Category Archives: Science

Paper: The Sibelius Project – E pluribus Unum

We introduce “Simulations Beyond The Local Universe” (SIBELIUS), a set of cosmological constraint simulations, that connect the Local Group to its cosmic environment. We show that introducing hierarchical small-scale perturbations to a density field constrained on large scales by observations provides an efficient way to explore the sample space of Local Group analogues.

Four Local Group realisations, created by varying intermediate scales. Being set on larger scales, the total mass, total energy and orientation are nearly identical. However, the separation of the two main haloes varies greatly between individual realisations.

Four Local Group realisations of the Sibelius project, created by varying intermediate scales. Being set on larger scales, the total mass, total energy and orientation are nearly identical. However, the separation of the two main haloes varies greatly between individual realisations.

Using the methods described in our previous paper, we create more than 60,000 simulations to identity we identify a hierarchy of Local Group characteristics emanating from different scales. We find that the total mass, orientation, orbital energy and the angular momentum are largely determined by modes above λ = 1.6 comoving Mpc (cMpc) in the primordial density field.

Smaller scale variations are mostly manifest as perturbations to the Milky Way – M31 orbit, and we find that the observables commonly used to describe the Local Group — the Milky Way – M31 separation and radial velocity — are transient, and depend on specifying very small scales down to 0.2 cMpc in the primordial density field.

View of a Local Group analogue from the Sibelius Project

Detailed view of a Local Group analogue from a dark matter simulation of the Sibelius project. The two large haloes are analogues of the Milky Way and M31.

We also find that the presence of M33/LMC analogues significantly affects the Milky Way – M31 orbit and its sensitivity to small-scale perturbations. We construct initial conditions that lead to the formation of a Local Group whose primary observables precisely match the current observations.

Paper: Setting the Stage – Structure Formation from Gaussian Random Fields

Cosmological simulations evolve a density field, specified at high redshift, forward in time. One of the tenets of the Inflationary Model is that the initial density fluctuations were created by quantum fluctuations within a fraction of a second during the Big Bang. Any density field can be described by the amplitudes of the fluctuations (the Power spectrum), which are determined by the cosmological model, and the phase information, which determines the specific peaks and dips in the density at different locations. A common way to parameterise this information is using Fourier modes.

In this paper, we use a new method for setting the initial conditions for cosmological simulations, building on earlier work by Adrian Jenkins, to specify the phase information. Instead of Fourier modes, we use orthogonal Octree basis functions, which have the benefit of being localised in space. By creating many variations of density fields, changing the phase information on different levels of the Octree (different spatial scales), we measure which scales in the very early universe effect the foŕmation of haloes we can observe today.

Dark matter density (left panel), an the difference in dark matter density in the same regions, when variations are introduced at different levels of the Octree.

Dark matter density (left panel), an the difference in dark matter density in the same regions, when variations are introduced at different levels of the Octree. Ast the scale of variations in the initial conditions increases from left to right, the difference in the final dark matter density grows. Smaller haloes disappear (and new ones appear), while larger haloes change in mass and position.

We quantify, for example, on what scales the information for objects like the Milky Way or the Virgo Galaxy cluster is defined. We also quantify how perturbations of the density field on smaller scales affect observable properties of the system, such as the mass, concentration, or position.

The level of the Octree (top axis) and the equivalent length scale (bottom axis) at which the density field needs to be specified in order for haloes of different masses to be uniquely defined. For example, Milky Way mass haloes (green line) are defined by density fluctuations of approximately 1.6 Mpc.

Beyond the theoretical interest, this method of setting the initial conditions, and varying the density field has important practical applications. By iteratively randomising the phase information, we can explore the sensitivity of observables to the initial conditions, and create objects that closely match observations – all while preserving the Gaussian nature of the initial density field created by the quantum fluctuations of the Big Bang.

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

 

English

  • The Daily Mail Online: Are we on the brink of finding dark matter?
  • Phys.org: ‘Cosmic own goal’ another clue in hunt for dark matter
  • Motherboard.vice.com: 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

Dutch

  • Astronomie.nl: Eerste sterren hinderden de vorming van sterrenstelsels

Spanish

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

Italian

  • INAF: Aloni sterili e materia oscura

 

Sources

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