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) `__