Isophote shape analysis and the unfortunate subtlety of dwarf galaxy structure

TL;DR

This study evaluates the effectiveness of isophotal parameters in distinguishing dwarf galaxy structures, finding high self-similarity and limited discriminatory power in traditional morphological metrics, thus emphasizing the need for high-dimensional analysis in future surveys.

Contribution

It introduces and assesses isophotal parameters for dwarf galaxy classification, revealing their limited power and highlighting the necessity of high-dimensional statistical methods for understanding dwarf galaxy evolution.

Findings

Dwarf spirals are structurally simpler than massive spirals.

Dwarf and massive ETGs show isophotal similarity, suggesting triaxiality.

No clear population separation in PCA or clustering analyses.

Abstract

Dwarf galaxies ($M_{*}/M_{\odot} \lesssim 10^{9.5}$), being sensitive to key evolutionary drivers like baryonic feedback and tidal perturbation, are crucial for understanding galaxy evolution as a whole. Their abundance and faintness, however, ensures that most will be studied primarily via broadband imaging for the foreseeable future. It is thus crucial to identify the most informative broadband-derivable quantities in the dwarf regime. As studies of widely used morphological parameters like concentration, asymmetry, and smoothness suggest these lack discriminatory power among dwarfs, we assess alternatives derived from isophotes: position angle twists, ellipticity, deviations from pure ellipses, and residuals to single-S\'{e}rsic profile fits. Using these parameters, we compare dwarf populations with massive galaxies of the same morphological class, and among themselves by morphological class. Only dwarf spirals may differ from their massive counterparts, being structurally simpler; dwarf and massive early type galaxy (ETG) isophotal similarity suggests all dwarf ETGs may be triaxial. Among only dwarfs, morphological classes are indistinguishable in this parameter space. A principal component analysis (PCA) using all available morphological, isophotal, and physical parameters expands on this: no PC explains more than $\sim$26% of the population variance, and no clear multimodality appears in any pairwise PC projection. We find similarly moderate spectral clustering, with a silhouette score of only 0.35. Given this self-similarity, parsing dwarf galaxy evolution from photometric parameters alone will likely require detailed statistical analysis of large dwarf populations in a high-dimensional parameter space, a task suitable for up-coming large-scale surveys like the Legacy Survey of Space and Time.