Galaxy Evolution in a New Era of HI Surveys

29 July - 23 August 2019

Andrew Baker, Jacqueline van Gorkom, Lister Staveley-Smith, Marc Verheijen, Martin Zwaan

Understanding the formation and evolution of galaxies is a central challenge of modern astrophysics and depends critically on understanding the ionised, neutral, and molecular phases of the interstellar medium as a function of galaxy mass, environment, and redshift. Optical and (sub)millimeter wavelength observations can now probe ionised and molecular gas reservoirs out to the highest redshifts; however, the best probe of galaxies' neutral atomic gas -- the 21cm HI transition -- has historically been difficult to observe past z = 0.2, owing to the line's faintness and the complications of radio frequency interference from terrestrial sources.

To push beyond this limit requires substantial investments of observing time, new facilities with large collecting areas and sensitive receivers, and protected radio-quiet sites. In anticipation of the Square Kilometre Array (SKA), pathfinder facilities in Australia (ASKAP) and South Africa (MeerKAT) as well as refurbished arrays (APERTIF, VLA) and large single-dish telescopes (Arecibo, FAST) are executing a diverse set of Galactic and extragalactic surveys of HI in the coming years.

As this new era of HI surveys dawns, it is important that survey teams and the broader astronomical communities studying gas and galaxy evolution remain in close contact with each other, in order to maximise the surveys' scientific impacts. This MIAPP programme will bring together a cross-cutting group of scientists, including both theorists and multi-wavelength observers, to exploit HI survey data that push to greater depths, areas, and redshift ranges than have ever been probed before. The programme will begin with a topical workshop ("Nine Billion Years of Neutral Gas Evolution") and spend roughly a week apiece examining four key questions:

  •  How do ionised and molecular gas cool into and condense out of galaxies' HI reservoirs as functions of redshift, environment, and other parameters?
  • How do HI emission-line and absorption-line observations relate to optical absorption-line measurements of the evolving cosmic gas density?
  • How can HI observations be exploited to understand the dynamical properties of disk galaxies and their possible evolution with redshift?
  • How do surveys of HI at z = 0 best inform the analysis of surveys of HI at z ~ 1, and vice versa?