The structure and characteristic scales of the H I gas in galactic disks

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: sami.dib@gmail.com
² Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
³ Indian Institute of Astrophysics (IIA), Sarjapur Road, Koramangala, Bangalore 560034, India
⁴ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
⁵ Sternberg Astronomical Institute, Lomonosov Moscow State University, University Avenue 13, 119899 Moscow, Russia
⁶ SOFIA Science Center, Universities Space Research Association, NASA Ames Research Center, M.S. N232-12, Moffett Field, CA 94035, USA
⁷ Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
⁸ School of Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
⁹ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁰ Departamento de Matemáticas, Universidad de Atacama Av. Copayapu 485, Copiapó, Atacama, Chile
¹¹ IPAC, California Institute of Technology, Pasadena, CA, 91125, USA
¹² CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
¹³ University of Chinese Academy of Sciences, Beijing 100049, PR China
¹⁴ NAOC-UKZN Computational Astrophysics Centre, University of KwaZulu-Natal, Durban 4000, South Africa

Received: 15 July 2021
Accepted: 30 September 2021

Abstract

The spatial distribution of the H I gas in galactic disks holds important clues about the physical processes that shape the structure and dynamics of the interstellar medium (ISM). The structure of the ISM could be affected by a variety of perturbations internal and external to the galaxy, and the unique signature of each of these perturbations could be visible in the structure of interstellar gas. In this work, we quantify the structure of the H I gas in a sample of 33 nearby galaxies taken from the HI Nearby Galaxy Survey (THINGS) using the delta-variance (Δ-variance) spectrum. The THINGS galaxies display a large diversity in their spectra, but there are a number of recurrent features. In many galaxies, we observe a bump in the spectrum on scales of a few to several hundred parsec. We find the characteristic scales associated with the bump to be correlated with the galactic star formation rate (SFR) for values of the SFR ≳0.5 M_⊙ yr⁻¹ and also with the median size of the H I shells detected in these galaxies. We interpret this characteristic scale as being associated with the effects of feedback from supernova explosions. On larger scales, we observe in most galaxies two self-similar, scale-free regimes. The first regime, on intermediate scales (≲0.5R₂₅), is shallow, and the power law that describes this regime has an exponent in the range [0.1–1] with a mean value of 0.55 that is compatible with the density field that is generated by supersonic turbulence in the cold phase of the H I gas. The second power law is steeper, with a range of exponents between 0.5 and 2.3 and a mean value of ≈1.5. These values are associated with subsonic to transonic turbulence, which is characteristic of the warm phase of the H I gas. The spatial scale at which the transition between the two self-similar regimes occurs is found to be ≈0.5R₂₅, which is very similar to the size of the molecular disk in the THINGS galaxies. Overall, our results suggest that on scales ≲0.5R₂₅, the structure of the ISM is affected by the effects of supernova explosions. On larger scales (≳0.5R₂₅), stellar feedback has no significant impact, and the structure of the ISM is determined by large-scale processes that govern the dynamics of the gas in the warm neutral medium, such as the flaring of the H I disk at large galactocentric radii and the effects of ram pressure stripping.