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name: galfits-manual description: 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.

GalfitS Configuration Guide

This skill provides comprehensive guidance for writing GalfitS configuration files (.lyric format). GalfitS extends GALFIT to simultaneously fit multiple photometric bands with SED constraints.

Quick Start

To create a new GalfitS config file:

1. Identify your fitting phase:

   • Phase 1: Image-only fitting (spatial parameters free, SED fixed)
   • Phase 2: SED-only fitting (spatial fixed, SED free)
   • Phase 3: Joint optimization (all parameters free)

2. Configure data input (Region → Images → Spectra → Atlas)

3. Define model components:

   • Profile components (P): Galaxy structural components with types specified by Pa2 parameter
     • sersic - Standard bulge/disk/bar profile
     • sersic_f - Fourier mode profile (spiral arms)
     • ferrer, edgeondisk, GauRing, const, Gaussian - Other profiles
   • Nuclei/AGN components (N): AGN with full SED model
   • Foreground stars (F): Stellar sources with blackbody SED
   • Galaxy components (G): Combine multiple Profile (P) components

4. Set parameter constraints (optional)

5. Run GalfitS with appropriate command-line options

Navigation

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

Component Type Quick Reference

GalfitS has TWO main categories of components. Understanding which category you need is the first step.

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📊 Category 1: Data Components (Input Configuration)

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

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🌌 Category 2: Model Components (What You're Fitting)

Purpose: Define the physical components that make up your source.

Component Hierarchy

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)

Detailed Model Component Reference

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

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Profile (P) Sub-Types (determined by Pa2 parameter)

Profile Type	Pa2 Value	Use When You See...	Documentation
Sersic	sersic	Elliptical galaxy, bulge, disk, bar, any axisymmetric structure	profile-sersic.md
Fourier Sersic	sersic_f	Spiral arms, 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

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🎯 Quick Decision Guide

What do you want to model?

Your Goal	Which Component?
Just the disk or elliptical (like a bulge) component	Single Profile (P) component
A galaxy with bulge + disk	Galaxy (G) containing 2+ Profile (P) components
Bar	Profile with Pa2) sersic and the Sersic index fixed to n=0.5
Spiral arms	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

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📝 Component Letter Rules

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     ← Same type (but Pc6 fixed to 0.5)!
Pd1) spiral   Pd2) sersic_f     ← Different type (Fourier)

# AGN is a DIFFERENT component type (N prefix, not P)
Na1) AGN      (not Pa1)       ← Separate component type

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Parameter Format Reminder

All parameters use: [value, min, max, step, vary]

Phase-Specific Configuration

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

Running GalfitS

Basic Command

galfits config.lyric --work ./output

For detailed command-line arguments, fitting methods, and usage examples, see running-galfits.md.

Quick Reference

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

Common Issues & Solutions

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)
Fitting fails to converge	See running-galfits.md for troubleshooting

File Structure

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)

Usage

To use this configuration guide, refer to the detailed documentation files:

• Data Configuration: data-config.md - Region, Images, Spectra, Atlas
• Model Components: model-components/ - Galaxy, Profile, Nuclei/AGN, Foreground Star
• Parameter Constraints: constraints/ - Parameter files, MSR, MMR, SFH, AGN, priors
• Configuration Examples: examples/ - Multi-band, SED-only, spectrum fitting
• Running GalfitS: running-galfits.md - Command-line options, fitting methods

Model Components Quick Links

• Galaxy - Combine profile components into galaxies
• Sersic Profile - Standard bulge/disk/bar profile
• Fourier Sersic - Spiral arms
• Other Profiles - Ferrer, edge-on, ring, Gaussian
• Nuclei/AGN - AGN with full SED model
• Foreground Star - Stellar sources
• Parameter Format - Parameter combining and format

Constraints Quick Links

• Parameter Files - .params and .constrain file usage
• Mass-Size Relation - MSR prior for galaxy size-mass correlation
• Mass-Metallicity Relation - MMR prior for stellar mass-metallicity
• SFH Constraints - Star formation history priors
• AGN Constraints - Black hole mass, emission line relations
• Gaussian Priors - GP parameter for Gaussian priors
• Energy Balance - EB parameter for dust-stellar energy balance

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