ELLIPSECT: A surface brightness analysis tool for GALFIT 3

EllipSect is a user-friendly analysis and measurement tool, implemented in Python, that operates on the imaging data together with the output of the widely used 2D surface-brightness fitting code GALFIT 3. It produces publication-quality figures and exportable data products to enable quantitative assessment of GALFIT 3 models and their individual components. In addition, EllipSect computes non-parametric measurements that are not provided by GALFIT 3, including the total effective radius resulting from multi-component fits, cusp radius, and the Petrosian radius. This paper provides examples and a quick guide for EllipSect.

💡 Research Summary

EllipSect is a new Python‑based post‑processing tool designed to work directly with the output of the widely used 2‑D surface‑brightness fitting code GALFIT 3. GALFIT 3 excels at multi‑component modelling of galaxies but lacks built‑in utilities for visual comparison, non‑parametric measurements, and convenient data export. Historically, users have relied on IRAF’s ellipse task or other external codes, which require cumbersome format conversion and are no longer easily accessible. EllipSect fills this gap by ingesting the FITS data cube (original image, model, residual) and the parameter file (galfit.nn) generated by GALFIT 3, eliminating any need for manual translation.

The core algorithm adopts the sectors_photometry routine from the mgefit library. It divides the galaxy image into angular sectors around the centre, using the ellipticity and position angle of the last GALFIT component (typically an exponential disk) as defaults, though all parameters are user‑configurable via command‑line options. Within each sector the code expands radially until a user‑defined minimum count level (default zero) is reached, then computes the mean surface‑brightness at each radius. Counts are converted to calibrated magnitudes using the standard relation
μ_I = zp_t + 5 log(PlateScale) + 2.5 log(ExpTime) – 2.5 log(C₀) + 0.1,
where the final term corrects for infinite aperture. This yields high‑quality 1‑D surface‑brightness profiles along the major axis and at several azimuthal angles.

Beyond the profiles, EllipSect provides a suite of non‑parametric quantities that GALFIT 3 does not output:

• Total effective radius (r_te) – calculated by summing the flux of all model components, finding the radius that encloses half of the total light via a bisection method. This supersedes the common practice of using the effective radius of a single Sérsic component as a proxy for the whole galaxy.
• Petrosian, Kron, and 90 % light radii – derived directly from the combined model.
• Bumpiness (B) – RMS deviation of the observed image from the smooth model within 2 r_e, useful for morphological classification (Blakeslee et al. 2006).
• Tidal parameter (T_gal) – quantifies residual structure by comparing observed and model pixel values, following Tal et al. (2009).
• AIC and BIC (including a version based on resolution elements) – statistical criteria that penalise extra free parameters, aiding model selection.
• Component‑wise light fractions, absolute magnitudes, luminosities, and sky‑subtracted residual statistics.

Sky background estimation, a critical source of systematic error, is handled by two independent methods. The “sky gradient” method follows Barden et al. (2012): concentric elliptical rings are constructed using the same geometry as the sectors, mean sky values are measured in each ring, extreme values are trimmed, and the gradient is followed outward until it becomes positive for the second time, at which point the sky level is adopted. The “random box” method places many randomly positioned boxes across the image and computes a robust median sky. Both methods respect user‑supplied masks.

All results are saved as (1) publication‑quality PNG images showing the galaxy, model, and residual side‑by‑side, (2) 1‑D surface‑brightness plots (major‑axis and multi‑azimuthal), and (3) a CSV file containing every derived parameter. Users can request component‑wise profiles with the –comp flag, or they can export the raw data for custom visualisation in other libraries. EllipSect also queries the NASA/IPAC Extragalactic Database (NED) via curl to retrieve distance modulus, Galactic extinction, and angular‑to‑physical conversion factors, automatically converting apparent magnitudes to absolute magnitudes and luminosities (k‑corrections are omitted and must be supplied by the user).

The paper details the software architecture, algorithmic flow, and provides real‑world examples, including a sky‑gradient analysis of the brightest cluster galaxy in Abell 2029. The authors demonstrate that EllipSect streamlines the post‑fit workflow, reduces human error, and adds scientifically valuable diagnostics that are otherwise unavailable or labor‑intensive. Planned future extensions include multi‑band simultaneous analysis, automated k‑correction using spectral templates, and GPU‑accelerated processing for large imaging surveys. In summary, EllipSect substantially augments GALFIT 3 by delivering a seamless, reproducible, and feature‑rich environment for surface‑brightness analysis, making it a valuable asset for both individual galaxy studies and large‑scale survey pipelines.