The evidence at present available points strongly to the conclusion that the spirals are individual galaxies, or island universes, comparable with our own galaxy in dimensions and in number of component units. – H.D. Curtis
... the extraplanar gas seems to consist of two parts: a large one from galactic fountains and a smaller part accreted from intergalactic space. There is direct (HVCs in our galaxy and filaments in external galaxies) and indirect (rotational velocity gradients) evidence for the accretion from outside. — Sancisi et al. 2008
Galaxies are like people. Every time you get to know one well, it turns out to be a little peculiar. – Sidney van den Bergh
Unlike the days of the Island Universe, when galaxies floated in solitary splendor on Hubble's Tuning Fork (Hubble, 1958), today's galaxies are a mess (Fig. 1). Evidence for the growth and evolution of galaxies by the capture of stellar systems is everywhere and there are arguments for continued accretion of gas as well. Certainly some gas will arrive with the small stellar systems that large galaxies devour, but how does it get to the disk, and is there neutral gas accreting from other sources? In this article I consider the evidence for accretion of neutral gas onto nearby galaxies, especially gas that is not associated with stars. This is not a comprehensive review, but instead explores the connection between gas likely to be accreting onto galaxies in the Local Group, where we can examine it with high sensitivity and linear resolution, and that seen in more distant systems with a better vantage but considerably less sensitivity. Is there local accretion of starless gas, and what form does it take?
Figure 1. Examples of stellar accretion in two galaxies from Carlin et al., (2016). Images on the left from the Sloan Digital Sky Survey of the nearby galaxies NGC 4013 (top) and M63 (bottom) show regular disks while the much deeper images on the right reveal streams of stars accreted from smaller galaxies.
Two comprehensive reviews are directly relevant to this topic. Sancisi et al. Sancisi et al., (2008) consider the evidence for accretion of neutral gas by analyzing high resolution H I maps of several dozen nearby galaxies. They find ample evidence of kinematic anomalies, tails and filaments, warps, lopsided disks, and interaction. They adopt the stance that any significant deviation of a galaxy's H I from symmetry is evidence for interaction, and that interaction implies accretion. Accretion also has a prominent place in the recent review of gaseous galaxy halos by Putman et al., (2012) which takes a thorough look at circumgalactic gas in all phases. I will refer to these reviews throughout this article for more complete discussion of some topics.
The volume around galactic disks contains material in many forms. Both the Milky Way (MW) and M31 have a hot 106−7 K circumgalactic medium (CGM) of enormous mass and extent, possibly dwarfing the baryon content of the disks themselves (Anderson & Bregman, 2011; Hodges-Kluck et al., 2016; Lehner et al., 2015). This gas may cool and condense, feeding the disk. There may also be cosmological accretion – cold flows – which may have to traverse the CGM to reach the disk and in that passage may be disrupted entirely (Putman et al., 2012). Then there is gas that has been stripped from one galaxy through interaction and ultimately ends up in the disk of another. Add to that the ejection of gas from a galaxy's disk from supernovae or a nuclear wind, and the processes can become quite difficult to disentangle. Considerable insight into the the CGM of galaxies has come through studies of Lyα and MgII absorption lines (Kacprzak et al., 2013; Wakker & Savage, 2009), but those data will not be discussed here. Instead, we will concentrate on 21cm H I observations, and work from the inside out, from the interstellar medium (ISM) disk-halo interface, to high velocity clouds (HVCs), to the products of interaction between galaxies, and finally to H I "clouds" that don't easily fit into any of these categories.
But first a note on the detectability of H I. Filled-aperture (single dish) radio telescopes can easily detect H I column densities of NHI = 1018 cm−2, and with a little work, 1017 cm−2. This comes at the expense of angular resolution, which for modern instruments is in the range 3′ − 10′. Hydrogen clouds with MHI ≥ 105 M⊙ can thus be detected anywhere in the Local Group at a linear resolution of 1–3 pc dkpc. Aperture synthesis instruments provide much higher angular resolution of < 1′ but at the cost of reduced sensitivity. The smallest object in the table of plumes, wings, and other peculiar H I structures of Sancisi et al. (2008) has 108 M⊙. The deepest aperture synthesis H I observations of galaxies so far have come from the HALOGAS survey (Heald et al., 2011) which reaches a limiting sensitivity of NHI = 1018.5−19.0 cm−2.
This sensitivity gap is not unbridgeable, but we should keep it in mind when comparing local with distant objects.