AMIGA Group: Isolated Galaxy & Cosmic Ray Studies

• AMIGA Group is a dual research collaboration studying isolated galaxies with stringent environmental criteria to establish baseline scaling laws and galaxy evolution benchmarks.
• Its cosmic ray branch employs underground muon counters with advanced electronics to directly measure muonic components in extensive air showers with high precision.
• The group integrates reproducible data-driven methodologies and semantic workflows, setting new standards for environmental diagnostics in galaxy evolution and astroparticle physics.

The AMIGA Group refers to two distinct but prominent research collaborations in contemporary astrophysics: (1) the Analysis of the interstellar Medium of Isolated GAlaxies—a leading consortium defining nature–dominated galaxy samples for benchmarking galaxy evolution, structure, and activity absent significant environmental perturbation; and (2) Auger Muons and Infill for the Ground Array, a project within the Pierre Auger Observatory specializing in the direct measurement of the muonic component of extensive cosmic-ray air showers. The following article surveys both, with explicit, technically detailed coverage strictly grounded in published results.

1. AMIGA: Isolated Galaxy Science and Benchmarking

AMIGA (Analysis of the interstellar Medium of Isolated GAlaxies) is a comprehensive, multi-wavelength project providing the astrophysical community with rigorously vetted samples of galaxies in extreme isolation. The reference sample is drawn from the Catalogue of Isolated Galaxies (CIG; Karachentseva 1973), further refined with quantitative environmental metrics and spectroscopic confirmation to ensure minimal recent or ongoing environmental influence (1311.0856, Bok et al., 2020).

Isolation Criteria:

• No neighbor of optical diameter between $0.25D_P$ and $4D_P$ is permitted within $20D_i$ projected distance (where $D_P$ and $D_i$ are the primary and neighbor diameters, respectively).
• Quantitative refinement uses (a) local number density $\eta_k$ (logarithmic density to the $k$th neighbor) and (b) tidal strength $Q$ (logarithm of the sum of tidal to binding forces from all neighbors within 1 Mpc and 500 km/s in velocity) (1311.0856).
• Only galaxies with $\eta_k<2.7$ and $Q<-2$ qualify as "truly isolated."

Post-SDSS-DR9, the baseline AMIGA sample comprises 426–481 galaxies depending on selection strictness, with an even more exclusive subset of $\sim$105 systems lacking any $m_r<17.77$ companion within 1 Mpc and $|\Delta v|<500$ km/s (1311.0856, Bok et al., 2020).

Scientific Mandate:

AMIGA isolates the "nature baseline" for (i) star formation and gas content scaling relations, (ii) morphology and structure (bars, rings, disc breaks), (iii) nuclear activity, and (iv) ISM chemistry, providing zero-point calibrations against which environmental effects (the "nurture" component) can be robustly quantified (Bok et al., 2020, Tuli et al., 2019, Sánchez-Alarcón et al., 2023).

2. Internal Structure, Evolution, and Activity in Isolated Galaxies

Morphological Distribution and Features

SDSS DR8-based re-classifications (Buta et al., 2019) confirm that the AMIGA sample exhibits a broad morphological spectrum, E–Im, with a strong predominance ($>$50%) of intermediate-to-late spirals (Sb–Sc). Detailed CVRHS analysis finds:

• Grand design spiral arms (Elmegreen AC 8/9/12) in $\sim$54% of classifiable spirals, challenging the view that such symmetry requires environmental triggers.
• Bar frequency is $\sim$50%, but only $\sim$16% are strongly barred.
• Rings and lenses: Inner rings are present in $\sim$8% of systems; lens-related outer varieties are rare.
• No significant dependences of bars, rings, or arm class on the isolation parameters $Q$ and $\eta_k$.
• Stellar mass correlates with structure: $⟨\log(M_s/M_\odot)⟩ \sim 10.7$ (E–S0), $10.6$ (Sb–Sc), $9.8$ (Scd–Im).

Galaxy Colors and Star Formation

AMIGA spirals show median SDSS $(g-r)$ values that are slightly redder (by $\sim0.02$–$0.05$ mag) and with distinctly lower dispersion (MAD, FWHM) than matched samples in pairs or groups (Lorenzo et al., 2012). Their color distributions are approximately Gaussian, indicating passive, unperturbed disk star formation, with few "green valley" systems. By contrast, environments with recent interaction history exhibit broader, non-Gaussian color spreads due to a mix of starburst and dust extinction effects.

Star Formation–Stellar Mass–Gas Scaling Laws

Using WISE mid-infrared diagnostics, AMIGA establishes scaling laws for star-forming main sequence (SFR vs $M_\star$), gas fraction ($M_{\mathrm{HI}}/M_\star$), and HI deficiency (Bok et al., 2020):

• Main sequence: $σ_{\rm AMIGA}=0.37$ dex, significantly lower than pairs ($σ=0.55$ dex); only 0.6% are quiescent ($>1$ dex below the MS).
• HI scaling relation: $$\log_{10}(M_\mathrm{HI}/M_\odot)=0.44\log_{10}(M_\star/M_\odot)+5.19$$, with scatter $σ=0.33$ dex. This provides the "gold standard" against which HI deficiency (environmental stripping/depletion) is gauged.
• Gas fraction scatter: $σ_{\rm AMIGA}=0.44$, vs $0.54$ in pairs.

This extremely low dispersion is interpreted as evidence for deterministic, environment-free evolution in the AMIGA sample.

HI Profile Asymmetry

AMIGA establishes the reference distribution for atomic hydrogen profile asymmetry ($A_{flux\,ratio}$), with $σ=0.13$ (possibly as low as $0.11$ intrinsic), and only $2\%$ of galaxies exceeding 3$σ$ (Espada et al., 2011). By comparison, field galaxy samples exhibit $10$–$20\%$ rates of strong asymmetry. No significant correlation exists between HI asymmetry and $Q$, $\eta_k$, or arm/bar strength; main deviations are attributed to stochastic minor mergers or accretion events.

ISM Chemistry

AMIGA galaxies such as CIG 638 display unusually high CCH-to-HCN, CCH-to-HCO$^+$, and CCH-to-HNC ratios compared to starburst or AGN galaxies. This is attributed to low dense-gas fractions and enhanced diffuse-phase photodissociation region chemistry, a direct consequence of evolutionary isolation (Martin et al., 2014).

Disc Breaks and Outer Structure

Ultra-deep imaging (down to $\mu_r\sim30$ mag arcsec$^{-2}$) in a subsample of AMIGA spirals (Sánchez-Alarcón et al., 2023) finds:

Environment	Type I (pure exp.)	Type II (down-bending)	Type III (up-bending)
AMIGA Isolated	$36\%$	$56\%$	$8\%$
Field	$6\%$	$66\%$	$29\%$
Cluster	$15\%$	$56\%$	$29\%$

A high Type I fraction and extreme paucity of Type III profiles in AMIGA imply that anti-truncations are merger-induced, while pure exponentials result from secular, quiescent evolution. Even among AMIGA, $\sim$40% show low-surface-brightness signs of past minor perturbation.

Nuclear Activity

Optical AGN (Seyfert 2, LINER) prevalence is $20.4\%$ (rising to $36.7\%$ including transition objects). After matching for host morphology and luminosity, the AGN fraction in AMIGA and in dense group samples (e.g., Hickson Compact Groups) is statistically indistinguishable, refuting any necessity for major interaction in triggering classical (optically selected) AGN (Sabater et al., 2012).

3. AMIGA at the Pierre Auger Observatory: Cosmic Ray Muon Detection

AMIGA (Auger Muons and Infill for the Ground Array) is a dedicated upgrade of the Pierre Auger Observatory, aimed at extending the primary energy range down to $\sim10^{16.5}$ eV and achieving direct, high-precision measurement of the muonic content in extensive air showers (EAS) (Collaboration et al., 2016, Taboada, 2019).

System Layout and Architecture

• Detector grid: Denser infill arrays (750 m, 433 m grid spacings) embedded in the main 1.5-km triangular array provide coverage at lower energies.
• Muon counters: Each station comprises three (production) or up to four (engineering) underground modules, each $10\,\mathrm{m^2}$ (total $30\,\mathrm{m^2}$), segmented into 64 plastic-scintillator bars read out via WLS fibers.
• Depth: Burial at $2.3\,\mathrm{m}$ depth ensures full electromagnetic component suppression.
• Optical sensor evolution: Initial multi-anode PMTs have been superseded by arrays of Hamamatsu silicon photomultipliers (SiPMs) for compactness, ruggedness, and low-voltage operation (Collaboration et al., 2020, Collaboration et al., 2017).

Electronics and Data Acquisition

• Front-end: Dual 32-channel CITIROC ASICs per module, offering per-channel programmable gain and bias, high-speed (15 ns peaking) shaper, discriminators for counter mode, and summed analog output for integrator mode (Collaboration et al., 2020).
• Dynamic Range: Counter mode (binary) is linear up to all 64 bars, high-gain analog integrator is linear up to $\sim$85 muons, and low-gain up to $\sim$362, ensuring coverage of all EAS densities encountered (Collaboration et al., 2020).
• Calibration: FPGA-driven remote calibration and bias tuning allow single-muon peak alignment and SiPM gain stabilization across the 9–44$^\circ$C field temperature range (Collaboration et al., 2017).
• Embedded System: Centralized acquisition and environment monitoring using Altera Cyclone IV FPGA and LEON3 CPU, synchronized with SD water-Cherenkov triggers over gigabit WLAN (Collaboration et al., 2021). Power systems use solar supply with a 19 W per-station typical draw.

Performance

• Detection Efficiency: Single-muon efficiency exceeds $98\%$ across the module (Collaboration et al., 2017).
• Noise Suppression: Optimized SiPM over-voltage ($\Delta V \sim 3.5\,\mathrm{V}$), discriminator threshold ($>2.5$ PE), and FPGA-based pulse timing maintain accidental rate $\sim2\%$ per module during EAS windows.
• Timing: 3.125 ns (counter mode) and 6.25 ns (integrator mode) sampling resolve arrival times and pulse structures for muon profile and depth estimation (Collaboration et al., 2016, Taboada, 2019).
• Dynamic Response: Laboratory non-linearity within $5\%$ up to designed saturation point; stable gain over diurnal/seasonal cycles (Collaboration et al., 2020).

4. Applications: High-Energy Astroparticle Physics and Extragalactic Benchmarks

Cosmic-Ray Composition and Hadronic Physics

AMIGA's direct muon counting, at 98% efficiency and with sub-10% statistical uncertainty up to $\sim$700 muons per station, enables:

• Primary mass composition inference at $10^{17}$–$10^{19}$ eV via $N_\mu$ discrimination.
• Cross-validation of hadronic interaction models by comparing observed $N_{\mu}^{opt}$ and $X_{\max,\mu}$ with simulations.
• Enhanced energy calibration of Auger's air-shower reconstructions, especially at lower energies inaccessible to fluorescence methods (Collaboration et al., 2016, Collaboration et al., 2020).

Galaxy Evolution: Calibration and Environmental Diagnostics

The strict zero-point characterization of isolated galaxies provided by AMIGA permits:

• Quantitative environmental diagnostics: any sample exceeding the AMIGA scaling relation's dispersions or HI/optical asymmetries must have nurture-induced modifications (Bok et al., 2020, Espada et al., 2011).
• Reference distributions for morphology, color, AGN incidence, ISM chemistry, and disc profile used in modeling galaxy transformation mechanisms across densities.
• Identification and study of secular evolutionary channels for structure formation (e.g., bar/spiral/lens formation absent interactions) (Buta et al., 2019).

Circumgalactic Medium: Project AMIGA (Andromeda)

A separate "Project AMIGA" (Absorption Maps In the Gas of Andromeda) targets the circumgalactic medium (CGM) of M31, leveraging HST/COS absorption observations and Green Bank 21-cm mapping. Key results include:

• A minimal covering factor $f_c<0.051$ for optically-thick HI within the M31 virial radius, consistent with low-redshift zoom-in simulation predictions (Howk et al., 2017).
• Cold accretion from the IGM, rather than outflows, is required to explain the observed column densities, kinematics, and coverage of Si II/III/IV in M31's halo (Afruni et al., 2021).

5. Data-Driven Methodologies and Big Data Reproducibility

The AMIGA group's recent work includes advanced techniques for Big Data reproducibility in radio astronomy and SKAO projects (Garrido et al., 12 Jan 2026):

• Semantic Data Models: SKA-ProvSDP (W3C PROV-DM based) for detailed workflow provenance and reproducibility across federated SRCNet infrastructures.
• Federated and serverless architectures: Containerized analysis services, workflow orchestration (COMPSs, Snakemake), and FaaS for scaling analysis near data.
• Validation protocols: Rule-level input/output checksums, embedded QA metrics, and publication of all workflows + containers with persistent identifiers (e.g., DOIs).
• Open Science: All science products are delivered with full provenance, versioning, and accessible semantic metadata, enabling reproducibility and verification at scale.

6. Significance and Legacy

AMIGA has produced the astrophysical community's leading reference sets for both environmental "zero-point" galaxy properties and for large-scale cosmic-ray muon detection. Its methodologies—quantitative isolation metrics, rigorous multiwavelength benchmarking, robust low-power detector design, highly modular electronics, and culture of reproducibility—define the standard for future large-sample extragalactic studies and precision astroparticle experiments. All results, measurements, and diagnostics are published with full technical detail, and databases (spectra, photometry, emission lines, environmental measures) are available for systematic cross-environment analyses (Buta et al., 2019, Sabater et al., 2012, Garrido et al., 12 Jan 2026).