The ATLAS3D project - XXII. Low-efficiency star formation in early-type galaxies: hydrodynamic models and observations

TL;DR

This study combines hydrodynamical simulations and observations to show that early-type galaxies have significantly lower star formation efficiency than spiral galaxies due to morphological quenching, which stabilizes gas discs against fragmentation.

Contribution

It demonstrates that morphological quenching reduces star formation efficiency in early-type galaxies through combined simulation and observational evidence, confirming the stability of gas discs in these galaxies.

Findings

Early-type galaxies form 2-5 times less efficiently than spirals.

Gas discs in early-types are more stable, preventing dense clump formation.

Simulations match observed star formation rates and gas properties in NGC524.

Abstract

We study the global efficiency of star formation in high resolution hydrodynamical simulations of gas discs embedded in isolated early-type and spiral galaxies. Despite using a universal local law to form stars in the simulations, we find that the early-type galaxies are offset from the spirals on the large-scale Kennicutt relation, and form stars 2 to 5 times less efficiently. This offset is in agreement with previous results on morphological quenching: gas discs are more stable against star formation when embedded in early-type galaxies due to the lower disc self-gravity and increased shear. As a result, these gas discs do not fragment into dense clumps and do not reach as high densities as in the spiral galaxies. Even if some molecular gas is present, the fraction of very dense gas (above 10^4 cm-3) is significantly reduced, which explains the overall lower star formation efficiency. We also analyse a sample of local early-type and spiral galaxies, measuring their CO and HI surface densities and their star formation rates as determined by their non-stellar 8um emission. As predicted by the simulations, we find that the early-type galaxies are offset from the Kennicutt relation compared to the spirals, with a twice lower efficiency. Finally, we validate our approach by performing a direct comparison between models and observations. We run a simulation designed to mimic the stellar and gaseous properties of NGC524, a lenticular galaxy, and find a gas disc structure and global star formation rate in good agreement with the observations. Morphological quenching thus seems to be a robust mechanism, and is also consistent with other observations of a reduced star formation efficiency in early-type galaxies in the COLD GASS survey. This lower efficiency of star formation is not enough to explain the formation of the whole Red Sequence, but can contribute to the reddening of some galaxies.