Star formation scaling relations at ∼100 pc from PHANGS: Impact of completeness and spatial scale

Abstract

Aims. The complexity of star formation at the physical scale of molecular clouds is not yet fully understood. We investigate the mechanisms regulating the formation of stars in different environments within nearby star-forming galaxies from the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) sample. Methods. Integral field spectroscopic data and radio-interferometric observations of 18 galaxies were combined to explore the existence of the resolved star formation main sequence (Σstellar versus ΣSFR), resolved Kennicutt–Schmidt relation (Σmol. gas versus ΣSFR), and resolved molecular gas main sequence (Σstellar versus Σmol. gas), and we derived their slope and scatter at spatial resolutions from 100 pc to 1 kpc (under various assumptions). Results. All three relations were recovered at the highest spatial resolution (100 pc). Furthermore, significant variations in these scaling relations were observed across different galactic environments. The exclusion of non-detections has a systematic impact on the inferred slope as a function of the spatial scale. Finally, the scatter of the Σmol. gas + stellar versus ΣSFR correlation is smaller than that of the resolved star formation main sequence, but higher than that found for the resolved Kennicutt–Schmidt relation. Conclusions. The resolved molecular gas main sequence has the tightest relation at a spatial scale of 100 pc (scatter of 0.34 dex), followed by the resolved Kennicutt–Schmidt relation (0.41 dex) and then the resolved star formation main sequence (0.51 dex). This is consistent with expectations from the timescales involved in the evolutionary cycle of molecular clouds. Surprisingly, the resolved Kennicutt–Schmidt relation shows the least variation across galaxies and environments, suggesting a tight link between molecular gas and subsequent star formation. The scatter of the three relations decreases at lower spatial resolutions, with the resolved Kennicutt–Schmidt relation being the tightest (0.27 dex) at a spatial scale of 1 kpc. Variation in the slope of the resolved star formation main sequence among galaxies is partially due to different detection fractions of ΣSFR with respect to Σstellar.

Implications of a spatially resolved main sequence for the size evolution of star forming galaxies

“…This is formally similar to (but physically distinct from) an effect reported byPessa et al (2021) in terms of 'detection fraction', which however originates in low surface density regions and appears related to sensitivity effects and the small probed spatial scales (100 pc).MNRAS 000,1-12 (2021) …”

The Gas-Star Formation Cycle in Nearby Star-forming Galaxies II. Resolved Distributions of CO and H$α$ Emission for 49 PHANGS Galaxies

“…At the highest common resolution of 150 pc, the median fraction of sight lines without any emission within 0.6 R 25 of our galaxies is as high as 70%, ranging from 13 -97%. Moreover, the fractions of empty sight lines decrease with increasing spatial scales (Figure B5, see also Pessa et al 2021). The distributions of empty pixels among galaxies is a potentially interesting diagnostic of ISM evolution and host-galaxy properties.…”

“…The picture becomes more complex when the resolution increases. When the resolution is high enough to separate molecular clouds and star-forming regions, molecular gas and star formation surface densities become loosely correlated (e.g., Blanc et al 2009;Onodera et al 2010;Kreckel et al 2018;Pessa et al 2021) or even anti-correlated (e.g., Schruba et al 2010) because the two components no longer coincide at all times. This spatial separation between different evolutionary stages of the star formation process is also evident in our sight line maps at the resolution of 150 pc (Figure 2 and Figure B5).…”

“…However, any relation between the sight line fractions and host galaxy properties seen at 150 pc resolution is gradually diminished as resolution decreases, and becomes non-identifiable when at resolutions coarser than ∼ 500 pc. Galactic structures add a further complexity to the Kennicutt-Schmidt relation as the relation varies among galactic environments (Pan & Kuno 2017;Pessa et al 2021;Querejeta et al 2021). Since the contribution of the different environments varies as a function of galactic radius, the impact of environments could be transferred to the Kennicutt-Schmidt relation when averaging the environments.…”

The impact of pre-supernova feedback and its dependence on environment

“…2 Physics at High Angular Resolution in Nearby Galaxies. Emsellem et al 2021, for early results see Kreckel et al 2019;Pessa et al 2021). For tens of thousands of regions, these surveys deliver spatially-resolved (for the nearest systems up to a few Mpc) and integrated (beyond a few Mpc) stellar and ionised gas properties, enabling environmental studies of stellar feedback in orders of magnitude more star-forming regions than previously possible.…”