Giant molecular cloud catalogues for PHANGS-ALMA: methods and initial results

Abstract

We present improved methods for segmenting CO emission from galaxies into individual molecular clouds, providing an update to the cprops algorithms presented by Rosolowsky & Leroy. The new code enables both homogenization of the noise and spatial resolution among data, which allows for rigorous comparative analysis. The code also models the completeness of the data via false source injection and includes an updated segmentation approach to better deal with blended emission. These improved algorithms are implemented in a publicly available Python package, pycprops. We apply these methods to 10 of the nearest galaxies in the PHANGS-ALMA survey, cataloguing CO emission at a common 90 pc resolution and a matched noise level. We measure the properties of 4986 individual clouds identified in these targets. We investigate the scaling relations among cloud properties and the cloud mass distributions in each galaxy. The physical properties of clouds vary among galaxies, both as a function of galactocentric radius and as a function of dynamical environment. Overall, the clouds in our target galaxies are well-described by approximate energy equipartition, although clouds in stellar bars and galaxy centres show elevated line widths and virial parameters. The mass distribution of clouds in spiral arms has a typical mass scale that is 2.5× larger than interarm clouds and spiral arms clouds show slightly lower median virial parameters compared to interarm clouds (1.2 versus 1.4).

Molecular gas and star formation within 12 strong galactic bars observed with IRAM-30 m

“…Even though we have used the largest sample to date for the stated scientific goals, the robustness of the analysis is still limited by sample size. Thus, we encourage this result to be verified elsewhere with larger surveys, such as EDGE-CALIFA (Bolatto et al 2017;Sánchez et al 2021) or Physics at High Angular Resolution in Nearby GalaxieS (PHANGS; e.g., Rosolowsky et al 2021;Anand et al 2021;Lee et al 2021). The analysis of SF scaling relations at ∼100 pc from PHANGS was released by Pessa et al (2021) during the review process of this paper and will be analysed elsewhere.…”

PHANGS–ALMA Data Processing and Pipeline

“…Table 3 lists the target resolutions and other details of the product creation process. These fixed-resolution cubes are intended to allow rigorous comparison among targets at different distances (e.g., Hughes et al 2013;Rosolowsky et al 2021). The processing described in Section 3 already places the cubes at nearly matched velocity resolution.…”

“…Second, convolving to multiple fixed physical resolutions plays a crucial role in testing scientific hypotheses. At the most basic level, this allows for rigorous comparison among galaxies observed with different beams and lying at different distances (e.g., Hughes et al 2013;Rosolowsky et al 2021). Increasingly, spatial scale is also by itself viewed as an important Notes.…”

The SEDIGISM survey: The influence of spiral arms on the molecular gas distribution of the inner Milky Way

“…Considering their beam sizes and the surface densities measured, and assuming that most of the molecular gas emission comes from spiral arms, we would get linear masses in the range of 10 5 10 7 M kpc 1 , more in line with our Milky Way values. Additionally, Rosolowsky et al (2021) performed molecular cloud segmentation for a sample of ten nearby galaxies, observing a spiral arm-inter-arm region molecular gas mass contrast of between ⇠ 1 2.3 (E. Rosolowsky, private communication) considering the mass within clouds, which is similar to what we measure here (1.5, see Section 5.3).…”

“…In the barred spiral galaxy M100, the situation is similar: central clouds are more massive, denser, and have higher velocity dispersions than objects in the inter-arm regions (Pan & Kuno 2017). Nevertheless, GMC properties in several other spiral galaxies do not di↵er signficantly from spiral arms to interarm regions (Donovan Meyer et al 2013), even if, more recently, Rosolowsky et al (2021) observed some slightly higher surface densities and lower virial parameters in clouds in the spiral arms compared to objects in inter-arm regions.…”