Isophote shape analysis and the unfortunate subtlety of dwarf galaxy structure

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-Sérsic 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.

Figures (10)

• Figure 1: Isophotal twistiness vs. stellar mass for the lazar24b dwarfs (black squares) and CS$^{4}$G galaxies (grey points), split by morphology as (from top-left to bottom-right): elliptical, lenticular, spiral, and featureless. The red dashed line and orange filled regions denote the mean and standard deviation for the lowest measurable value of $T$ as estimated using mock Sérsic profile injections (see text). The top-left of each panel also shows the Pearson r correlation coefficients and associated p-values for all galaxies in each morphological class. The inset in the bottom-right panel shows a zoom-in on the F dwarfs trend.
• Figure 2: Left: median values of twistiness for each morphological class. Black squares denote the lazar24b dwarfs, while grey points denote the CS$^{4}$G galaxies. Errorbars are bootstrapped errors on the medians. Right: histograms of $\log(T)$ for each morphology class between the two samples, with dwarfs shown in blue and CS$^{4}$G galaxies shown in gold.
• Figure 3: As Fig. \ref{['fig:twist1']}, but showing the mean residuals between the measured surface brightness profiles and the best-fit single-Sérsic index profiles. Stronger residuals imply the profiles require more components to fit properly. The inset in the bottom-right panel shows a zoom-in on the F dwarfs trend.
• Figure 4: Histograms of the maximum deviation in $a_{4}/a$ from zero, split by morphological class. Blue histograms denote the lazar24b dwarfs, while gold histograms denote the CS$^{4}$G galaxies. Negative values of $a_{4}/a$ denote boxy isophotes, while positive values denote disky or eye-shaped isophotes. Typical uncertainty is $a_{4}/a\times100 \approx1$ within the measurement bounds (see text).
• Figure 5: Luminosity-weighted mean ellipticity vs. the maximum measured isophote twist for the lazar24b dwarfs (left panel) and CS$^{4}$G galaxies (right panel). Maximum twist is defined as the maximum difference between each measured position angle and the luminosity-weighted mean. Dwarf ETGs (circles, squares, and plus-signs for E, S0, and F, respectively) are shown in varying shades of gold, while spirals are shown as blue triangles. Massive ETGs and spirals are shown as brown and blue points, respectively, following the same symbol scheme. We show a representative error bar in the left panel at $(0.5,50)$ equal to $(\sigma_{\epsilon}\sim0.03,\sigma_{\theta}\sim0.6^{\circ})$, derived from mock Sérsic profile injections (see text).