VLT-MOONRISE: MOONS Redshift Survey

• VLT-MOONRISE is the main MOONS Guaranteed Time Observation extragalactic survey on the ESO VLT, designed to capture SDSS-like high-redshift spectra during Cosmic Noon.
• The survey leverages MOONS’ multiplexing, broad optical-to-near-IR spectral coverage, and rapid fiber positioning to target up to half a million galaxies between z ~ 0.9–2.6 and beyond.
• It delivers key diagnostics of galaxy evolution, quenching, and environmental effects using advanced sky subtraction and multi-resolution spectroscopic techniques.

VLT-MOONRISE is the main MOONS Guaranteed Time Observation extragalactic survey on the ESO Very Large Telescope, where the acronym denotes the MOONS Redshift-Intensive Survey Experiment. It is presented as the flagship extragalactic component of the MOONS GTO programme, with 190 nights dedicated to obtaining an SDSS-like survey at high redshift, especially around Cosmic Noon, approximately $z \sim 1{-}2.5$, when the cosmic star-formation rate density peaked (Maiolino et al., 2020). Within the broader VLT/MOONS programme, MOONRISE is designed to exploit the large multiplexing, high sensitivity, broad simultaneous optical-to-near-infrared spectral coverage, large patrol area, and high fibre density of MOONS in order to secure key spectroscopic information for a few hundred thousand galaxies, possibly up to about half a million galaxies at $0.9 < z < 2.6$, plus a few thousand galaxies at $z \sim 6{-}8$ (Maiolino et al., 2020).

1. Definition and programme context

MOONRISE is explicitly defined as the main MOONS Guaranteed Time Observation (GTO) extragalactic survey on the VLT (Maiolino et al., 2020). In the 2020 MOONS overview, the broader GTO allocation is described as 300 nights of GTO, divided into about 100 nights for Galactic surveys and about 200 nights for extragalactic surveys, situating MOONRISE within a large facility-survey framework (Cirasuolo et al., 2020). The MOONRISE-specific paper sharpens that context by identifying it as the principal GTO extragalactic effort and the core extragalactic legacy component of early MOONS operations (Maiolino et al., 2020).

The survey is embedded in the capabilities of MOONS, the Multi-Object Optical and Near-infrared Spectrograph for the VLT (Cirasuolo et al., 2020). MOONS is a fibre-fed spectrograph for an 8 m Unit Telescope on the VLT, using the Nasmyth platform and the full 25-arcminute-diameter field of view delivered by a dedicated field corrector (Cirasuolo et al., 2020). Earlier design work had already framed MOONS as a multi-object optical and near-infrared spectrograph selected by ESO for a Phase A study, with approximately 1000 fibers deployable over a field of view of 500 square arcmin (Cirasuolo et al., 2012). In the mature construction-era description, the instrument has 1000 fibres with individual robotic positioners, more precisely 1001 miniature fibre positioning units, and simultaneous spectral coverage across 0.64–1.8 um (Cirasuolo et al., 2020).

Within this landscape, MOONRISE is not an instrument but a survey. That distinction matters because several foundational papers describe MOONS itself rather than MOONRISE specifically [(Cirasuolo et al., 2012); (Cirasuolo et al., 2020)]. A plausible implication is that “VLT-MOONRISE” is best understood as the survey layer built upon the MOONS instrumental platform, rather than as a separate hardware system.

2. Scientific rationale at Cosmic Noon

The central scientific motivation is to obtain, for the first time, a survey at $z \sim 1{-}2.5$ with scale and diagnostic power analogous to local spectroscopic surveys such as SDSS and GAMA (Maiolino et al., 2020). The logic is that the same rest-frame optical nebular and stellar diagnostics used in nearby-galaxy surveys become accessible in the near-infrared for galaxies at these redshifts (Maiolino et al., 2020). This makes it possible to pursue an accurate and consistent description of the evolution of galaxy physical properties across the epoch when galaxy growth and star formation were near their maximum.

The MOONRISE paper emphasizes the redshift interval around Cosmic Noon because it coincides with the peak of the cosmic star-formation rate density (Maiolino et al., 2020). MOONS is described as particularly well matched to this interval because H$\alpha$ is detectable out to $z = 1.74$, [O III] 5007 Å out to $z = 2.6$, and [O II] 3727 Å enters the MOONS band at $z = 0.7$ and is observable up to $z = 3.8$ (Maiolino et al., 2020). This directly addresses the long-standing “redshift desert” around $z \sim 1.5$, where optical spectrographs have traditionally struggled because prominent spectral features move out of the optical window [(Maiolino et al., 2020); (Cirasuolo et al., 2012)].

The scientific agenda is broad. The survey is designed to address metallicity evolution and scaling relations, chemical abundance ratios, AGN demographics and black-hole growth, stellar and gas kinematics, the build-up of the passive galaxy population, galaxy transformation, quenching, and the role of environment, from voids to proto-clusters (Maiolino et al., 2020). It also includes, opportunistically, galaxies at the dawn of the Universe (Maiolino et al., 2020). The MOONS instrument papers place these goals in a wider programme that also includes environment and large-scale structure studies, black hole and AGN feedback, and the epoch of reionisation (Cirasuolo et al., 2020).

The later prediction study on quenching reframes MOONRISE as the first wide-field spectroscopic galaxy survey to target cosmic noon and stresses that it should probe environments over volumes approaching the local homogeneity scale and span a density range of more than four orders of magnitude from voids to dense clusters (Goubert et al., 11 Sep 2025). This suggests that one of the survey’s distinctive contributions is not merely spectroscopy at high redshift, but spectroscopy at high redshift with enough volume and environmental leverage to separate intrinsic and environmental drivers of galaxy evolution.

3. Instrumental basis in MOONS

MOONRISE depends on MOONS’s combination of multiplex, wavelength coverage, and spectral resolution. The survey paper summarizes MOONS as opening an “unprecedented discovery space” through large multiplexing, high sensitivity, broad simultaneous spectral coverage from optical to near-infrared, large patrol area, and high fibre density (Maiolino et al., 2020). The instrument overview makes those properties more concrete: MOONS combines the collecting power of an 8-m telescope, 1000 fibres with individual robotic positioners, and simultaneous low- and high-resolution coverage across 0.64–1.8 micron (Cirasuolo et al., 2020).

In low-resolution mode, the instrument covers the entire 0.64–1.8 um range simultaneously through the RI, YJ, and H channels, with resolving powers $0.9 < z < 2.6$0, $0.9 < z < 2.6$1, and $0.9 < z < 2.6$2 (Cirasuolo et al., 2020). In the earlier conceptual design, the medium-resolution mode was described as $0.9 < z < 2.6$3 across $0.9 < z < 2.6$4 (Cirasuolo et al., 2012). In high-resolution mode, MOONS provides selected windows with $0.9 < z < 2.6$5 around the Ca triplet, $0.9 < z < 2.6$6, and $0.9 < z < 2.6$7 over $0.9 < z < 2.6$8 (Cirasuolo et al., 2020). The 2012 design paper described the high-resolution concept as a CaII-triplet region at $0.9 < z < 2.6$9 plus J and H windows at $z \sim 6{-}8$0 for detailed chemical abundances (Cirasuolo et al., 2012).

Several engineering characteristics are directly relevant to survey execution. The field corrector creates a fully corrected 25-arcminute-diameter field and is made of two large lenses, each of almost 1 m in diameter and about 110 mm thick (Cirasuolo et al., 2020). The 1001 miniature fibre positioning units can configure the whole field in less than two minutes, each fibre has an on-sky aperture of 1.2 arcseconds, and more than 3 fibres can reach any point in the focal plane because of the overlapping patrol geometry (Cirasuolo et al., 2020). MOONS uses two identical triple-arm spectrographs, enclosed in a single cryostat vessel and operated at $z \sim 6{-}8$1 degrees C, with light split by dichroics into RI, YJ, and H channels (Cirasuolo et al., 2020).

Sky subtraction is a major systems driver for faint near-infrared spectroscopy. The MOONS papers state that at $z \sim 6{-}8$2, and $z \sim 6{-}8$3 in the H band, at least 60–70% of the observed regions in Y, J, or H are free of OH airglow contamination (Cirasuolo et al., 2020). Three observing strategies are described: stare, stare+nod, and XSwitch, where XSwitch is presented as the most accurate sky-subtraction mode and uses paired object-sky fibres separated by $z \sim 6{-}8$4 arcseconds (Cirasuolo et al., 2020). The 2012 design paper adds that cross-beam switching can subtract the sky to better than 1% level (Cirasuolo et al., 2012). These instrumental and observing characteristics are the operational substrate on which MOONRISE’s high-redshift survey design rests.

4. Survey scale, diagnostics, and observing domain

The intended MOONRISE sample size is exceptionally large by high-redshift spectroscopic standards. The survey paper states that MOONRISE is expected to obtain key spectroscopic information for a few hundred thousand galaxies, possibly up to about half a million galaxies at $z \sim 6{-}8$5, plus a few thousand galaxies at $z \sim 6{-}8$6 (Maiolino et al., 2020). The quenching-prediction paper restates this scale as roughly half a million galaxies over $z \sim 6{-}8$7 (Goubert et al., 11 Sep 2025).

The survey is motivated by direct access to rest-frame optical diagnostics at high redshift, but its scientific use case is broader than line detection alone. The prediction study argues that MOONRISE should provide robust central–satellite classification and halo-mass estimates, and that the spectra should constrain stellar masses, star-formation rates, and even black hole masses via empirical calibrations, while also identifying optical AGN (Goubert et al., 11 Sep 2025). This is important because earlier high-redshift surveys were often limited by photometric-redshift uncertainties and projection effects that complicated group identification and environmental measures (Goubert et al., 11 Sep 2025).

The MOONS instrument overview also gives sensitivity benchmarks that contextualize the survey’s operational reach. In low resolution, the instrument targets $z \sim 6{-}8$8 at $z \sim 6{-}8$9 when rebinned to $z \sim 1{-}2.5$0 after sky subtraction, and $z \sim 1{-}2.5$1 for an emission-line flux of $z \sim 1{-}2.5$2 with FWHM = 200 km s$z \sim 1{-}2.5$3 (Cirasuolo et al., 2020). In high resolution, the quoted benchmark is $z \sim 1{-}2.5$4 at $z \sim 1{-}2.5$5 and $z \sim 1{-}2.5$6 (Cirasuolo et al., 2020). The same paper reports throughput figures of $z \sim 1{-}2.5$7 in low resolution and $z \sim 1{-}2.5$8 in high resolution (Cirasuolo et al., 2020).

The long-term institutional ambition is larger than MOONRISE alone. The MOONRISE survey paper notes that additional future open-time surveys over the first decade of MOONS operations are expected to extend the MOONS legacy to millions of galaxy spectra over wider redshift ranges and parameter space (Maiolino et al., 2020). The MOONS overview similarly states that over 10 years, MOONS surveys would provide spectra for millions of galaxies at $z \sim 1{-}2.5$9 (Cirasuolo et al., 2020). A plausible implication is that MOONRISE is intended both as a self-contained survey and as the anchor dataset for a broader MOONS-era extragalactic legacy programme.

5. Environmental and cluster science in precursor fields

Cluster and environment science is one of the explicit applications of MOONRISE. The survey is intended to spectroscopically identify hundreds of thousands of galaxies at high redshift in order to determine the environments in which primeval galaxies lived and reveal how those environments affected galaxy evolution (Maiolino et al., 2020). This theme is developed in detail by the 2026 precursor study of cluster detection in the VIDEO fields (Galois et al., 11 Jun 2026).

That study analyzes the XMM-LSS and CDFS VIDEO fields, which are expected to be partially covered by MOONRISE, as testbeds for cluster confirmation and characterization (Galois et al., 11 Jun 2026). After masks and wavelength-coverage cuts, the effective areas are 4.55 deg$\alpha$0 in XMM-LSS and 4.11 deg$\alpha$1 in CDFS, with H-band-selected catalogues limited to $\alpha$2 and galaxy densities of 38.0 arcmin$\alpha$3 in XMM-LSS and 36.1 arcmin$\alpha$4 in CDFS (Galois et al., 11 Jun 2026). Photometric redshifts are derived as the median of seven SED-fitting runs, five with LePhare and two with EAZY, and spectroscopic cross-matching yields 28,292 matches in XMM-LSS and 12,367 in CDFS (Galois et al., 11 Jun 2026).

Two photometric-redshift-based cluster finders, AMICO and WaZP, are used to build a robust intersection sample suitable for MOONRISE targeting (Galois et al., 11 Jun 2026). The final joint sample contains 519 cluster candidates over the combined 8.66 deg$\alpha$5 effective area, spanning $\alpha$6–3.0, with 271 in XMM-LSS and 248 in CDFS (Galois et al., 11 Jun 2026). Of these, 74 lie at $\alpha$7, specifically 31 in XMM-LSS and 43 in CDFS (Galois et al., 11 Jun 2026). The likely central galaxy is operationally defined as the brightest member in $\alpha$8 within 400 kpc of the cluster center, termed the brightest central member (Galois et al., 11 Jun 2026).

The precursor study then evaluates how these clusters would be detected and characterized under various MOONRISE strategies. Its central conclusion is that cluster spectroscopic confirmation and characterisation could be efficiently achieved up to $\alpha$9 even with the shallowest survey strategy (Galois et al., 11 Jun 2026). This does not constitute a demonstrated survey result, but it establishes a feasibility basis for one of MOONRISE’s environmental goals: the assembly of clusters and the evolution of galaxies in dense environments across the redshift range where MOONS offers access to rest-frame optical diagnostics.

6. Predictions for quenching studies and model discrimination

A major theoretical forecast for VLT-MOONRISE is provided by the 2025 simulation paper on environmental vs. intrinsic quenching at cosmic noon (Goubert et al., 11 Sep 2025). That work is explicitly framed as a prediction paper for MOONRISE and uses IllustrisTNG and EAGLE, specifically TNG100-1 and EAGLE RefL0100N1504, at $z = 1.74$0 to forecast what the survey should be able to test (Goubert et al., 11 Sep 2025).

To mimic the survey, the authors construct a “MOONRISE-Like” mock sample. They impose $z = 1.74$1, move galaxies into observer-like 2D+$z = 1.74$2 space, and randomly remove 30% of galaxies to mimic an expected 70% spectroscopic completeness (Goubert et al., 11 Sep 2025). Pair selection in redshift space uses

$z = 1.74$3

Quenched galaxies are defined through a redshift-dependent threshold,

$z = 1.74$4

with a galaxy classified as quenched if

$z = 1.74$5

Using this criterion, the total quenched fraction in both simulations is about $z = 1.74$6 at $z = 1.74$7 and falls to $z = 1.74$8, roughly $z = 1.74$9, by $z = 2.6$0 (Goubert et al., 11 Sep 2025).

The main inference tool is Random Forest classification, using input features including black hole mass $z = 2.6$1, stellar mass $z = 2.6$2, halo mass $z = 2.6$3, local over-density $z = 2.6$4, and distance to the central $z = 2.6$5 (Goubert et al., 11 Sep 2025). The environmental densities are defined by

$z = 2.6$6

for 6D simulation space, and

$z = 2.6$7

for 2D+$z = 2.6$8 observer space (Goubert et al., 11 Sep 2025). Feature importance is derived from Gini-impurity reduction, with

$z = 2.6$9

The forecast is structurally simple but physically consequential. For central galaxies, both simulations predict that quiescence is dominated by intrinsic quenching, and black hole mass is by far the best predictor at all redshifts (Goubert et al., 11 Sep 2025). For high-mass satellites, the same paper predicts that they behave more like centrals than like low-mass satellites: $z = 0.7$0 is again the strongest predictor of quiescence, with environmental parameters contributing a secondary signal (Goubert et al., 11 Sep 2025). For low-mass satellites, by contrast, the Random Forest analysis predicts that quiescence is governed by environmental parameters, with host halo mass $z = 0.7$1 as the strongest predictor at all epochs (Goubert et al., 11 Sep 2025).

The sharpest model discrepancy concerns whether environmentally quenched low-mass satellites remain observable within MOONRISE’s mass limits. The paper reports that TNG predicts the existence of environmentally quenched satellites visible within the survey limits of MOONRISE, whereas EAGLE does not (Goubert et al., 11 Sep 2025). In the MOONRISE-like samples, TNG retains 1307 low-mass satellites at $z = 0.7$2 with 298 quenched, and 530 at $z = 0.7$3 with 51 quenched; EAGLE has 406 low-mass satellites at $z = 0.7$4 with only 45 quenched, and none at $z = 0.7$5 (Goubert et al., 11 Sep 2025). The authors therefore cast MOONRISE as a direct discriminator between competing quenching models.

A second discriminant is rejuvenation. The prediction paper states that TNG shows little sign of rejuvenation, whereas EAGLE shows strong evidence that quenched galaxies can restart star formation at late times (Goubert et al., 11 Sep 2025). This suggests that MOONRISE’s measurement of quenched fractions as functions of stellar mass, inferred black hole mass, and environment across $z = 0.7$6 could test not only the relative roles of AGN-linked and environment-linked quenching, but also the temporal persistence of quiescence itself (Goubert et al., 11 Sep 2025).

7. Position within high-redshift survey astronomy

MOONRISE is repeatedly characterized in comparison with local spectroscopic surveys. The key formulation is that it seeks an SDSS-like survey at high redshift, especially across the redshift desert and around Cosmic Noon [(Maiolino et al., 2020); (Cirasuolo et al., 2012)]. In that sense, it is not merely a redshift-collection project: it is intended to extend the diagnostic regime of large local surveys into the epoch when galaxy assembly, black-hole growth, quenching, and environment were evolving rapidly.

The survey also occupies a specific place within the VLT facility ecosystem. MOONS is described as filling a crucial gap that could not be addressed with only optical spectroscopy or with low-multiplex near-infrared spectroscopy, and as a facility instrument that complements existing VLT capabilities such as FLAMES and VIMOS [(Cirasuolo et al., 2012); (Cirasuolo et al., 2020)]. The 2020 MOONS overview adds that MOONS is expected to follow up major survey and mission datasets, including Gaia, VISTA, UKIDSS, VST, Pan-STARRS, Dark Energy Survey, LSST, Euclid, and a range of multiwavelength facilities (Cirasuolo et al., 2020).

A common misconception is to treat “MOONRISE” as the instrument. The source literature does not support that usage. MOONS is the instrument; MOONRISE is the main extragalactic GTO survey built around it (Cirasuolo et al., 2020, Maiolino et al., 2020). Another misconception is that the survey’s role is exhausted by measuring redshifts. The cluster-feasibility and quenching-prediction papers indicate a broader function: environmental reconstruction, central–satellite classification, halo-mass estimation, cluster confirmation to $z = 0.7$7 in precursor analyses, and model discrimination for quenching physics at cosmic noon (Galois et al., 11 Jun 2026, Goubert et al., 11 Sep 2025).

Taken together, the available literature presents VLT-MOONRISE as the central extragalactic realization of the MOONS concept: a large, near-infrared-enabled spectroscopic survey on the VLT designed to recover the physical diagnostics, environmental information, and statistical scale needed to study galaxy evolution at $z = 0.7$8, while also extending toward the reionization era (Maiolino et al., 2020).