--- title: 'VLT-MOONRISE: MOONS Redshift Survey' url: https://www.emergentmind.com/topics/vlt-moonrise type: topic --- # VLT-MOONRISE: MOONS Redshift Survey 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 [2009.00644]. 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 \(z \sim 6{-}8\)** [2009.00644]. ## 1. Definition and programme context MOONRISE is explicitly defined as the **main MOONS Guaranteed Time Observation (GTO) extragalactic survey** on the **VLT** [2009.00644]. 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 [2009.00628]. 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 [2009.00644]. The survey is embedded in the capabilities of **MOONS**, the **Multi-Object Optical and Near-infrared Spectrograph** for the VLT [2009.00628]. 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 [2009.00628]. 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** [1208.5780]. 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** [2009.00628]. 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 [1208.5780; 2009.00628]. 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** [2009.00644]. 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 [2009.00644]. 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 [2009.00644]. 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 **\(z = 3.8\)** [2009.00644]. 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 [2009.00644; 1208.5780]. 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** [2009.00644]. It also includes, opportunistically, galaxies at the **dawn of the Universe** [2009.00644]. 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** [2009.00628]. 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 [2509.09626]. 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** [2009.00644]. 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** [2009.00628]. 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 **\(R_{RI} = 4100\)**, **\(R_{YJ} = 4300\)**, and **\(R_H = 6600\)** [2009.00628]. In the earlier conceptual design, the medium-resolution mode was described as **\(R \sim 4{,}000-6{,}000\)** across **\(0.8\,\mu\mathrm{m} - 1.8\,\mu\mathrm{m}\)** [1208.5780]. In high-resolution mode, MOONS provides selected windows with **\(R_{RI} = 9200\)** around the Ca triplet, **\(R_{YJ} = 4300\)**, and **\(R_H = 19\,000\)** over **\(1.52 - 1.64\ \mathrm{\mu m}\)** [2009.00628]. The 2012 design paper described the high-resolution concept as a CaII-triplet region at **\(R \sim 8{,}000\)** plus **J** and **H** windows at **\(R \sim 20{,}000\)** for detailed chemical abundances [1208.5780]. 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 [2009.00628]. 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 [2009.00628]. MOONS uses **two identical triple-arm spectrographs**, enclosed in a **single cryostat vessel** and operated at **\(-130\) degrees C**, with light split by dichroics into **RI**, **YJ**, and **H** channels [2009.00628]. Sky subtraction is a major systems driver for faint near-infrared spectroscopy. The MOONS papers state that at **\(R>4000\)**, and **\(R>6500\)** in the **H** band, at least **60–70%** of the observed regions in **Y, J, or H** are free of OH airglow contamination [2009.00628]. 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 **\(10 < d < 30\) arcseconds** [2009.00628]. The 2012 design paper adds that cross-beam switching can subtract the sky to **better than 1% level** [1208.5780]. 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 \(0.9 < z < 2.6\)**, plus **a few thousand galaxies at \(z \sim 6{-}8\)** [2009.00644]. The quenching-prediction paper restates this scale as roughly **half a million galaxies** over **\(0.9 < z < 2.6\)** [2509.09626]. 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 [2509.09626]. 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 [2509.09626]. The MOONS instrument overview also gives sensitivity benchmarks that contextualize the survey’s operational reach. In low resolution, the instrument targets **\( \mathrm{S/N} > 5 \) at \( \mathrm{mag}(AB) \sim 23 \)** when rebinned to **\( R = 1000 \)** after sky subtraction, and **\( \mathrm{S/N} > 5 \)** for an emission-line flux of **\(> 2 \times 10^{-17}\ \mathrm{erg\ s^{-1}\ cm^{-2}} \)** with **FWHM = 200 km s\(^{-1}\)** [2009.00628]. In high resolution, the quoted benchmark is **\( \mathrm{S/N} > 60 \) at \( H_{AB} \sim 17 \) and \( RI_{AB} \sim 17.5 \)** [2009.00628]. The same paper reports throughput figures of **\(> 30\%\)** in low resolution and **\(> 25\%\)** in high resolution [2009.00628]. 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 [2009.00644]. The MOONS overview similarly states that over **10 years**, MOONS surveys would provide spectra for **millions of galaxies at \(0 < z < 10\)** [2009.00628]. 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 [2009.00644]. This theme is developed in detail by the 2026 precursor study of cluster detection in the **VIDEO** fields [2606.13359]. 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 [2606.13359]. After masks and wavelength-coverage cuts, the effective areas are **4.55 deg\(^2\)** in XMM-LSS and **4.11 deg\(^2\)** in CDFS, with H-band-selected catalogues limited to **\(m_H<24\)** and galaxy densities of **38.0 arcmin\(^{-2}\)** in XMM-LSS and **36.1 arcmin\(^{-2}\)** in CDFS [2606.13359]. 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 [2606.13359]. Two photometric-redshift-based cluster finders, **AMICO** and **WaZP**, are used to build a robust intersection sample suitable for MOONRISE targeting [2606.13359]. The final joint sample contains **519 cluster candidates** over the combined **8.66 deg\(^2\)** effective area, spanning **\(z=0.1\)–3.0**, with **271** in XMM-LSS and **248** in CDFS [2606.13359]. Of these, **74** lie at **\(z>1.5\)**, specifically **31** in XMM-LSS and **43** in CDFS [2606.13359]. The likely central galaxy is operationally defined as the **brightest member in \(m_H\) within 400 kpc of the cluster center**, termed the **brightest central member** [2606.13359]. 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 \(z\sim1.7\) even with the shallowest survey strategy** [2606.13359]. 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** [2509.09626]. That work is explicitly framed as a prediction paper for MOONRISE and uses **IllustrisTNG** and **EAGLE**, specifically **TNG100-1** and **EAGLE RefL0100N1504**, at **\(z=0,1,2\)** to forecast what the survey should be able to test [2509.09626]. To mimic the survey, the authors construct a **“MOONRISE-Like”** mock sample. They impose **\(M_* > 10^{9.5}\,M_\odot\)**, move galaxies into observer-like **2D+\(z\)** space, and randomly remove **30% of galaxies** to mimic an expected **70% spectroscopic completeness** [2509.09626]. Pair selection in redshift space uses \[ \Delta V_{\rm los} = \frac{c|\Delta z|}{1+\bar z} \le 1500\,{\rm km\,s^{-1}}. \] Quenched galaxies are defined through a redshift-dependent threshold, \[ {\rm sSFR}_{\rm QT}(z) \equiv \mu_{\rm SF}(z) - 3\sigma_{\rm SF}(z), \] with a galaxy classified as quenched if \[ {\rm sSFR}_i(z_i) \le {\rm sSFR}_{\rm QT}(z_i). \] Using this criterion, the total quenched fraction in both simulations is about **\(f_Q \sim 30\%\)** at \(z=0\) and falls to **\(<10\%\)**, roughly **\(\sim 7\%\)**, by \(z=2\) [2509.09626]. The main inference tool is **Random Forest classification**, using input features including black hole mass \(M_{\rm BH}\), stellar mass \(M_*\), halo mass \(M_{\rm Halo}\), local over-density \(\delta\), and distance to the central \(D_{\rm cen}\) [2509.09626]. The environmental densities are defined by \[ \delta_n = \log(\rho_n) - \langle \log \rho_n \rangle,\qquad \rho_n = \frac{3n}{4\pi R^3} \] for 6D simulation space, and \[ \delta_n = \log(\Sigma_n) - \langle \log \Sigma_n \rangle,\qquad \Sigma_n = \frac{n}{\pi R^2} \] for 2D+\(z\) observer space [2509.09626]. Feature importance is derived from Gini-impurity reduction, with \[ G(n) = 1 - \sum_i^c p_i(n)^2. \] 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 [2509.09626]. For **high-mass satellites**, the same paper predicts that they behave more like centrals than like low-mass satellites: **\(M_{\rm BH}\)** is again the strongest predictor of quiescence, with environmental parameters contributing a secondary signal [2509.09626]. For **low-mass satellites**, by contrast, the Random Forest analysis predicts that quiescence is governed by **environmental parameters**, with **host halo mass \(M_{\rm Halo}\)** as the strongest predictor at all epochs [2509.09626]. 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** [2509.09626]. In the MOONRISE-like samples, **TNG** retains **1307 low-mass satellites at \(z=1\)** with **298 quenched**, and **530 at \(z=2\)** with **51 quenched**; **EAGLE** has **406 low-mass satellites at \(z=1\)** with only **45 quenched**, and **none at \(z=2\)** [2509.09626]. 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 [2509.09626]. This suggests that MOONRISE’s measurement of quenched fractions as functions of **stellar mass**, **inferred black hole mass**, and **environment** across **\(0.9