When Is a Bulge Not a Bulge? Revealing the Satellite Nature of NGC 5474's Bulge

TL;DR

This study uses integral field spectroscopy to analyze NGC 5474's bulge, revealing it is likely a satellite galaxy rather than a classical bulge within the disk, based on stellar and gas kinematics.

Contribution

First detailed kinematic analysis showing NGC 5474's bulge is a separate satellite galaxy, not a classical bulge within the disk.

Findings

Radial velocity offset of ~24 km/s between disk and bulge components

Evidence supporting the bulge as a satellite galaxy orbiting the disk

Implications for understanding galaxy interactions in the M101 group

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 and portions of the disk. We find a radial velocity offset of ~24 km/s between the emission lines produced by the disk HII regions and the absorption lines produced by the putative bulge stellar component. We interpret this velocity offset as evidence that the putative bulge 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.