Stellar Tidal Streams in Spiral Galaxies of the Local Volume: A Pilot Survey with Modest Aperture Telescopes

TL;DR

This pilot survey used modest telescopes to detect diverse, faint stellar tidal features around isolated spiral galaxies, providing new observational evidence supporting galaxy formation models involving minor mergers.

Contribution

First ultra deep imaging survey of nearby spiral galaxies revealing extensive tidal features, confirming their prevalence and diversity in the Local Volume.

Findings

Discovered six new stellar structures in galaxy halos.

Confirmed and clarified large stellar over-densities as tidal streams.

Observed diverse morphologies matching cosmological simulations.

Abstract

[Abridged] Within the hierarchical framework for galaxy formation, minor merging and tidal interactions are expected to shape all large galaxies to the present day. As a consequence, most seemingly normal disk galaxies should be surrounded by spatially extended stellar 'tidal features' of low surface brightness. As part of a pilot survey for such interaction signatures, we have carried out ultra deep, wide field imaging of 8 isolated spiral galaxies in the Local Volume, with data taken at small (D=0.1-0.5m) robotic telescopes that provide exquisite surface brightness sensitivity (mu_V)~28.5$ mag/arcsec^2). This initial observational effort has led to the discovery of six previously undetected extensive (to ~30 kpc) stellar structures in the halos surrounding these galaxies, likely debris from tidally disrupted satellites. In addition, we confirm and clarify several enormous stellar over-densities previously reported in the literature, but never before interpreted as tidal streams. Even this pilot sample of galaxies exhibits strikingly diverse morphological characteristics of these extended stellar features: great circle-like features that resemble the Sagittarius stream surrounding the Milky Way, remote shells and giant clouds of presumed tidal debris far beyond the main stelar body, as well as jet-like features emerging from galactic disks. A qualitative comparison with available simulations set in a Lambda-Cold Dark Matter cosmology shows that the extraordinary variety of stellar morphologies detected in this pilot survey matches that seen in those simulations. The common existence of these tidal features around 'normal' disk galaxies and the morphological match to the simulations constitutes new evidence that these theoretical models also apply to a large number of other Milky Way-mass disk galaxies in the Local Volume.