A physically motivated approach to measure diffuse emission fractions and ISM fragmentation lengths
Alex Hygate
Poster -- Galactic scale ISM and star formation
Comparisons between interferometric and single-dish CO line observations reveal that observed molecular gas consists of both a concentrated clumpy component (molecular clouds) and a significant component spread out over large scales (diffuse emission). Thus far, efforts to separate the diffuse and clumpy emission in molecular or ionised gas have had to rely on arbitrary separation boundaries (such as between the flux missing in an interferometric map versus a single dish map). I will present a physically-motivated method for separating diffuse and concentrated emission components from tracer maps that does not rely on an arbitrary spatial boundary. The statistical method developed by Kruijssen, & Longmore (2014) and Kruijssen, Schruba, Hygate et al. (2018) provides a way to measure the mean separation length of star-forming regions in galaxies. This “separation length” represents the boundary between concentrated emission strongly associated with dense star-forming regions of the ISM and emission from a diffuse reservoir. We then use this length to decompose the emission into concentrated and diffuse components through Fourier filtering. This method allows us to probe diffuse emission fractions across a wide array of multi-wavelength tracers including those such as Hα and UV, for which we do not have interferometric observations. I will present the validation of this method on a large number of simulated data sets along with the initial results from a systematic application of the method to a large sample of local-Universe galaxies from the PHANGS collaboration. These observational results reveal the environmental variation of the diffuse emission fraction and the separation length. I will interpret these in terms of physical fragmentation length scales, such as the Toomre length, disc scale height, turbulence driving scale, etc., revealing the dynamical mechanisms that regulate the structure of the ISM in galaxies.