Imfit: A Fast, Flexible New Program for Astronomical Image Fitting

Abstract

I describe a new, open-source astronomical image-fitting program called Imfit, specialized for galaxies but potentially useful for other sources, which is fast, flexible, and highly extensible. A key characteristic of the program is an object-oriented design which allows new types of image components (2D surface-brightness functions) to be easily written and added to the program. Image functions provided with Imfit include the usual suspects for galaxy decompositions (Sersic, exponential, Gaussian), along with Core-Sersic and broken-exponential profiles, elliptical rings, and three components which perform line-of-sight integration through 3D luminosity-density models of disks and rings seen at arbitrary inclinations. Available minimization algorithms include Levenberg-Marquardt, Nelder-Mead simplex, and Differential Evolution, allowing trade-offs between speed and decreased sensitivity to local minima in the fit landscape. Minimization can be done using the standard chi^2 statistic (using either data or model values to estimate per-pixel Gaussian errors, or else user-supplied error images) or Poisson-based maximum-likelihood statistics; the latter approach is particularly appropriate for cases of Poisson data in the low-count regime. I show that fitting low-S/N galaxy images using chi^2 minimization and individual-pixel Gaussian uncertainties can lead to significant biases in fitted parameter values, which are avoided if a Poisson-based statistic is used; this is true even when Gaussian read noise is present.

pdflatex, 27 pages, 19 figures. Revised version, accepted by ApJ. Programs, source code, and documentation available at: http://www.mpe.mpg.de/~erwin/code/imfit/

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Cited by in corpus (8)

• CALIFA, the Calar Alto Legacy Integral Field Area survey. III. Second public data release
• Two-dimensional multi-component photometric decomposition of CALIFA galaxies
• Galaxy And Mass Assembly (GAMA): ${\mathcal{M}}_{\star }-R_{\mathrm{e}}$ relations of $z=0$ bulges, discs and spheroids
• ProFit: Bayesian Profile Fitting of Galaxy Images
• The nearest ultra diffuse galaxy: UGC2162
• Galaxy And Mass Assembly (GAMA): the Stellar Mass Budget of Galaxy Spheroids and Disks
• Galaxies with conspicuous optical warps
• Testing the presence of multiple photometric components in nearby early-type galaxies using SDSS