Brought to Light II: Revealing the Origins of Cloaked Spiral Features in Cluster Passive Dwarf Galaxies

TL;DR

This paper investigates the origins of spiral features in passive dwarf galaxies within clusters, proposing that hidden cold stellar disks revealed by tidal interactions explain observed structures, supported by high-resolution simulations.

Contribution

It introduces a new simulation approach showing that tidal triggering can reveal cold disks in dwarf galaxies, explaining observed spiral features in cluster environments.

Findings

Tidal interactions can generate spiral features resembling observations.

Dwarf galaxies may host hidden cold stellar disks.

High rotational support is necessary for tidal features to appear.

Abstract

In our companion paper (Brought to Light I: Michea et al. 2021), 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$_{\rm{eff}}$. 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$_{\rm{peak}}/\sigma_0 \sim 3$), 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 which 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$_{\rm{peri}}<0.25$r$_{\rm{vir}}$) 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.