Gas-to-star formation cycle as a potential contributor to the scatter of the star-forming main sequence

Hsi-An Pan^1,2*, Eva Schinnerer², Annie Hughes³, Adam Leroy⁴, Brent Groves⁵

¹Department of Physics, Tamkang University, New Taipei City, Taiwan
²Department of Galaxies and Cosmology, The Max Planck Institute for Astronomy, Heidelberg, Germany
³MICMAC, L’Institut de Recherche en Astrophysique et Planétologie (IRAP), Toulouse, France
⁴Department of Astronomy, Ohio State University, Columbus, Ohio, USA
⁵International Centre for Radio Astronomy Research, The University of Western Australia, Perth, Australia

* Presenter:Hsi-An Pan, email:hapan@gms.tku.edu.tw

Molecular gas is a crucial element to understand the star-forming main sequence as it acts as a go-between for the two axes. Molecular gas mass is correlated with stellar mass, both globally and locally, either by the local potential well of the galactic disk set by stellar mass or both stellar and gas mass responding to the gravity set by dark matter. On the other hand, molecular gas mass is correlated with star formation rate to some extent as a natural consequence of stars forming in molecular gas.

We quantify the spatial correlation between molecular gas and star-forming regions for ~50 galaxies using the ALMA and narrowband-Hα imaging from the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. At the resolution (150 pc) at which the individual molecular clouds and star-forming regions can be identified, we find that molecular gas and star-forming regions do not necessarily coexist. The decoupled molecular gas and star-forming regions are a signature of evolutionary cycling and feedback of the star formation process. Moreover, the evolution of star formation is not universal but varies among galaxy populations.

Such a spatial (non-)correlation between molecular gas and star-forming regions will contribute to the scatter of star-forming main sequence due to the inevitable correlation between stellar and molecular gas mass. We conclude that molecular gas-to-star formation cycle, which is set by the host-galaxy properties, must be considered when interpreting the star-forming main sequence. Finally, we also find that the way that molecular gas and star-forming regions (de)correlate with each other is sensitive to observing resolution. Therefore, the scatter of star-forming main sequence may also respond to observational effect.

Keywords: nearby galaxies, interstellar medium, galaxy evolution