Molecular gas in Tidal Dwarf Galaxies: Exploring the conditions for star formation

TL;DR

This study investigates the conditions for star formation in Tidal Dwarf Galaxies, examining molecular gas and star formation laws, revealing that high molecular gas density alone does not guarantee star formation, with gas kinematics also playing a role.

Contribution

It provides new insights into star formation conditions in TDGs, highlighting the importance of gas kinematics alongside molecular gas density, and tests star formation laws in this unique environment.

Findings

Good agreement with the Schmidt law for total and molecular gas

High molecular gas density alone does not trigger star formation

Star formation correlates with narrow gas line widths in some regions

Abstract

Tidal Dwarf Galaxies (TDGs), produced from material expelled in galactic interactions, are well--suited to test the laws of star formation (SF) due to their simple structure, high metallicity -- making CO a reliable tracer of the molecular gas content -- and recent SF. Here, we study the conditions for the onset of SF and for the rate at which SF proceeds once above a threshold in a small sample of TDGs. We use data for the gas (atomic and molecular) surface density and SF rate per area to test the laws of SF found for spiral and dwarf galaxies in this more extreme environment. We find in general a good agreement with the Schmidt law found for the total gas and for the molecular gas but note that higher resolution CO observations are necessary to clarify some possible discrepancies. We find, down to a scale of $\sim$1 kpc, in general a good agreement between the peaks of SF and of the molecular gas, but also find in some objects surprisingly large quantities of molecular gas at places where no SF is occuring. A high column density of molecular gas is therefore not a sufficient condition for the onset of SF. We find that the kinematical properties of the gas are also relevant: in two objects our observations showed that SF only occured in regions with a narrow line width.