IRAC Near-Infrared Features in the Outer Parts of S4G Galaxies

TL;DR

This paper catalogs faint outer features in nearby galaxies using Spitzer IR data, revealing asymmetries and signs of interactions, and compares observations with cosmological simulations to understand galaxy evolution.

Contribution

It provides a new catalog of outer galaxy features at 3.6 microns and compares observed asymmetries with simulated galaxies to explore their origins.

Findings

22% of galaxies show outer asymmetries.

Simulations exhibit a higher asymmetry fraction (33%), possibly due to selection effects.

Outer asymmetries are not strongly linked to galaxy interactions or companions.

Abstract

We present a catalogue and images of visually detected features, such as asymmetries, extensions, warps, shells, tidal tails, polar rings, and obvious signs of mergers or interactions, in the faint outer regions (at and outside of R_25) of nearby galaxies. This catalogue can be used in future quantitative studies that examine galaxy evolution due to internal and external factors. We are able to reliably detect outer region features down to a brightness level of 0.03 MJy/sr per pixel at 3.6 microns in the Spitzer Survey of Stellar Structure in Galaxies (S4G). We also tabulate companion galaxies. We find asymmetries in the outer isophotes in 22+/-1 per cent of the sample. The asymmetry fraction does not correlate with galaxy classification as an interacting galaxy or merger remnant, or with the presence of companions. We also compare the detected features to similar features in galaxies taken from cosmological zoom re-simulations. The simulated images have a higher fraction (33 per cent) of outer disc asymmetries, which may be due to selection effects and an uncertain star formation threshold in the models. The asymmetries may have either an internal (e.g., lopsidedness due to dark halo asymmetry) or external origin.