VESTIGE XXI. HII regions in perturbed galaxies

The difference in the two distributions comes principally from the outer disc (outside the effective radius). Perturbed and unperturbed systems share a similar HII size distribution, while gas-poor objects hosts higher electron density regions than HI-rich systems. The analysis of the scaling relations indicates that perturbed objects have, on average, a lower number of HII regions per unit stellar mass and disc surface than unperturbed systems, with differences increasing with the HI-deficiency parameter, principally in the outer disc where HII regions are less present in gas-poor systems.

This systematic difference is also observed in the Halpha luminosity of the first ranked and first three ranked HII regions, which is reduced in HI-deficient systems with respect to gas-rich objects. All these differences can be explained in the framework of galaxy evolution in rich environments, where their hydrodynamic interaction with the surrounding intracluster medium (ram pressure) removes the gas outside-in quenching the star formation activity in the outer disc once the atomic hydrogen is removed.