Category Archives: Dwarf Galaxies

The APOSTLE collaboration

The APOSTLE collaboration is “A Project Of Simulating The Local Environment”.

An offshoot of the EAGLE collaboration, it was started by myself, Carlos Frenk, Azadeh Fattahi and Julio Navarro in 2013, and has now grown to a large international collaboration. To date, more than 20 papers based directly on APOSTLE data have been written by more than 30 different co-authors from Argentina, Belgium, Canada, Chile, China, Finland, Germany, Iran, Ireland, Italy, Mexico, the Netherlands, Poland, Romania, Switzerland, the UK and the US.

Click below for a list of APOSTLE papers published to date. We also keep a repository for ongoing projects (password required). If you are interested in using APOSTLE data for your own project, please send me an email (till+DOT+sawala+AT+helsinki+DOT+fi)

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Beyond LCDM – science, scenery and a prize!

From January 14-17, Oslo’s famous Holmenkollen played a fitting backdrop to the remarkable Beyond LCDM conference, an assembly of cosmologists from all over the world, working within and without the so-called standard model.

The conference venue on the Holmenkollen View over Oslo from the Holmenkollen

For those of us who have learned to refer to LCDM as the “standard model” of cosmology, it gave an insight into the range of alternatives out there, while those in the community hoping to slay the LCDM dragon were reminded that, outrageous and ugly as it may seem, LCDM continues to be notoriously tough to beat with observational evidence.

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IAU Symposium 311: Galaxy Masses as Constraints of Formation Models

For IAU Symposium 311, I have prepared an augmented reality poster using DARO. The augmented reality elements will work if the poster is viewed on your screen just like the printed poster.

If you have just scanned the QR code using your phone, but you are reading this message, you probably have not installed the DARO viewer yet. Please speak to me at the IAU meeting – I’ll be happy to lend you my tablet. Alternatively, you can download the latest version of DARO for your phone from the DARO project website:

DownloadDaro DownloadDARO.vuforia


DARO is developed by Jascha Schewtschenko at Durham University. By downloading and installing DARO, you agree to its license. In the future, please always visit the project website for the latest version.

Paper: which low mass halos host galaxies?

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.

V1_MR_reion_z0 V1_MR_no_reion_z0   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. FrenkAzadeh FattahiJulio F. NavarroTom TheunsRichard G. BowerRobert A. CrainMichelle FurlongAdrian JenkinsMatthieu SchallerJoop Schaye


In the news: cosmic own goal leaves galaxies dark

Our paper “The chosen few: the low mass halos that host faint galaxies” has been in the news!

Collaborators: Carlos S. FrenkAzadeh FattahiJulio F. NavarroTom TheunsRichard G. BowerRobert A. CrainMichelle FurlongAdrian JenkinsMatthieu SchallerJoop Schaye



  • The Daily Mail Online: Are we on the brink of finding dark matter?
  • ‘Cosmic own goal’ another clue in hunt for dark matter
  • Dark Matter Halos are Sad Would-Be Galaxies
  • Red OrbitIn the Hunt For Dark Matter, New Simulations Show Evolution Of “Local Universe”
  • ANI News: Understanding formation of galaxies could solve mystery of dark matter
  • Science World Report: Supercomputer Simulations Chart the Evolution of the Local Universe
  • Science 2.0: Just In Time For The World Cup, The Cosmos Scores A Dark Matter Own Goal


  • Eerste sterren hinderden de vorming van sterrenstelsels


  • Tendencias 21: Nuevos descubrimientos nos acercan a la materia oscura


  • INAF: Aloni sterili e materia oscura



Paper: bent by baryons

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.

Bent By Baryons

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. FrenkAzadeh FattahiJulio F. NavarroRichard G. BowerRobert A. CrainClaudio Dalla VecchiaMichelle FurlongAdrian JenkinsIan G. McCarthyYan QuMatthieu SchallerJoop SchayeTom Theuns