Galaxy imaging spectra fitting tool for multi-band image decomposition, SED modeling, and joint analysis. Covers .lyric config files, command-line arguments, and MCP interface.
This skill provides comprehensive guidance for writing GalfitS configuration files (.lyric format). GalfitS extends GALFIT to simultaneously fit multiple photometric bands with SED constraints.
To create a new GalfitS config file:
Identify your fitting phase:
Configure data input (Region → Images → Spectra → Atlas)
Define model components:
Pa2 parameter
sersic - Standard bulge/disk profilesersic_f - Fourier mode profile (spiral arms, bars, etc.)ferrer, edgeondisk, GauRing, const, Gaussian - Other profilesSet parameter constraints (optional)
Run GalfitS with appropriate command-line options
| Topic | File | Description |
|---|---|---|
| Data Configuration | data-config.md | Region (R1-R3), Images (Ia1-Ia15), Spectra (Sa1-Sa4), Atlas (Aa1-Aa7) |
| Model Components | model-components/ | Galaxy, Profile (Sersic, Fourier), Nuclei/AGN, Foreground Star |
| Parameter Constraints | constraints/ | MSR, MMR, SFH, AGN relations, parameter files, priors |
| Configuration Examples | examples/ | Multi-band, SED-only, spectrum, joint fitting examples |
| Running GalfitS | running-galfits.md | Command-line arguments, fitting methods, usage examples |
GalfitS has TWO main categories of components. Understanding which category you need is the first step.
Purpose: Define your observational data (images, spectra) before fitting.
| Component | Prefix | Parameters | Use For | Docs |
|---|---|---|---|---|
| Region | R | R1-R3 | Target name, coordinates, redshift | data-config.md |
| Image | I | Ia1-Ia15 | Single-band image data, PSF, sigma, mask | data-config.md |
| Spectrum | S | Sa1-Sa4 | Optical/IR spectrum data | data-config.md |
| Atlas | A | Aa1-Aa7 | Group images/spectra for joint fitting | data-config.md |
Purpose: Define the physical components that make up your source.
Model Components
│
├── 🔹 Galaxy (G) ──────────────► Combines Profiles into ONE physical galaxy
│ └── contains Profile (P) components
│
├── 🔹 Profile (P) ─────────────► Galaxy structural component (bulge, disk, bar...)
│ └── Type determined by Pa2 parameter
│
├── 🔹 Nuclei/AGN (N) ──────────► AGN with full SED model (independent component)
│
└── 🔹 Foreground Star (F) ────► Star in Milky Way (independent component)
| Component | Prefix | Parameters | Purpose | Documentation |
|---|---|---|---|---|
| Galaxy | G | Ga1-Ga7 | Container that combines multiple Profile (P) components into one physical galaxy | galaxy.md |
| Profile | P | Pa1-Pa32 | Structural component (bulge, disk, bar). Type set by Pa2: | See below ↓ |
| Nuclei/AGN | N | Na1-Na27 | Central AGN with continuum, emission lines, torus | nuclei-agn.md |
| Foreground Star | F | Fa1-Fa8 | Milky Way star with blackbody SED | foreground-star.md |
Pa2 parameter)| Profile Type | Pa2 Value | Use When You See... | Documentation |
|---|---|---|---|
| Sersic | sersic | Elliptical galaxy, bulge, disk, any axisymmetric structure | profile-sersic.md |
| Fourier Sersic | sersic_f | Spiral arms, bar, non-axisymmetric features | profile-fourier.md |
| Ferrer Bar | ferrer | Bar with flat inner core | profile-other.md |
| Edge-on Disk | edgeondisk | Galaxy viewed edge-on | profile-other.md |
| Gaussian Ring | GauRing | Ring or lens structure | profile-other.md |
| Gaussian | Gaussian | Unresolved point source | profile-other.md |
| Constant | const | Flat background | profile-other.md |
What do you want to model?
| Your Goal | Which Component? |
|---|---|
| A galaxy with bulge + disk | Galaxy (G) containing 2+ Profile (P) components |
| Just the bulge or disk | Single Profile (P) component |
| Spiral arms or a bar | Profile with Pa2) sersic_f |
| An AGN / central black hole | Nuclei/AGN (N) component |
| A star in the field | Foreground Star (F) component |
| Multiple galaxies | Multiple Galaxy (G) components: Ga, Gb... |
| Galaxy + AGN together | Galaxy (G) for host + Nuclei/AGN (N) for center |
Important: Letters (a, b, c...) are user-defined, NOT fixed to types.
| ❌ Wrong | ✅ Correct |
|---|---|
| Pa = Sersic<br>Pb = Fourier<br>Pc = AGN | Pa2) determines type:<br>- Pa2) sersic → Sersic<br>- Pa2) sersic_f → Fourier<br>- Use Na prefix for AGN |
Example:
# All three are Sersic profiles, just with different parameters
Pa1) bulge Pa2) sersic
Pb1) disk Pb2) sersic ← Same type!
Pc1) bar Pc2) sersic_f ← Different type (Fourier)
# AGN is a DIFFERENT component type (N prefix, not P)
Na1) AGN (not Pa1) ← Separate component type
All parameters use: [value, min, max, step, vary]
| Phase | Ia15 (Use SED) | Spatial (Pa3-Pa8) | SED (Pa9-Pa16) |
|---|---|---|---|
| 1 | 0 | vary=1 | vary=0 |
| 2 | 1 | vary=0 | vary=1 |
| 3 | 1 | vary=1 | vary=1 |
galfits config.lyric --work ./output
For detailed command-line arguments, fitting methods, and usage examples, see running-galfits.md.
| Task | Command |
|---|---|
| Quick fit | galfits config.lyric --work ./output |
| Bayesian analysis | galfits config.lyric --work ./output --fit_method dynesty --nlive 200 |
| Refine previous | galfits config.lyric --work ./output --readpar previous.params |
| With constraints | galfits config.lyric --work ./output --priorpath priors.txt |
| Issue | Solution |
|---|---|
| Parameter hits limit | Expand min/max bounds |
| Band misalignment | Set Ia13) 1 and adjust Ia14) shift ranges |
| Off-center residual | Add new component at that position |
| Circular center residual | Add bulge/AGN component |
| Bar-like residual | Add bar component (set Pa2) sersic_f) |
| Fitting fails to converge | See running-galfits.md for troubleshooting |
your-config/
├── SKILL.md # This file - main entry point
├── running-galfits.md # Command-line arguments & usage
├── data-config.md # Data input configuration
├── model-components/ # Model component documentation
│ ├── index.md # Component overview
│ ├── galaxy.md # Galaxy configuration
│ ├── profile-sersic.md # Sersic profile
│ ├── profile-fourier.md # Fourier Sersic profile
│ ├── profile-other.md # Other profile types
│ ├── nuclei-agn.md # Nuclei/AGN configuration
│ ├── foreground-star.md # Foreground star configuration
│ └── parameter-format.md # Parameter format and combining
├── constraints/ # Parameter constraints & priors
│ ├── index.md # Constraints overview
│ ├── parameter-files.md # .params and .constrain file usage
│ ├── mass-size-relation.md # MSR prior for size-mass correlation
│ ├── mass-metallicity-relation.md # MMR prior for stellar mass-metallicity
│ ├── sfh-constraints.md # Star formation history priors
│ ├── agn-constraints.md # AGN-specific constraints
│ ├── gaussian-priors.md # GP parameter for Gaussian priors
│ └── energy-balance.md # EB parameter for dust-stellar energy balance
├── examples/ # Configuration examples
│ ├── index.md # Examples overview
│ ├── multi-band-imaging.md # Multi-band bulge-disk decomposition
│ ├── pure-sed-fitting.md # Photometric SED fitting
│ ├── spectrum-fitting.md # AGN/host spectrum decomposition
│ ├── imaging-spectrum-joint.md # Joint imaging+spectra fitting
│ └── grism-imaging.md # Grism data analysis (placeholder)
└── templates/ # Config templates (optional)
To use this configuration guide, refer to the detailed documentation files: