Concepts¶
This page provides a short overview of common astronomy and image–analysis terms used throughout the GALFITools documentation. The goal is to make the package more accessible for users who may be less familiar with astronomical jargon.
Computational Tools¶
GALFIT¶
GALFIT is a widely used program for two–dimensional fitting of galaxy images (Peng et al. 2002). It models galaxies with analytic functions (e.g., Sérsic, exponential disks, Nuker profiles) convolved with the point–spread function (PSF). GALFIT is the external engine that GALFITools interfaces with.
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GALFIT file¶
A GALFIT file (often called a feedme file) is a text file that contains all of the information required for GALFIT to run a model fit. It specifies the image properties (such as image filename, plate scale, and photometric zero point), the fitting and convolution regions, and the initial parameters of each component (e.g., magnitude, effective radius, Sérsic index). After a run, the file can also record the final fitted parameters. GALFITools provides utilities to create and manage these files programmatically.
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Example of a GALFIT file¶
Below is a simplified snippet of a GALFIT input file (feedme file). Each line specifies a property of the image or a parameter of a model component. The symbols in brackets (A, B, etc.) indicate whether a parameter is free or fixed during fitting.
=============================================================================
# IMAGE and GALFIT CONTROL PARAMETERS
A) galaxy.fits # Input data image (FITS file)
B) output.fits # Output data image block
C) none # Sigma image name (or "none")
D) psf.fits # Input PSF image
E) 1 # PSF oversampling factor
F) mask.fits # Bad pixel mask (or "none")
G) none # Parameter constraint file (or "none")
H) 1 512 1 512 # Image fitting region (xmin xmax ymin ymax)
I) 100 100 # Size of convolution box (x y)
J) 25.0000 # Magnitude photometric zeropoint
K) 0.2500 0.2500 # Plate scale (dx dy) [arcsec per pixel]
O) regular # Display type (regular, curses, both)
P) 0 # Choose: 0=optimize, 1=model, 2=imgblock, 3=subcomps
# Component 1: Sérsic profile
0) sersic # Object type
1) 256.0 256.0 1 1 # Position x, y [pixel]
3) 18.5 1 # Total magnitude
4) 20.0 1 # Effective radius [pixels]
5) 2.5 1 # Sérsic index
9) 0.9 1 # Axis ratio (b/a)
10) 45.0 1 # Position angle (degrees)
Z) 0 # Skip this model in output image? (no=0)
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SaoImage DS9¶
SaoImage DS9 is an astronomical imaging and visualization application. It is commonly used to inspect FITS images, define regions of interest, and interactively examine astronomical data.
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DS9 regions¶
In DS9, a region is a user–defined geometric shape (circle, box, polygon, etc.) drawn on an image. Regions can mark sources, masks, or fitting boundaries, and can be saved to files that GALFITools can read. In GALFITools, DS9 regions must be saved in physical or image coordinates.
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Mask¶
A mask is an image that flags which pixels should be ignored during fitting (for example, bright foreground stars, cosmic rays, or image defects). GALFITools includes utilities to create masks automatically or manually.
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galfitools.galin.MaskSky.skyRem()
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Initial parameters¶
Initial parameters are starting guesses for the fitting procedure, such as galaxy magnitude, effective radius, axis ratio, and Sérsic index. Providing reasonable initial values helps GALFIT converge to the correct solution.
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Surface brightness model¶
A surface brightness model is a mathematical description of how the light distribution of a galaxy is represented. Models are constructed by combining analytic functions such as Sérsic profiles, de Vaucouleurs laws, exponential disks, Gaussians, or Nuker profiles. Each function contributes to the total brightness distribution, and the complete model can include one or many components.
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Component¶
A component refers to a single analytic function used within a surface brightness model, for example one Sérsic profile, one exponential disk, or one Gaussian. A model may consist of a single component (e.g. one Sérsic function) or several components combined (e.g. bulge + disk, or bulge + disk + bar). Components allow complex galaxy structures to be described in a modular way.
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Surface brightness profile¶
A surface brightness profile is a one–dimensional curve showing how the surface brightness of a galaxy changes with radius. Profiles are often used to characterize galaxy structure and to fit analytic models.
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Simultaneous fitting¶
In GALFIT, simultaneous fitting refers to fitting two or more galaxies at the same time within the same image. Each galaxy may have its own surface brightness model (bulge, disk, bar, etc.), and all models are optimized together in a single run.
Why use it. - To correctly account for light contamination when galaxies overlap or lie
close to one another in projection.
To avoid bias in parameter estimates that would occur if neighboring galaxies were ignored or subtracted beforehand.
To ensure that the global sky, PSF, and noise properties are treated consistently across the field.
How GALFITools interprets simultaneous fitting. - Components belonging to the same galaxy are grouped by a common center
(within a distance tolerance given by
-d/--dis).
Components belonging to different galaxies have distinct centers.
Many GALFITools routines allow you to specify which galaxy (set of components) to analyze via
--numcompor similar options.The software can therefore compute quantities for one galaxy (e.g., bulge to total ratio) even if several galaxies were fit simultaneously.
Note
This is different from fitting multiple components of a single galaxy. Here, entire galaxies (each possibly with multiple components) are fit together.
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Related GALFITools CLI commands
routine-getReComp
routine-getMeRad
Photometric Parameters¶
Star (in an image)¶
In the context of astronomical images, a star usually appears as a point–like source broadened by the PSF. Stars are often used to construct PSFs or to calibrate the image.
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Point–spread function (PSF)¶
The point–spread function represents the response of the telescope and detector to a point source of light (such as a star). It describes the blurring of the image due to the instrument and atmosphere (for ground–based observations). GALFIT requires a PSF to deconvolve models.
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Full Width at Half Maximum (FWHM)¶
FWHM is a measure of the width of a profile. It is the distance between the two points on the profile where the value falls to half of its maximum. In astronomy, the FWHM of a stellar image provides an estimate of the PSF size and image resolution.
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Surface brightness (SB)¶
Surface brightness is the flux received per unit area on the sky, commonly reported in magnitudes per square arcsecond (mag arcsec⁻²). It describes how light is distributed across a galaxy image.
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Photometry¶
Photometry is the measurement of fluxes or magnitudes of astronomical objects. It can be performed with apertures, PSF fitting, or model fitting methods such as GALFIT.
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Simulated galaxy (photometric)¶
A simulated galaxy is a synthetic image constructed using analytic profiles (e.g., Sérsic, exponential disk) and observational effects such as PSF convolution and noise. Simulated galaxies are used for testing, teaching, and validating analysis pipelines.
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Sky background¶
The sky background is the level of light in an image not associated with the target object. Accurate background estimation is important because it strongly affects measured magnitudes and profiles.
Related GALFITools API
galfitools.sky.GalfitSky.galfitsky()
Magnitude¶
Magnitude is a logarithmic measure of the brightness of an astronomical object. A decrease of 1 magnitude corresponds to an increase in brightness by a factor of about 2.512. Fainter objects have larger magnitude values, while brighter objects have smaller values.
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Convolution¶
In image analysis, convolution is the process of combining two functions, such as a model galaxy image and the point–spread function (PSF), to simulate how the model would appear through a telescope and detector. GALFIT uses convolution to compare model components with the observed data.
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Sigma image¶
A sigma image is an auxiliary image where each pixel value represents the estimated standard deviation (uncertainty) of the corresponding pixel in the science image. GALFIT can use a sigma image to weight the fit, giving less importance to noisy pixels.
Magnitude zero point¶
The magnitude zero point is a calibration constant that converts between instrumental fluxes (in counts or electrons) and standard magnitudes. It depends on the instrument, filter, and exposure time. A correct zero point ensures that fitted magnitudes can be compared with standard photometric systems.
Plate scale¶
The plate scale is the conversion factor between pixel units in the image and angular units on the sky, usually expressed in arcseconds per pixel. It depends on the telescope optics and detector.
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Galactic Components¶
Bulge¶
The bulge is the central, spheroidal component of a galaxy. It is generally more concentrated and has higher surface brightness than the surrounding disk.
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Disk¶
The disk is the flattened, rotating component of a galaxy, typically hosting spiral arms and ongoing star formation. Its brightness profile is often well described by an exponential law.
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Galactic bar¶
A bar is an elongated structure of stars crossing the central region of a disk galaxy. Bars redistribute angular momentum and can drive gas inflows toward the galaxy center.
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Surface Brightness Modeling¶
Sérsic profile and index¶
A Sérsic profile is a mathematical function that describes how the brightness of a galaxy varies with radius. The Sérsic index (n) controls the shape of the profile: low n values represent disk–like, exponential profiles, while high n values represent more concentrated, bulge–like profiles.
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Multi–Gaussian Expansion (MGE)¶
The Multi–Gaussian Expansion (MGE) method represents a complex two–dimensional light distribution as a sum of multiple two–dimensional Gaussian functions. This approach provides a flexible but compact way to model galaxy surface brightness profiles and is often used as input for dynamical modeling.
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Nuker profile¶
The Nuker profile is a broken power–law function used to describe the inner surface brightness distribution of galaxies, especially elliptical galaxies and bulges. It is defined by an inner slope, an outer slope, a break radius where the transition occurs, and a smoothness parameter that controls how sharp the transition is. The Nuker profile was introduced by Lauer et al. (1995) to fit the central light profiles of early–type galaxies observed with the Hubble Space Telescope.
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Structural Parameters¶
Bulge–to–total luminosity ratio (B/T)¶
The bulge–to–total luminosity ratio is the fraction of a galaxy’s total light that comes from the bulge compared to the sum of bulge and disk. It is commonly used to quantify galaxy morphology.
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Effective radius (Re)¶
The effective radius is the radius of a circular aperture that contains half of the total light of a galaxy or model component. It is a standard measure of galaxy size.
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Axis ratio¶
The axis ratio (b/a) is the ratio of the minor axis length (b) to the major axis length (a) of an ellipse that describes the projected shape of a galaxy component. Values near 1 correspond to nearly circular objects, while smaller values indicate more elongated shapes.
Break radius (Nuker function)¶
In the Nuker profile, the break radius is the scale at which the slope of the surface brightness profile changes from the inner power–law to the outer power–law regime.
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Gamma radius (Nuker function)¶
The gamma radius is defined as the radius where the negative logarithmic slope of the Nuker profile equals 0.5. It is used as a scale indicator for the transition between the core and outer regions.
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Slope of a function¶
The slope of a function is the rate at which the function changes with respect to its variable. In logarithmic surface brightness profiles, slopes describe how steeply brightness declines with radius.
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Kappa (κ)¶
In mathematics, κ (kappa) is commonly used to denote curvature. For a curve, curvature measures how quickly the direction of the tangent changes with position. In galaxy dynamics, κ often appears as the epicyclic frequency, describing the radial oscillations of stars in a disk around their guiding center orbit. See Wikipedia: Curvature for the mathematical definition.
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