Bar-Driven Disk Evolution: Grand Design Nuclear Spirals

Abstract

Our study of the grand-design spiral galaxy NGC 5248 reveals that the feature previously thought to be an inclined disk is in fact an extended bar or semi-major axis 7.1 kpc, embedded within a fainter outer disk which is visible out to a radius of 17.2 kpc. NGC 5248 provides a classic demonstration of how an extended large-scale stellar bar embedded within a faint outer disk can be missed if imaging studies lack the sensitivity detect the outer disk. This effect may well lead high redshift studies to underestimate the fraction of barred galaxies. Through multi-wavelength observations and hydrodynamical modeling, we also demonstrate that the grand-design nuclear spiral on scales of 75-225 pc in NGC 5248 forms part of an exyended grand-design spiral which can be traced from 8 kpc down to 80 pc and appears to be driven by the large-scale stellar bar. This study suggests that grand-design spirals on scales of several 10-100 pc, which are increasingly common in HST images of galaxies, can be generated by bar-driven gaseous spiral density waves which propagate inside the OILR. This propagation is particularly effective when the central mass concentration is low. Conversely, an existing large central mass concentration favors other bar-driven gas transport mechanisms such as dynamically decoupled secondary nuclear bars.