The Nature of Turbulence in the LITTLE THINGS Dwarf Irregular Galaxies

Abstract

We present probability density functions and higher order (skewness and kurtosis) analyses of the galaxy-wide and spatially-resolved H i column density distributions in the LITTLE THINGS sample of dwarf irregular galaxies. This analysis follows that of Burkhart et al. (2010) for the Small Magellanic Cloud. About 60% of our sample have galaxy-wide values of kurtosis that are similar to that found for the Small Magellanic Cloud, with a range up to much higher values, and kurtosis increases with integrated star formation rate. Kurtosis and skewness were calculated for radial annuli and for a grid of 32 pixel × 32 pixel kernels across each galaxy. For most galaxies, kurtosis correlates with skewness. For about half of the galaxies, there is a trend of increasing kurtosis with radius. The range of kurtosis and skewness values is modeled by small variations in the Mach number close to the sonic limit and by conversion of H i to molecules at high column density. The maximum H i column densities decrease with increasing radius in a way that suggests molecules are forming in the weak field limit, where H 2 formation balances photodissociation in optically thin gas at the edges of clouds.

The density structure of supersonic self-gravitating turbulence

“…Hopkins (2013b) argues that these appear because the density fluctuations are not uncorrelated and thus the central limit theorem is not fully applicable. Despite these caveats, the LN distribution remains a good approximation for turbulent supersonic media, and is consistent with observations of molecular clouds (Berkhuijsen & Fletcher 2008;Hill et al 2008;Burkhart et al 2010;Burkhart & Lazarian 2012;Maier et al 2016;Federrath et al 2016b;Maier et al 2017;Chen et al 2018; however, for a contrary view, see Lombardi et al 2015;Alves et al 2017 who suggest that the distribution follows a power law modified at low densities by the finite size of a given map).…”

The Self-gravitating Gas Fraction and the Critical Density for Star Formation

“…Atomic gas transitions to molecular gas at lower densities than the critical density for collapse. This is naturally explained by 21-cm observations in the local universe where atomic gas demonstrates lognormal PDFs, high virial parameters and statistics of supersonic turbulence without signs of collapse (Burkhart et al 2009(Burkhart et al , 2010Zhang et al 2012;Burkhart et al 2015b;Pingel et al 2013;Maier et al 2016Maier et al , 2017Nestingen-Palm et al 2017;Bialy et al 2017;Pingel et al 2018). Furthermore, dwarf galaxies with essentially no star formation and no molecular gas are observed in the extreme environment of galaxy clusters (Taylor et al 2012;Janowiecki et al 2015;Cannon et al 2015;Burkhart & Loeb 2016;Bellazzini et al 2018) again confirming that HI is unbound or pressure bound.…”

The H i-to-H2 Transition in a Turbulent Medium

“…The product αG may be small or large for realistic astronomical environments. For example, for the starforming region W43 and for the Perseus molecular cloud, Bialy et al (2017) and derived αG ∼ 20 and αG ∼ 10, respectively, whereas for a sample of dwarf irregular galaxies in the LITTLE THINGS survey (Hunter et al 2012), Maier et al (2017) deduced αG < 1. For cold neutral medium (CNM) which is in pressure equilibrium with the warm neutral medium (WNM), the n/I UV ratio is restricted to the range ≈ 8 − 70 cm −3 (Wolfire et al 2003), giving (αG) CNM ≈ 1 − 8.…”

“…Bialy & Sternberg (2016, hereafter BS16) presented an analytic procedure for generating atomic (HI) to molecular (H 2 ) density profiles for optically thick hydrogen gas clouds in Galactic star-forming regions. These studies thus far have been instrumental in interpreting emission line observations of HI /H 2 interfaces (Lee et al 2012;Burkhart et al 2015;Bihr et al 2015;Bialy et al 2017;Maier et al 2017), for estimating starformation thresholds in external galaxies (Leroy et al 2008;Lada et al 2012;Clark & Glover 2014;Bialy & Sternberg 2016;Burkhart & Loeb 2016), and for sub-grid components in hydrodynamics simulations (Pelupessy et al 2006;Thompson et al 2014;Tomassetti et al 2015).…”