One of the Key Science Questions for the Square Kilometre Array (SKA) is "How do galaxies assemble and evolve?” The SKA will be able to directly trace the gradual, global transformation from primordial neutral hydrogen (HI) gas into galaxies over cosmic time. However, direct detailed observations of the sub-kpc-scale physical processes that cause this transformation, taking place both in and around these evolving galaxies, can best be made in the nearby universe; it is the only place where we can study, in detail, the “Galactic Ecosystem”: the flow of gas into galaxies, its physical conditions, its transformation into stars, and how it, in turn, is affected by feedback from these stars. The HI kinematics tell us about the distribution of dark matter, angular momentum, the shape of the halo potential, the disk gravitational potential, and, ultimately, how the dark and visible matter together determine and regulate the evolution of galaxies. 

These local galaxies are the “fossil records” of the distant, high-redshift galaxies, and provide a wealth of information that can further refine models of galaxy formation and evolution. This knowledge can guide the interpretation of similar, but necessarily less detailed, observations with the SKA and MeerKAT at higher redshifts. The study of nearby galaxies thus provides the foundations on which studies of higher redshift galaxies must be built.

MeerKAT is the ideal instrument for this kind of study. The simultaneous combination of exquisite column density sensitivity, high spatial resolution and a large primary beam make it possible to efficiently and comprehensively survey galaxy evolution in our nearby universe. In particular, studying the elusive gas component in the outer disk, with column densities some two orders of magnitude lower than in the star-forming part of the galaxy, will yield clues on gas flows in and out of the disk, accretion from the intergalactic medium (IGM), the fueling of star formation, the connection with the cosmic web and even the possible existence of low-mass cold dark matter (CDM) halos.

The high angular resolution enables a detailed study of the structure of the HI component in the inner disk. In combination with multi-wavelength data this will enable comprehensive studies of the detailed, local links between the gas and star formation in a galaxy. In the outer parts of galaxies, where HI dominates, the ratio of UV or Hα to HI column density is the key tracer of the timescale and efficiency of star formation. With a large, comprehensive sample, the angular resolution to isolate specific conditions of the ISM, and the velocity resolution to separate warm and cold HI, MHONGOOSE promises to be the ultimate data set to study star formation at and beyond the edges of galaxies. 

© Erwin de Blok 2017