Evidence for a large stellar bar in the Low Surface Brightness galaxy UGC 7321

Abstract

Late-type spiral galaxies are thought to be the dynamically simplest type of disk galaxy and our understanding of their properties plays a key role in galaxy formation and evolution scenarios. The low surface brightness (LSB) galaxy UGC 7321, a nearby, isolated, “superthin” edge-on galaxy, is an ideal object to study these purely disk-dominated bulge-less galaxies. Although late type spirals are believed to exhibit the simplest possible structure, even prior observations showed deviations from a pure single component exponential disk in the case of UGC 7321. We present for the first time photometric evidence for peanut-shaped outer isophotes from a deep optical ( R -band) image of UGC 7321. Observations and dynamical modeling suggest that boxy/peanut-shaped (b/p) bulges in general form through the buckling instability in bars of the parent galaxy disks. Together with recent HI observations supporting the presence of a stellar bar in UGC 7321, this could be the earliest known case of the buckling process during the evolutionary life of a LSB galaxy, whereby material in the disk-bar has started to be pumped up above the disk, but a genuine bulge has not yet formed.