Dwarf early-type galaxies (ETGs) display a rich diversity in their photometric, structural, and dynamical properties. In this work, we address their structural complexity by studying with deep imaging a sample of nine dwarf ETGs from the Virgo galaxy cluster, characterized by having faint disk features such as bars and spiral arms, that lie mostly hidden within the bright diffuse light of the galaxies. We present a new, robust method that aims to identify and extract the disk substructure embedded in these dwarf ETGs. The method consists in an iterative procedure that gradually separates a galaxy image into two components: the bright, dominant, diffuse component, and the much fainter, underlying disk component. By applying it to the dwarf ETG sample, we quantify their disk substructure and find that its relative contribution to the total galaxy light ranges between 2.2% and 6.4% within two effective radii. We test the reliability of the method, and prove that it is accurate in recovering the disk substructure we introduce in mock galaxy images, even at low disk-to-total light fractions of a few percent. As a potential application of the method, we perform a Fourier analysis on the extracted disk substructures and measure the orientation, length, and strength of the bars, as well as the pitch angle and strength of the spiral arms. We also briefly discuss a scenario based on the numerical simulations presented in our companion paper, Brought to Light II: Smith et al., in which we investigate the origins of the substructure in such dwarf systems.

In our companion paper (Brought to Light I: Michea et al.), we reveal spectacular spiral-galaxy-like features in deep optical imaging of nine Virgo early-type dwarf galaxies, hidden beneath a dominating smooth stellar disk. Using a new combination of approaches, we find that bar- and spiral-like features contribute 2.2%–6.4% of the total flux within 2 R eff. In this study, we conduct high-resolution simulations of cluster harassment of passive dwarf galaxies. Following close pericenter passages of the cluster core, tidal triggering generates features in our model disks that bear a striking resemblance to the observed features. However, we find the disks must be highly rotationally supported (V peak/σ 0 ∼ 3), much higher than typically observed. We propose that some early-type dwarfs may contain a few percent of their mass in a cold, thin disk that is buried in the light of a hot, diffuse disk and only revealed when they undergo tidal triggering. The red optical colors of our sample do not indicate any recent significant star formation, and our simulations show that very plunging pericenter passages (r peri < 0.25r vir) are required for tidal triggering. Thus, many cluster early-type dwarfs with less-plunging orbits may host a yet-undetected cold stellar disk component. We discuss possible origin scenarios and consider why similar-mass star-forming galaxies in the field are significantly more thin-disk dominated than in our cluster sample.

It has been well established that dwarf early-type galaxies (ETGs) can often exhibit a complex morphology, whereby faint spiral arms, bars, edge-on disks, or clumps are embedded in their main, brighter diffuse body. In our first paper (“Brought to Light I”), we developed a new method for robustly identifying and extracting substructures in deep imaging data of dwarf ETGs in the Virgo galaxy cluster. Here we apply our method to a sample of 23 dwarf ETGs in the Fornax galaxy cluster, out of which 9 have disk-like and 14 have clump-like substructures. According to Fornax Deep Survey (FDS) data, our sample constitutes 12% of all dwarf ETGs in Fornax brighter than M r = − 13 mag, and contains all cases that unequivocally exhibit substructure features. We use g- and r-band FDS images to measure the relative contribution of the substructures to the total galaxy light and to estimate their g − r colors. We find that the substructures typically contribute 8.7% and 5.3% of the total galaxy light in the g and r bands, respectively, within two effective radii. Disk substructures are usually found in dwarf ETGs with redder global colors, and they can be either as red as or bluer than their galaxy’s diffuse component. In contrast, the clump substructures are found in comparatively bluer dwarf ETGs, and they are always bluer than their galaxy’s diffuse component. These results provide further evidence that dwarf ETGs can hide diverse complex substructures, with stellar populations that can greatly differ from those of the dominant diffuse light in which they are embedded.