Another baryon miracle? Testing solutions to the "missing dwarfs" problem

Abstract

The dearth of dwarf galaxies in the local universe is hard to reconcile with the large number of low mass haloes expected within the concordance $\Lambda$CDM paradigm. In this paper we perform a systematic evaluation of the uncertainties affecting the measurement of DM halo abundance using galaxy kinematics. Using a large sample of dwarf galaxies with spatially resolved kinematic data we derive a correction to obtain the observed abundance of galaxies as a function of their halo maximum circular velocity from the line-of-sight velocity function in the Local Volume. This estimate provides a direct means of comparing the predictions of theoretical models and simulations (including nonstandard cosmologies and novel galaxy formation physics) to the observational constraints. The new "galactic $V_{max}$" function is steeper than the line-of-sight velocity function but still shallower than the theoretical CDM expectation, showing that some unaccounted physical process is necessary to reduce the abundance of galaxies and/or drastically modify their density profiles compared to CDM haloes. Using this new galactic $V_{max}$ function, we investigate the viability of baryonic solutions such as feedback-powered outflows and photoevaporation of gas from an ionising radiation background. At the 3-$\sigma$ confidence level neither energetic feedback nor photoevaporation are effective enough to reconcile the disagreement. In the case of maximum baryonic effects, the theoretical estimate still deviates significantly from the observations for $V_{max} < 60$ km/s. CDM predicts at least 1.8 times more galaxies with $V_{max} = 50$ km/s and 2.5 times more than observed at $30$ km/s. Recent hydrodynamic simulations seem to resolve the discrepancy but disagree with the properties of observed galaxies with resolved kinematics. (abridged)