The Porosity of the neutral ISM in 20 THINGS Galaxies

¹ Centre for Astrophysics Research, Science & Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB, UK
² Max–Planck–Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁴ Observatorio Astronómico Nacional, C/ Alfonso XII 3, Madrid 28014, Spain
⁵ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁶ Research School of Astronomy & Astrophysics, ANU, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Abstract

We present an analysis of the properties of H i holes detected in 20 galaxies that are part of “The H i Nearby Galaxy Survey”. We detected more than 1000 holes in total in the sampled galaxies. The holes are found throughout the disks of the galaxies, out to the edge of the H i disk. We find that shear limits the age of holes in spirals. Shear is less important in dwarf galaxies which explains why H i holes in dwarfs are rounder, on average than in spirals. Shear is particularly strong in the inner part of spiral galaxies, limiting the lifespan of holes there and explaining why we find that holes outside R₂₅ are larger and older. We proceed to derive the surface and volume porosity and find that this correlates with the type of the host galaxy: later Hubble types tend to be more porous. The size distribution of the holes in our sample follows a power law with a slope of a_ν ~ −2.9. Assuming that the holes are the result of massive star formation, we derive values for the supernova rate (SNR) and star formation rate (SFR) which scales with the SFR derived based on other tracers. If we extrapolate the observed number of holes to include those that fall below our resolution limit, down to holes created by a single supernova, we find that our results are compatible with the hypothesis that H i holes result from star formation.