Defining Models

PyImfit uses models which are instances of the ModelDescription class (or subclasses thereof).

A “model” is defined as a collection of “image functions”, grouped into one or more “function sets”. Each function set (a.k.a. “function block”) is a collection of one or more image functions with the same central coordinates (X0,Y0) within the image. (The SimpleModelDescription class is a subclass which holds just one function set.)

A ModelDescription object can be instantiated using a pre-existing Imfit configuration file; it can also be constructed programmatically within Python.

Image Functions

A list of the available image functions can be found in the module-level variable pyimfit.imageFunctionList, or by calling the function pyimfit.get_function_list(), and a dict containing lists of the parameter names for individual image functions can be found in pyimfit.imageFunctionDict (this dict can also be obtained by calling the function pyimfit.get_function_dict()). E.g.,

In [1]: pyimfit.imageFunctionDict['Exponential']
Out[1]: ['PA', 'ell', 'I_0', 'h']

Detailed descriptions of the individual image functions can be found in Chapter 6 of the Imfit manual (PDF), and background information on most can be found in Section 6 of Erwin (2015). (Note that the latter reference won’t include the more recent functions.)

The following is a brief list of the available image functions; see the Imfit manual for more details.

  • 2D image functions: Most of these have a position-angle parameter (“PA”) which defines their orientation on the image (measured in degrees counter-clockwise from the +x image axis). Many also have an ellipticity parameter (“ell”) defining their shape. The most common type of 2D image function has elliptical isophotes with a particular radial surface-brightness profile (e.g., BrokenExponential, Core-Sersic, Exponential, etc.).

    • BrokenExponential – Elliptical isophotes with a radial surface-brightness profile following a broken-exponential function. Geometric parameters: PA, ell

    • BrokenExponential2D – Isophotes for a perfectly edge-on disk, similar to “EdgeOnDisk” (below) but with a radial broken-exponential profile. Geometric parameters: PA

    • Core-Sersic – Elliptical isophotes with a Core-Sérsic (REFS) radial surface-brightness profile. Geometric parameters: PA, ell

    • EdgeOnDisk – The analytic form of an edge-on exponential disk (van der Kruit & Searle 1981), using the Bessel-function solution of van der Kruit & Searle (1981) for the radial profile and the generalized sech function of van der Kruit (1988) for the vertical profile. Geometric parameters: PA

    • EdgeOnRing – A simplistic model for an edge-on ring, using an off-center Gaussian for the radial profile and another Gaussian (with different sigma) for the vertical profile. Geometric parameters: PA

    • EdgeOnRing2Side – As for “EdgeOnRing”, but with the radial profile similar to that of “GaussianRing2Side” (asymmetric Gaussian). Geometric parameters: PA

    • Exponential – Elliptical isophotes with a radial surface-brightness profile following an exponential function. Geometric parameters: PA, ell

    • Exponential_GenEllipse – As for the “Exponential” function, but with isophotes having generalized ellipse shapes (boxy to disky). Geometric parameters: PA, ell, c0 (boxy/disk isophote-shape parameter)

    • FerrersBar2D – Isophotes (generalized elliptical shapes) for a 2D version of the Ferrers ellipsoid. Geometric parameters: PA, c0 (boxy/disk isophote-shape parameter)

    • FlatBar – An elongated structure with a broken-exponential profile along its major axis, suitable for the outer parts of (some) bars in massive disk galaxies; see Erwin et al. (2021) for more details and examples of use. Geometric parameters: PA, ell, deltaPA_max

    • FlatSky – Produces a constant background for the entire image.

    • Gaussian – Elliptical isophotes with a radial surface-brightness profile following a Gaussian function. Geometric parameters: PA, ell

    • GaussianRing – An elliptical ring with a radial Gaussian profile (peaking at the user-specified semi-major axis). Geometric parameters: PA, ell

    • GaussianRing2Side – As for “GaussianRing”, except that the ring profile is an asymmetric Gaussian, with different widths on the inner and outer sides. Geometric parameters: PA, ell

    • GaussianRingAz – As for “GaussianRing”, except that the surface brightness in the ring varies as a function of azimuth. Geometric parameters: PA, ell

    • ModifiedKing – Elliptical isophotes with a radial surface-brightness profile following the “modified King” function (Elson 1999; Peng et al. 2010), which is a generalization of the original King (1962) profile. Geometric parameters: PA, ell

    • ModifiedKing2 – Identical to “ModifiedKing”, except that the tidal/truncation radius parameter is replaced by a concentration parameter. Geometric parameters: PA, ell

    • Moffat – Elliptical isophotes with a radial surface-brightness profile following the Moffat profile. Geometric parameters: PA, ell

    • PointSource – This produces an interpolated, scaled copy of the user-suppled PSF image.

    • Sersic – Elliptical isophotes with a radial surface-brightness profile following the Sérsic function. Geometric parameters: PA, ell

    • Sersic_GenEllipse – As for the “Sersic” function, but with isophotes having generalized ellipse shapes (boxy to disky). Geometric parameters: PA, ell, c0 (boxy/disk isophote-shape parameter)

    • TiltedSkyPlane – Produces a background for the entire image in the form of ain inclined plane.

  • 3D image functions (luminosity-density functions): These generate a 2D image via line-of-sight integration through a 3D luminosity-density model, seen at arbitrary inclination.

    • ExponentialDisk3D – A disk model where the luminosity density follows a radial exponential profile and a vertical generalized sech (van der Kruit 1988) profile.

    • BrokenExponentialDisk3D – As for “ExponentialDisk3D”, but with the radial profile specified by a broken-exponential function.

    • FerrersBar3D – The classic Ferrers (1877) triaxial ellipsoid.

    • GaussianRing3D – A 3D ring, where the luminosity density follows a radial Gaussian profile and a vertical exponential profile.

More Information

See Chapters 5 and 6 of the Imfit manual (PDF)