When is a Bulge Not a Bulge? Revealing the Satellite Nature of NGC 5474’s Bulge

Abstract

A satellite galaxy of the nearby spiral M101, NGC 5474 has a prominent bulge offset from the kinematic center of the underlying star-forming disk that has gained attention in recent years. Recent studies have proposed that this putative offset bulge is not a classical bulge within the plane of the disk but instead a dwarf companion galaxy along the line of sight. Using integral field spectroscopy data taken as part of the PPak IFS Nearby Galaxies Survey (PINGS), we perform the first analysis of the stellar and gas kinematics of this putative bulge (PB) and portions of the disk. We find a radial velocity offset of ∼24 km s ^−1 between the emission lines produced by the disk H ii regions and the absorption lines produced by the PB stellar component. We interpret this velocity offset as evidence that the PB and disk are two separate objects, the former orbiting around the latter, supporting simulations and observations of this peculiar system. We attempt to place this external companion into the context of the M101 Group and the M101-NGC 5474 interaction.

Keywords

• Galaxy interactions
• Galaxy evolution
• Galaxy kinematics
• Galaxy spectroscopy