The Structure and Content of Galaxy Outskirts

The outer regions of galaxies are expected to contain important clues about the way in which galaxies are assembled. Although quantitative study of these parts has been severely limited in the past, breakthroughs are now being made thanks to the combination of wide-area star counts, deep HST imagery and 8-m class spectroscopy. I review here several recent results concerning substructure, star clusters and stellar halos in the outer regions of our nearest large neighbours, M31 and M33. 1. The Outskirts of Galaxies: Motivation and History The study of the faint outskirts of galaxies has become increasingly important in recent years. From a theoretical perspective, it has been realised that many important clues about the galaxy assembly process should lie buried in these parts. Cosmological simulations of disk galaxy formation incorporating baryons now yield predictions for the large-scale structure and stellar content at large radii – for example, the abundance and nature of stellar substructure and the ubiquity, structure and content of stellar halos and thick disks. These models generally predict a wealth of (sub)structure at levels of μV ∼ 30 mag/⊓⊔ ′′ and fainter (e.g. Bullock & Johnson 2005); their verification thus requires imagery and spectroscopy of galaxies to ultra-faint surface brightness levels. Since Malin first applied his photographic stacking and amplification technique (e.g. Malin et al. 1983), it has been known that some galaxies possess unusual low surface brightness (LSB) structures – shells, loops, asymmetric envelopes – in their outer regions. Although limited to μB ∼< 28 mag/⊓⊔ , these images were sufficient to demonstrate that even the most “normal” nearby galaxies could become very abnormal when viewed at faint light levels (e.g. Weil et al 1997). Follow-up study of Malin’s LSB features, and several more recentlydiscovered examples (e.g. Shang et al. 1999), has been severely limited due to the technical difficulties associated with detecting and quantifying diffuse light at surface brightnesses ∼ 10 magnitudes below sky. The currently most viable technique to probe the very low surface brightness regions of galaxies is that of wide-area resolved star counts. I review here recent results from studies of the resolved populations in the outer regions of M31 and M33, focusing on substructure, star clusters and stellar halos.