[Paper Review] Structural classification of galaxies in clusters

S. Andreon, E. Davoust|arXiv (Cornell University)|Oct 29, 1996

Astronomy and Astrophysical Research2 references3 citations

TL;DR

This paper proposes a quantitative structural classification method for galaxies in clusters based on isophotal analysis of luminosity, ellipticity, and shape deviations, offering higher reproducibility than traditional visual morphological classification. It demonstrates 15–20% agreement with traditional morphologists on the Coma cluster sample and shows superior consistency, especially for faint or feature-rich galaxies often missed visually.

ABSTRACT

The traditional method of morphological classification, by visual inspection of images of uniform quality and by reference to standards for each type, is critically examined. The rate of agreement among traditional morphologists on the morphological type of galaxies is estimated from published classification works, and is estimated at about 20 %, when galaxies are classified into three bins (E, S0, S+Irr). The advantages of the quantitative method of structural classification for classifying galaxies in clusters are outlined. This method is based on the isophotal analysis of galaxy images, and on the examination of quantitative structural parameters derived from this analysis, such as the profiles of luminosity, ellipticity and deviations from ellipticity of the galaxy. The structural and traditional methods are compared on a complete sample of 190 galaxies in the Coma cluster. The morphological types derived by both methods agree to within 15 or 20 %, the same rate as among traditional morphologists alone, thus showing that our morphological classes do coincide with the traditional ones. The galaxies with discrepant types are mostly faint (mag_B > 16.0), or have features typical of spirals, but which have not been detected, noticed or taken into account by traditional morphologists. The rate of agreement is also good for galaxies in a distant cluster (z ~0.4). The structural method, gives highly reproducible results, never reached by traditional estimations of the morphological type. Thus, the morphological types obtained with this method should be preferred. The advantages of the method are further demonstrated by new results on the properties of galaxies in clusters.

Motivation & Objective

• To address the low reproducibility and agreement rate in traditional visual morphological classification of galaxies.
• To develop a quantitative, objective method for classifying galaxies in clusters based on measurable structural parameters.
• To compare the new structural classification method with traditional morphological classification on a complete galaxy sample.
• To evaluate the performance of the structural method on both nearby (Coma cluster) and distant (z ~ 0.4) galaxy clusters.
• To demonstrate that the structural method yields more consistent and reliable morphological classifications than visual inspection alone.

Proposed method

• Perform isophotal analysis on galaxy images to extract structural parameters such as luminosity profiles, ellipticity, and deviations from elliptical shape.
• Use quantitative measures of galaxy structure—luminosity distribution, shape, and isophotal distortions—as the basis for classification.
• Classify galaxies into morphological types (E, S0, S+Irr) based on these derived structural parameters rather than visual inspection.
• Compare the results of the structural classification with traditional morphological classifications from published works.
• Apply the method to a complete sample of 190 galaxies in the Coma cluster and validate it on a distant cluster at redshift z ~ 0.4.
• Assess agreement rates between structural and traditional classifications using statistical comparison across morphological bins.

Experimental results

Research questions

• RQ1 How does the agreement rate between traditional morphologists compare when classifying galaxies into three morphological types (E, S0, S+Irr)?
• RQ2 To what extent does the structural classification method reproduce traditional morphological types on a complete galaxy sample?
• RQ3 What factors contribute to discrepancies between structural and traditional morphological classifications?
• RQ4 How reproducible is the structural classification method compared to traditional visual classification?
• RQ5 Does the structural method maintain high agreement with traditional classification in distant clusters (z ~ 0.4) as in nearby clusters?

Key findings

• The agreement rate among traditional morphologists on galaxy morphological types (E, S0, S+Irr) is approximately 20% when classifying galaxies into three bins.
• The structural classification method achieves 15–20% agreement with traditional morphological classifications on the Coma cluster sample, matching the level of agreement among traditional morphologists.
• Discrepancies between structural and traditional classifications are primarily due to faint galaxies (mag_B > 16.0) or spiral features not detected or ignored in visual inspection.
• The structural method produces highly reproducible results, a level of consistency not achieved by traditional visual classification methods.
• The method performs well on distant clusters (z ~ 0.4), maintaining strong agreement with traditional classifications despite observational challenges.
• The structural method is recommended over traditional classification due to its superior reproducibility and objectivity, especially for complex or faint galaxies.