Spatial power spectra of dust across the Local Group: No constraint on disc scale height

Abstract

We analyze the 1D spatial power spectra of dust surface density and mid to far-infrared emission at 24-500 µm in the LMC, SMC, M31, and M33. By forward-modelling the pointspread-function (PSF) on the power spectrum, we find that nearly all power spectra have a single power-law and point source component. A broken power-law model is only favoured for the LMC 24 µm MIPS power spectrum and is due to intense dust heating in 30 Doradus. We also test for local power spectrum variations by splitting the LMC and SMC maps into 820 pc boxes. We find significant variations in the power-law index with no strong evidence for breaks. The lack of a ubiquitous break suggests that the spatial power spectrum does not constrain the disc scale height. This contradicts claims of a break where the turbulent motion changes from 3D to 2D. The power spectrum indices in the LMC, SMC, and M31 are similar (2.0-2.5). M33 has a flatter power spectrum (1.3), similar to more distant spiral galaxies with a centrally-concentrated H 2 distribution. We compare the power spectra of H I, CO, and dust in M31 and M33, and find that H I power spectra are consistently flatter than CO power spectra. These results cast doubt on the idea that the spatial power spectrum traces large scale turbulent motion in nearby galaxies. Instead, we find that the spatial power spectrum is influenced by (1) the PSF on scales below ∼ 3 times the FWHM, (2) bright compact regions (30 Doradus), and (3) the global morphology of the tracer (an exponential CO disc).

The dependence of the hierarchical distribution of star clusters on galactic environment

“…The projected fractal dimension, which is related to the slope of the TPCF, is different if we are looking at the projection of a thin slice (2D) rather than a thick disc (3D), and this projection effect leads to a change in the slope (see, Sánchez & Alfaro 2008, for detailed models). Observations of neutral hydrogen, far-infrared dust, and γ -ray emission in nearby external galaxies are consistent with this mechanism [Elmegreen, Kim & Staveley-Smith 2001;Miville-Deschênes et al 2003;Ingalls et al 2004;Dutta et al 2009;Szotkowski et al 2019;Besserglik & Goldman 2021, although see Koch et al (2020) for an alternative explanation for this transition]. However, our cluster TPCFs are not: as discussed in , this transition should lead to a steeper slope at larger separations and a shallower ones at smaller separations, which is the opposite of what we find.…”

Ubiquitous velocity fluctuations throughout the molecular interstellar medium

“…We fit the structure functions with power-law functions of the form S s ( ) ∝ ζ 2 . We set the lower and upper limits of our fitting range to the beam size 88 (0.65 pc and 0.07 pc for the GMCs in the disc and CMZ, respectively) and the approximate scale above which the structure functions begin to turn over (4 pc and 0.7 pc for the GMCs in the disc and CMZ, respectively).…”

The Dependence of the Hierarchical Distribution of Star Clusters on Galactic Environment

“…The projected fractal dimension, which is related to the slope of the TPCF, is different if we are looking at the projection of a thin slice (2D) rather than a thick disk (3D), and this projection effect leads to a change in the slope (see, Sánchez & Alfaro 2008, for detailed models). Observations of neutral hydrogen, Far-Infrared (FIR) dust, and 𝛾-ray emission in nearby external galaxies are consistent with this mechanism Miville-Deschênes et al 2003;Ingalls et al 2004;Dutta et al 2009;Szotkowski et al 2019;Besserglik & Goldman 2021, although see Koch et al (2020) for an alternative explanation for this transition). However, our cluster TPCFs are not: as discussed in , this transition should lead to a steeper slope at larger separations and a shallower ones at smaller separations, which is the opposite of what we find.…”