Discovery of a Giant Stellar Tidal Stream Around the Disk Galaxy NGC 4013

TL;DR

A giant stellar tidal stream was discovered around galaxy NGC 4013, indicating a recent minor merger and revealing complex accretion signatures in seemingly undisturbed disk galaxies.

Contribution

This is the first detailed detection and analysis of a stellar tidal stream around NGC 4013, linking it to a minor merger event and expanding understanding of galaxy evolution.

Findings

Detected a low surface-brightness stellar stream extending 26 kpc from NGC 4013

The stream's morphology matches predictions for a tidal stream from a dwarf galaxy

Estimated the progenitor galaxy's initial mass as 6×10^8 solar masses

Abstract

We report the discovery of a giant, loop-like stellar structure around the edge-on spiral galaxy NGC 4013. This arcing feature extends 6 arcmin (~26 kpc in projected distance) northeast from the center and 3 arcmin (~=12 kpc) from the disk plane; likely related features are also apparent on the southwest side of the disk, extending to 4 arcmin (~17 kpc). The detection of this low surface-brightness muR= 27.0+0.3-0.2 mag/sqarcsec) structure is independently confirmed in three separate datasets from three different telescopes. Although its true three dimensional geometry is unknown, the sky- projected morphology of this structure displays a match with the theoretical predictions for the edge-on, projected view of a stellar tidal streams of a dwarf satellite moving in a low inclined (~25deg), nearly circular orbit. Using the recent model of the Monoceros tidal stream in the Milky Way by Penarrubia et al. as template, we find that the progenitor system may have been a galaxy with an initial mass 6*10^8 Msun, of which current position and final fate is unknown. According to this simulation, the tidal stream may be approximately ~2.8 Gyr of age. Our results demonstrate that NGC 4013, previously considered a prototypical isolated disk galaxy in spite of having one of the most prominent HI warps detected thus far, may have in fact suffered a recent minor merger. This discovery highlights that undisturbed disks at high surface brightness levels in the optical but warped in HI maps may in fact reveal complex signatures of recent accretion events in deep photometric surveys.