Galaxy formation and evolution

Abstract:

Star formation in galaxies is governed by the amount of molecular gas and the efficiency that gas is converted into stars. However, assessing the amount of molecular gas relies on the CO-to-H2 conversion factor (alpha_CO), which is known to vary with molecular gas conditions like density, temperature, and dynamical state -- the same conditions that also alter star formation efficiency. The variation of alpha_CO, particularly in galaxy centers where alpha_CO can drop by nearly an order of magnitude, thus causes major uncertainties in current molecular gas and star formation efficiency measurements. Using ALMA observations of multiple low-J 12CO, 13CO, and C18O lines in several barred galaxy centers, we found that alpha_CO is primarily driven by CO opacity changes and therefore shows strong correlations with observables like velocity dispersion and 12CO/13CO line ratio. Motivated by these results, we have constructed a new alpha_CO prescription which accounts for emissivity effects in galaxy centers and verified it on a set of barred and non-barred galaxies with measured alpha_CO values from dust. Applying our new prescription to 65 galaxies from the PHANGS-ALMA survey, we found an overall three times higher star formation efficiency in barred galaxy centers than in non-barred galaxy centers, and such a trend is obscured when using a constant alpha_CO or other existing prescriptions. Our results suggest that the high star formation rate observed in barred galaxy centers is due to an enhanced star formation efficiency compared to non-barred galaxy centers or the disk regions, rather than a substantially increased amount of molecular gas in barred galaxy centers

Abstract:

[[abstract]]The relative distribution of molecular gas and star formation in galaxies gives insight into the physical processes and timescales of the cycle between gas and stars. In this work, we track the relative spatial configuration of CO and Hα emission at high resolution in each of our galaxy targets and use these measurements to quantify the distributions of regions in different evolutionary stages of star formation: from molecular gas without star formation traced by Hα to star-forming gas, and to H ii regions. The large sample, drawn from the Physics at High Angular resolution in Nearby GalaxieS ALMA and narrowband Hα (PHANGS-ALMA and PHANGS-Hα) surveys, spans a wide range of stellar masses and morphological types, allowing us to investigate the dependencies of the gas‒star formation cycle on global galaxy properties. At a resolution of 150 pc, the incidence of regions in different stages shows a dependence on stellar mass and Hubble type of galaxies over the radial range probed. Massive and/or earlier-type galaxies in our sample exhibit a significant reservoir of molecular gas without star formation traced by Hα, while lower-mass galaxies harbor substantial H ii regions that may have dispersed their birth clouds or formed from low-mass, more isolated clouds. Galactic structures add a further layer of complexity to the relative distribution of CO and Hα emission. Trends between galaxy properties and distributions of gas traced by CO and Hα are visible only when the observed spatial scale is ≪500 pc, reflecting the critical resolution requirement to distinguish stages of the star formation process.[[notice]]補正完