The halo by halo missing baryon problem

TL;DR

This paper investigates the discrepancy between observed baryons and expected cosmic baryon content at both global and individual galaxy scales, highlighting a critical velocity scale and reviewing potential baryon reservoirs.

Contribution

It provides an empirical estimate of the baryon deficit function in galaxies and discusses the implications of a critical velocity scale around 900 km/s.

Findings

Baryon deficit varies with galaxy circular velocity

Identifies a critical velocity scale of ~900 km/s

Discusses potential dark baryon reservoirs in galaxies and clusters

Abstract

The global missing baryon problem - that the sum of observed baryons falls short of the number expected form BBN - is well known. In addition to this, there is also a local missing baryon problem that applies to individual dark matter halos. This halo by halo missing baryon problem is such that the observed mass fraction of baryons in individual galaxies falls short of the cosmic baryon fraction. This deficit is a strong function of circular velocity. I give an empirical estimate of this function, and note the presence of a critical scale of ~900 km/s therein. I also briefly review Omega_b from BBN, highlighting the persistent tension between lithium and the CMB, and discuss some pros and cons of individual galaxies and clusters of galaxies as potential reservoirs of dark baryons.