.