Structural and Dynamical Analysis of the Quiescent Molecular Ridge in the Large Magellanic Cloud

TL;DR

This study compares the physical and dynamical properties of the quiescent Molecular Ridge with active star-forming regions in the LMC, revealing lower density and kinetic energy, which explains its lack of massive star formation.

Contribution

It provides a detailed hierarchical analysis of molecular structures in the LMC, highlighting differences in physical conditions that influence star formation activity.

Findings

The Molecular Ridge has lower surface densities than active regions.

Lower kinetic energy at given scales in the Ridge correlates with less star formation.

Physical conditions vary within the Ridge and are weakly linked to proximity to star-forming sites.

Abstract

We present a comparison of low-J 13CO and CS observations of four different regions in the LMC -- the quiescent Molecular Ridge, 30 Doradus, N159, and N113, all at a resolution of $\sim3$ pc. The regions 30 Dor, N159, and N113 are actively forming massive stars, while the Molecular Ridge is forming almost no massive stars, despite its large reservoir of molecular gas and proximity to N159 and 30 Dor. We segment the emission from each region into hierarchical structures using dendrograms and analyze the sizes, masses, and linewidths of these structures. We find that the Ridge has significantly lower kinetic energy at a given size scale and also lower surface densities than the other regions, resulting in higher virial parameters. This suggests that the Ridge is not forming massive stars as actively as the other regions because it has less dense gas and not because collapse is suppressed by excess kinetic energy. We also find that these physical conditions and energy balance vary significantly within the Ridge and that this variation appears only weakly correlated with distance from sites of massive star formation such as R136 in 30 Dor, which is $\sim1$ kpc away. These variations also show only a weak correlation with local star formation activity within the clouds.