The low-mass end of the baryonic Tully-Fisher relation

TL;DR

This paper demonstrates that the low-mass end of the baryonic Tully-Fisher relation can be accurately reproduced by LCDM simulations, revealing insights into galaxy formation efficiency and the impact of feedback mechanisms.

Contribution

It shows that LCDM simulations match the low-mass BTF relation and predicts a steepening at the faint end, addressing previous discrepancies with observations.

Findings

Simulations reproduce the BTF relation well across galaxy masses.

Feedback mechanisms do not induce large scatter in the BTF relation.

Predicted steepening of the BTF at low velocities is not fully observed.

Abstract

The scaling of disk galaxy rotation velocity with baryonic mass (the "Baryonic Tully-Fisher" relation, BTF) has long confounded galaxy formation models. It is steeper than the M ~ V^3 scaling relating halo virial masses and circular velocities and its zero point implies that galaxies comprise a very small fraction of available baryons. Such low galaxy formation efficiencies may in principle be explained by winds driven by evolving stars, but the tightness of the BTF relation argues against the substantial scatter expected from such vigorous feedback mechanism. We use the APOSTLE/EAGLE simulations to show that the BTF relation is well reproduced in LCDM simulations that match the size and number of galaxies as a function of stellar mass. In such models, galaxy rotation velocities are proportional to halo virial velocity and the steep velocity-mass dependence results from the decline in galaxy formation efficiency with decreasing halo mass needed to reconcile the CDM halo mass function with the galaxy luminosity function. Despite the strong feedback, the scatter in the simulated BTF is smaller than observed, even when considering all simulated galaxies and not just rotationally-supported ones. The simulations predict that the BTF should become increasingly steep at the faint end, although the velocity scatter at fixed mass should remain small. Observed galaxies with rotation speeds below ~40 km/s seem to deviate from this prediction. We discuss observational biases and modeling uncertainties that may help to explain this disagreement in the context of LCDM models of dwarf galaxy formation.