HI Disks in the high-redshift Universe; Size and Quantified Morphology

Abstract

The upcoming new perspective of the high redshift Universe in the 21 cm line of atomic hydrogen opens possibilities to explore topics of spiral disk evolution, hitherto reserved for the optical regime. The growth of spiral gas disks over Cosmic time can be explored with the new generation of radio telescopes, notably the SKA, and its precursors, as accurately as with the Hubble Space Telescope for stellar disks. Since the atomic hydrogen gas is the building block of these disks, it should trace their formation accurately. Morphology of H I disks can now equally be quantified over Cosmic time. In studies of HST deep fields, the optical or UV morphology of high-redshift galaxy disks have been characterised using a few quantities: concentration (C), asymmetry (A), smoothness (S), second-order-moment (M 20 ), the GINI coefficient (G), and Ellipticity (E). We have applied these parameters across wavelengths and compared them to the HI morphology over the THINGS sample. NGC 3184, an unperturbed disk, and NGC 5194, the canonical 3:1 interaction, serve as examples for quantified morphology. We find that morphology parameters determined in H I are as good or better a tracer of interaction compared to those in any other wavelength, notably in Asymmetry, Gini and M 20 . This opens the possibility of using them in the parameterization pipeline for SKA precursor catalogues to select interacting or harassed galaxies from their H I morphology. Asymmetry, Gini and M 20 may be redefined for use on data-cubes rather than H I column density image.