MUSE-Faint Survey: Faint Galaxies & Dark Matter

• MUSE-Faint Survey is a coordinated deep integral-field spectroscopic study that targets ultra-faint galaxies and nebular regions beyond conventional detection limits.
• It employs tiered 'wedding-cake' and cluster lensing strategies to measure key properties like Lyα flux, metallicity, and dark matter distributions with high precision.
• The findings reveal a steep faint-end luminosity function, low-metallicity trends in low-mass galaxies, and stringent constraints on dark matter core sizes and alternative dark matter models.

The MUSE-Faint Survey refers to a coordinated set of very deep spectroscopic investigations of extremely faint galaxies and nebular emission regions using the Multi Unit Spectroscopic Explorer (MUSE) integral-field spectrograph at the ESO Very Large Telescope. The survey’s defining feature is the targeted exploration of parameter spaces in galaxy luminosity, emission line flux, surface brightness, or stellar mass that are inaccessible to traditional narrow-band, slit, or broadband photometric surveys. MUSE-Faint leverages both blank fields and lensing cluster observations, as well as specific programs targeting local ultra-faint dwarf galaxies and crowded resolved stellar fields, to produce quantitative constraints on galaxy evolution, cosmic reionization, interstellar and circumgalactic medium enrichment, low-mass star formation, and dark matter microphysics.

1. Survey Strategies and Instrumentation

The MUSE-Faint Survey design exploits the unique combination of MUSE’s $1'\times1'$ field-of-view, moderate spectral resolution ($R\sim3000$), and high sensitivity to both continuum and emission lines. Two major observational modes are employed:

• Blank field “wedding-cake” surveys deploy a tiered area–depth strategy:
  • Shallow Field (SF): $\sim 100$ arcmin$^2$, $F_{\mathrm{Ly}\alpha} \gtrsim 10^{-17}$ erg s$^{-1}$ cm$^{-2}$ for bright LAEs.
  • Medium-Deep Field (MDF): $\sim 10$ arcmin$^2$, probing down to $F_{\mathrm{Ly}\alpha} \gtrsim 10^{-18}$ erg s$^{-1}$ cm$^{-2}$.
  • Deep Field (DF): $\sim 1$ arcmin$^2$, reaching $F_{\mathrm{Ly}\alpha} \gtrsim 4\times10^{-19}$ erg s$^{-1}$ cm$^{-2}$ (Garel et al., 2015).
• Cluster lensing surveys observe massive clusters to capitalize on flux magnification, enabling the paper of LAEs down to $L_{\mathrm{Ly}\alpha} \lesssim 10^{40.5}$ erg s$^{-1}$. A tailored $1/V_{\mathrm{max}}$ approach is developed to account for spatial, spectral, and lensing-dependent detection thresholds (Vieuville et al., 2019, Richard et al., 2020).

The MUSE instrument preserves 3D spectral–spatial information, enabling all-source and blind emission-line searches within each cube and the recovery of spectral line fluxes, continuum levels, and robust redshifts. In crowded or low-S/N regimes, methods such as full-spectrum fitting (e.g., spexxy, pPXF) are used for precise velocity and metallicity determinations.

2. Census of Faint Galaxies and Emission Regions

The MUSE-Faint Survey identifies several key populations:

• Ultra-faint Lyman-$\alpha$ Emitters (LAEs): The survey robustly identifies LAEs at $2.8\lesssim z\lesssim 6.7$ down to $\sim 4\times10^{-19}$ erg s$^{-1}$cm$^{-2}$ (Garel et al., 2015, Maseda et al., 2018). This extends the known Ly$\alpha$ emitter luminosity function (LF) by >1 dex below previous narrow-band limits, revealing hundreds of objects per DF pointing.
• Extremely Faint Continuum Sources: Many LAEs are spectroscopically detected in Ly$\alpha$ but remain undetected in HST continuum imaging to $M_{\mathrm{UV}}\sim -15$ or fainter—comparable to local blue compact dwarfs and the presumed reionization progenitors (Maseda et al., 2018).
• Low-Metallicity Emission-Line Galaxies: The survey combines HST grism and MUSE follow-up to assemble a direct metallicity-selected sample of star-forming galaxies at $0.3$M_*\sim10^{7.9}\ M_\odot$, probing the low-mass, low-metallicity regime inaccessible to continuum-limited samples (Pharo et al., 2018).
• Local Faint HII and Diffuse Ionized Gas (DIG) Regions: In nearby spirals, MUSE’s spatial and spectral fidelity enables characterization of HII regions with $L({\rm H}\alpha)\sim10^{34.7}$ erg s$^{-1}$ and their associated DIG, providing new insights into ionization structure and energetics at the extreme faint limit (Micheva et al., 2022).

3. Key Physical Results and Scientific Impact

Faint-Luminosity LAEs and Reionization

Deep MUSE-Faint data shows that the faint-end of the LAE LF rises steeply, with measured slopes $\alpha = -1.69$ to $-1.87$ for $z\sim3$–7 (Vieuville et al., 2019), suggesting a dominant contribution of low-luminosity ($L\ll L^*$) LAEs to the total Ly$\alpha$ and star-formation rate density at high-$z$. MDF and DF tiers increase the SFRD captured by $2$–$7\times$ compared to narrow-band limits.

Faint Sources and the UV Luminosity Function

The detection of LAEs invisible to HST, down to $M_{\mathrm{UV}}\sim-15$, bridges the gap between UV-selected Lyman-break galaxies and the fainter ionizing sources necessary for cosmic reionization models. The abundance of these “continuum-dark” LAEs matches expectations from the high-EW tail of the Ly$\alpha$ emitter population (Maseda et al., 2018).

Metallicity and Evolution of Low-Mass Galaxies

Spectroscopic detection of auroral [OIII]$\,\lambda4363$ in faint emission-line galaxies enables “direct” (T$_e$-based) gas-phase oxygen abundances of $7.4<12+\log({\rm O/H})<8.4$ at $M_*\sim10^{7.9}$–$10^{10.4}M_\odot$. The observed mass–metallicity relation is systematically offset to lower metallicities ($0.6$–$0.7$ dex below continuum-selected samples). There is a trend towards lower metallicity at higher specific SFR, consistent with a combination of metal-poor gas inflow and enriched outflows (Pharo et al., 2018).

Star Formation in the Faintest Dwarfs

Integral field data in ultra-faint dwarfs such as Leo T reveals two kinematically distinct stellar populations (young, $\lesssim500$ Myr, and old, $>5$ Gyr), with the young population’s dispersion ($\sim2.3$ km s$^{-1}$) tightly matching that of the cold neutral gas, implying recent star formation directly from the CNM. No extended emission-line (star-forming) regions are detected down to $\Sigma_{\rm SFR}\sim 10^{-11}$ M$_\odot$ yr$^{-1}$ pc$^{-2}$, signifying the cessation of current star formation (Vaz et al., 2023, Vaz et al., 2023).

4. Implications for Galaxy Formation and Cosmic Structure

Hierarchical Assembly and Progenitors of L* Galaxies

Semi-analytic models anchored to the MUSE-Faint selection indicate that faint LAEs detected in deep and medium-deep surveys at $z\sim3$ reside in halos that evolve into typical sub-$L^*$ and $L^*$ galaxy halos at $z=0$, i.e., galaxies akin to the Milky Way. Deep fields can probe up to 87% of the total stellar mass density in MW halo progenitors at $z\sim3$ (Garel et al., 2015).

Circumgalactic Enrichment and Metal Transport

Faint LAEs in close proximity to high-$z$ CIV absorbers demonstrate that sub-$L^*$ galaxies are directly associated with metal-enriched circum- and intergalactic media (CGM/IGM). The detected LAE–CIV pairs—with impact parameters $11$–$200$ pkpc—support the view that energetic feedback from low-mass galaxies is a dominant driver of early metal enrichment (Diaz et al., 2020).

Faint Galaxies and the Ly$\alpha$ Halo Phenomenon

Clustering analyses combined with LF integration demonstrate that undetected faint LAEs can substantially contribute to the extended Ly$\alpha$ halos (LAHs) of brighter systems. At radii $R\gtrsim50$ pkpc, the integrated surface brightness from clustered faint LAEs is predicted to match or exceed observed LAH values unless the LF faint-end slope flattens considerably below observed values (Alonso et al., 2023).

5. Constraints on Dark Matter and Fundamental Physics

MUSE-Faint’s targeting of nearby ultrafaint dwarfs provides stringent tests of dark matter models:

• Cuspy vs. Cored Halos: Jeans-based dynamical modeling (CJAM, GravSphere) of stars in Eridanus 2, Antlia B, Leo T, and others shows no evidence for large dark matter cores (core radii $r_c\lesssim66$–$95$ pc, 68% CL) and constrains possible soliton/core radii expected in fuzzy or scalar field dark matter models to $<13$–$180$ pc (Zoutendijk et al., 2021, Zoutendijk et al., 2021, Júlio et al., 2023).
• Alternative DM Scenarios: Analysis rules out self-interaction cross-sections and scalar field self-couplings large enough to produce $\sim$kpc-size cores as the origin of observed shallower profiles in more massive dwarfs; limits on the fuzzy dark matter particle mass exceed $4\times10^{-20}$ eV (Zoutendijk et al., 2021).
• Indirect Dark Matter Searches: MUSE-Faint data yield the most stringent optical bounds on axion-like particle (ALP) dark matter with $m_a=2.65$–$5.3$ eV, placing $g_{a\gamma\gamma}\lesssim10^{-12}$ GeV$^{-1}$, more constraining than previous cluster or globular cluster stellar evolution bounds in this mass range (Regis et al., 2020, Todarello et al., 2023, Todarello, 2023).

6. Methodological Innovations and Data Products

• Completeness Control in Lensing Fields: The survey advanced 3D detection mask techniques sensitive to local noise, magnification, and spatial–spectral profile for each lensed background source, incorporating asymmetric magnification PDFs to account for lens-model uncertainties (Vieuville et al., 2019, Richard et al., 2020).
• Automated and Visual Identification: Blind searches using software like ORIGIN, cross-checked with stacking and S/N profiles, ensure robust emission-line detection even in the absence of clear continuum.
• Direct Empirical Stacking: Stacked mean UV–optical spectra are released and recommended as templates for JWST and ELT spectroscopic planning, providing empirical guidance on line ratios, nebular emission strengths, and ISM diagnostics for the faintest galaxy regimes (Feltre et al., 2020).
• Photometric+Spectroscopic Procedures in Dense Environments: For UDGs and crowded Galactic fields, multi-band photometry is combined with spectroscopic velocity/morphological vetting to statistically correct for completeness and contamination, yielding GC counts and M/L ratios for dark matter mass estimation (Mirabile et al., 17 Sep 2025).

7. Legacy and Broader Impact

The MUSE-Faint Survey redefines the observational frontier for faint and low-mass galaxies, faint HII regions, and the search for dark sector physics using integral-field spectroscopy. Its results inform hierarchical galaxy formation models, reionization histories, baryon/metal ejection processes, and the microphysical properties of dark matter. The suite of technical and empirical catalog products (deep redshift catalogs, empirical stacked spectra, completeness-corrected LFs, dynamical modeling frameworks) underpins future studies with JWST, ELT, ALMA, and wide-field spectroscopic facilities targeting the faint universe.