We have just written a new paper, where we study the impact of reionization on galaxy formation in the Local Group using computer simulations.

We find that most of the dark matter halos of similar mass to the observed dwarf galaxies are in fact completely dark; unable to form stars after reionization heats up the intergalactic gas. Those halos that do form stars are not only rare, but also special: they formed much earlier than “typical” halos, and if they are satellites, they follow different orbits than a pure dark matter simulation would predict. We conclude that if we want to understand dark matter by only studying the halos that host the observable galaxies, we have to be aware that we are dealing with a very special selection. We call them The Chosen Few.

  From Fig. 1: Gas density in the Local Group simulation with reionization (left) and without reionization (right). Without reionization, many more “clumps” of gas can cool and form dwarf galaxies, but with reionization included, only a small fraction of low-mass halos keep enough cold gas for star formation, leaving most dark matter halos completely dark.

If you’d like to read more, please take a look at our paper. Also, see Durham University’s press release.

Collaborators: Carlos S. Frenk, Azadeh Fattahi, Julio F. Navarro, Tom Theuns, Richard G. Bower, Robert A. Crain, Michelle Furlong, Adrian Jenkins, Matthieu Schaller, Joop Schaye

In this paper, we look at how the appearance of dark halos that have failed to form galaxies changes the relation between galaxies and dark matter halos.

Abundance matching is a very neat method of statistically linking (simulated) dark matter halos to (observed). It requires no detailed knowledge about galaxy formation physics and just assumes that each halo contains exactly one galaxy, with brighter galaxies living in more massive halos. From these  simple assumptions, one can derive average the stellar mass – halo mass relation for all galaxies.

It had been argued (including by myself) that the average relation inferred from abundance matching does not match the values measured for individual dwarf galaxies, whether by observations or direct simulations. This has been interpreted as a problem for the LCDM model, which seemed to produce too many halos. However, what we show in our new paper is that the simple assumption of one galaxy per halo breaks down for low mass halos, because many of them do not host a galaxy at all. We find that once these “dark” halos and other baryonic effects are taken into account, the stellar-halo mass relation bends upwards and matches the observations.

From Fig. 4: The classic abundance matching relation (black line) does not match the data (squares and triangles) at the low mass end. However, after the relation gets bent by baryons (red line), the disagreement is resolved.

If you’d like to read more, please take a look at our paper.

Collaborators: Carlos S. Frenk, Azadeh Fattahi, Julio F. Navarro, Richard G. Bower, Robert A. Crain, Claudio Dalla Vecchia, Michelle Furlong, Adrian Jenkins, Ian G. McCarthy, Yan Qu, Matthieu Schaller, Joop Schaye, Tom Theuns