Relationship between the line width of the atomic and molecular ISM in M33

Abstract

We investigate how the spectral properties of atomic (H I) and molecular (H 2 ) gas, traced by CO(2-1), are related in M33 on 80 pc scales. We find the H I and CO(2-1) velocity at peak intensity to be highly correlated, consistent with previous studies. By stacking spectra aligned to the velocity of H I peak intensity, we find that the CO line width (σ HWHM = 4.6 ± 0.9 km s −1 ; σ HWHM is the effective Gaussian width) is consistently smaller than the H I line width (σ HWHM = 6.6 ± 0.1 km s −1 ), with a ratio of ∼0.7, in agreement with Druard et al. (2014). The ratio of the line widths remains less than unity when the data are smoothed to a coarser spatial resolution. In other nearby galaxies, this line width ratio is close to unity which has been used as evidence for a thick, diffuse molecular disk that is distinct from the thin molecular disk dominated by molecular clouds. The smaller line width ratio found here suggests that M33 has a marginal thick molecular disk. From modelling individual lines-ofsight, we recover a strong correlation between H I and CO line widths when only the H I located closest to the CO component is considered. The median line width ratio of the lineof-sight line widths is 0.56 ± 0.01. There is substantial scatter in the H I-CO(2-1) line width relation, larger than the uncertainties, that results from regional variations on < 500 pc scales, and there is no significant trend in the line widths, or their ratios, with galactocentric radius. These regional line width variations may be a useful probe of changes in the local cloud environment or the evolutionary state of molecular clouds.

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

“…Approximating the disc as a uniform exponential disc, the power spectrum should have a break near the disc scale length 4 and a flat power spectrum on larger scales. While this is a plausible explanation for the CO(2-1) power spectrum from Combes et al (2012) (see Koch et al 2019b), this simple model does not explain the lack of a power spectrum break near the disc scale length nor the shallow power spectrum measured for the dust surface density and IR bands. This implies that the more diffuse and predominantly atomic gas plays an important role in setting the large scale power spectrum shape.…”

“…In M33, the H 2 distribution is centrally-concentrated in the galaxy and primarily from GMC scale emission (Rosolowsky et al 2003). Koch et al (2019b) show that the distribution of GMCs in M33 can provide an excess in power on scales up to ∼ 2 kpc, and can therefore affect the CO and dust power spectra shape on similar scales. The final fourth point will tend to flatten the dust power spectrum on large scales, as the dust abundance decreases with metallicity.…”

“…The latter does not flatten on large-scales. Koch et al (2019b) shows that an excess of power on large scales is due to the clustering of GMCs in the inner few kpc. We also explore these tracers in M33 in §4.4 and compare the recovered indices there.…”

The Dawes Review 9: The role of cold gas stripping on the star formation quenching of satellite galaxies

“…However, it is important to acknowledge that there are also cases where no evidence for a significant thick CO component has been found (e.g., M33; Koch et al 2019), and that we are still missing an extensive investigation of the frequency and properties of CO diffuse, thick discs, primarily due to the lack of CO observations able to trace emission at all physical scales. Moreover, it remains unclear if the fraction of diffuse CO emission maps directly into diffuse H 2 or not, which would clearly impact the overall physical scenario of direct molecular hydrogen stripping.…”

Evidence for supernova feedback sustaining gas turbulence in nearby star-forming galaxies

“…moments maps, position-velocity diagrams, rotation curves) in Appendix A. In B19a, we did not analyse the molecular gas kinematics for each galaxy in the sample, as we relied on previous works in the literature that showed that the velocity dispersion of CO is about half of σ HI (Mogotsi et al 2016;Marasco et al 2017;Koch et al 2019). For this work, a robust measurement of the velocity dispersion is desirable to accurately calculate Eq.…”