The stellar mass-halo mass relation of isolated field dwarfs: a critical test of $\Lambda$CDM at the edge of galaxy formation

TL;DR

This study measures the stellar mass-halo mass relation of isolated dwarf galaxies, finds strong agreement with $ m extit{ extbf{Lambda}}$CDM predictions in the field, and highlights environment-dependent discrepancies related to galaxy formation physics.

Contribution

It provides the first direct measurement of the $M_*-M_{200}$ relation for isolated dwarfs, testing $ m extit{ extbf{Lambda}}$CDM at the low-mass end with new rotation curve data.

Findings

Agreement between observed and predicted $M_*-M_{200}$ in the field down to $5 imes 10^8 M_ ext{sun}$

Discrepancies in group environments suggest galaxy formation physics issues, not cosmology

Warm dark matter models with $m_{WDM} < 1.25$ keV are inconsistent with the data

Abstract

We fit the rotation curves of isolated dwarf galaxies to directly measure the stellar mass-halo mass relation ($M_*-M_{200}$) over the mass range $5 \times 10^5 < M_{*}/{\rm M}_\odot < 10^{8}$. By accounting for cusp-core transformations due to stellar feedback, we find a monotonic relation with little scatter. Such monotonicity implies that abundance matching should yield a similar $M_*-M_{200}$ if the cosmological model is correct. Using the 'field galaxy' stellar mass function from the Sloan Digital Sky Survey (SDSS) and the halo mass function from the $\Lambda$ Cold Dark Matter Bolshoi simulation, we find remarkable agreement between the two. This holds down to $M_{200} \sim 5 \times 10^9$M$_\odot$, and to $M_{200} \sim 5 \times 10^8$M$_\odot$ if we assume a power law extrapolation of the SDSS stellar mass function below $M_* \sim 10^7$M$_\odot$. However, if instead of SDSS we use the stellar mass function of nearby galaxy groups, then the agreement is poor. This occurs because the group stellar mass function is shallower than that of the field below $M_* \sim 10^9$M$_\odot$, recovering the familiar 'missing satellites' and 'too big to fail' problems. Our result demonstrates that both problems are confined to group environments and must, therefore, owe to 'galaxy formation physics' rather than exotic cosmology. Finally, we repeat our analysis for a $\Lambda$ Warm Dark Matter cosmology, finding that it fails at 68% confidence for a thermal relic mass of $m_{\rm WDM} < 1.25$keV, and $m_{\rm WDM} < 2$keV if we use the power law extrapolation of SDSS. We conclude by making a number of predictions for future surveys based on these results.