On the morphologies, gas fractions, and star formation rates of small galaxies

TL;DR

This paper uses simulations and analytic arguments to explain why small galaxies tend to be spheroidal, gas-rich, and less efficient at star formation, due to an interstellar medium temperature floor affecting gas pressure support.

Contribution

It demonstrates that an interstellar temperature floor naturally accounts for morphological and star formation differences between small and large galaxies, linking gas physics to galaxy evolution.

Findings

Low-mass galaxies are more spheroidal and gas-rich.

Pressure support dominates in small haloes, affecting galaxy morphology.

Massive galaxies have thinner discs and higher star formation efficiency.

Abstract

We use a series of N-body/smoothed particle hydrodynamics simulations and analytic arguments to show that the presence of an effective temperature floor in the interstellar medium at T_F ~ 10^4 K naturally explains the tendency for low-mass galaxies to be more spheroidal, more gas rich, and less efficient in converting baryons into stars than larger galaxies. The trend arises because gas pressure support becomes important compared to angular momentum support in small dark matter haloes. We suggest that dwarf galaxies with rotational velocities ~ 40 km/s do not originate as thin discs, but rather are born as thick, puffy systems. If accreted on to larger haloes, tenuous dwarfs of this kind will be more susceptible to gas loss or tidal transformation than scaled-down versions of larger spirals. For a constant temperature floor, pressure support becomes less important in large haloes, and this produces a tendency for massive isolated galaxies to have thinner discs and more efficient star formation than their less massive counterparts, as observed.