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.
Highlights
Performing studies of the internal or external factors that cause galaxies to evolve is predicated on the availability of statistically significant numbers of target galaxies that exhibit resolvable, implicative signs of these processes
The main approach to detecting faint features in the outer regions of galaxies is through visual classification (e.g. Sandage 2005, and references therein)
In which we look at features mostly outside the main galaxy bodies or at the edges of them, the effects of dust are generally thought to be less important than closer to the centre of galaxies, but some of the features that we classify, such as shells, polar rings, or even warps, may be blocked from view at least partially at visible light wavelengths
Summary
Performing studies of the internal or external factors that cause galaxies to evolve is predicated on the availability of statistically significant numbers of target galaxies that exhibit resolvable, implicative signs of these processes. One way to do this is by observing a large sample of nearby galaxies and searching for faint features that exist at or outside their outer ‘edges’ (at or outside the 25 mag arcsec−2 B-band isophotes; ‘R25’). Gas. The main approach to detecting faint features in the outer regions of galaxies is through visual classification In which we look at features mostly outside the main galaxy bodies or at the edges of them, the effects of dust are generally thought to be less important than closer to the centre of galaxies, but some of the features that we classify, such as shells, polar rings, or even warps, may be blocked from view at least partially at visible light wavelengths. Early-type galaxies may not have such obvious signs of interaction
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