2011MNRAS.416.2447H

We explore the quantified morphology of atomic hydrogen (H I) discs in the Virgo cluster. These galaxies display a wealth of phenomena in their H I morphology, e.g., tails, truncation and warps. These morphological disturbances are related to the ram pressure stripping and tidal interaction that galaxies undergo in this dense cluster environment. To quantify the morphological transformation of the H I discs, we compute the morphological parameters of Concentration, Asymmetry, Smoothness, Gini and M₂₀ and our own G_M for 51 galaxies in 48 H I column density maps from the VLA Imaging of Virgo spirals in Atomic gas (VIVA) project.

Some morphological phenomena can be identified in this space of relatively low-resolution H I data. Truncation of the H I disc can be cleanly identified via the Concentration parameter (C < 1), and Concentration can also be used to identify H I deficient discs (1 < C < 5). Tidal interaction is typically identified using combinations of these morphological parameters, applied to (optical) images of galaxies. We find that some selection criteria (Gini–M₂₀, Asymmetry and a modified Concentration–M₂₀) are still applicable for the coarse (∼15 arcsec full width at half - maximum) VIVA H I data. We note that Asymmetry is strongly affected by the choice for the centre of these galaxies. The phenomena of tidal tails can be reasonably well identified using the Gini–M₂₀ criterion (60 per cent of galaxies with tails identified but with as many contaminants).

Ram pressure does move H I discs into and out of most of our interaction criteria: the ram pressure sequence identified by previous authors tracks into and out of some of these criteria (Asymmetry-based and the Gini–M₂₀selections, but not the Concentration–M₂₀ or the G_M-based ones). Therefore, future searches for interaction using H I morphologies should take ram pressure into account as a mechanism to disturb H I discs enough to make them appear as gravitationally interacting. One mechanism would be to remove all the H I deficient (C < 5) discs from the sample, as these have undergone more than one H I removal mechanism.