Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

The small scale structure of the Universe Stefan Gottl¨ober (Astrophysikalisches Institut Potsdam)

Barcelona May 12th, 2010

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Collaborators Gustavo Yepes (UAM, Madrid) Yehuda Hoffman (HU, Jerusalem) Anatoly Klypin (NMSU, Las Cruces) Matthias Steinmetz (AIP, Potsdam) Jesus Zavala (MPA, Garching) Anton Tikhonov (St. Petersburg) Noam Libeskind (AIP, Potsdam) Jaime Forero (AIP, Potsdam) Juan Carlos Munoz (AIP, Potsdam) Kristin Riebe (AIP, Potsdam)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

1

Cosmological simulations Observational background What are the ingredients of numerical simulations? Dark matter simulations

2

Constrained simulations

3

The Local Group simulations

4

Small scale structure and Warm Dark Matter

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Cosmic evolution

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Cosmic Microwave Background (CMB) radiation

first observed structures, tiny density fluctuations (10−5 )

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

What causes the formation of the observed structure?

gravitational instability =⇒ density perturbations grow they become nonlinear we need (super-)computers to follow their evolution

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Dark Matter Observed by gravitational lensing Galaxy cluster Abell 1689 Gravitational lensing as predicted by Einstein’s General Relativity The gravitative mass is much larger than the mass seen in light and X-ray =⇒ Dark Matter

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Dark Matter What do we know? about 85 per cent of all matter must be dark dark matter is responsible for the large scale structure all galaxies and clusters live in dark matter halos probably not in all dark matter halos live galaxies dark matter must be made of weakly interacting non-baryonic more or less cold =⇒ small scale structure

particles

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Dark Matter (an artist’s view)

Cornelia Parker Cold Dark Matter: An Exploded View 1991, Mixed media Presented by the Patrons of New Art (Special Purchase Fund) through the Tate Gallery Foundation 1995 http://www.tate.org.uk/modern/exhibitions/cinema/parker.htm

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Dark Energy

Supernova Cosmology Project Knop et al. (2003)

ΩΜ , ΩΛ 0.25,0.75 0.25, 0 1, 0

24

effective mB

observed magnitude vs. distance

Supernova Cosmology Project

22

Observed by distant supernovae

20

high redshift supernovae are fainter than expected in standard cosmology

18

Calan/Tololo & CfA 16

14 0.0

0.2

0.4

0.6

0.8

1.0

=⇒ Accelerating force

redshift z

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Dark Energy What do we know? about 70 per cent of the total energy in the universe is unknown this dark energy is responsible for the acceleration of expansion experiments are planned to measure the evolution of the dark energy Simplest explanation: Cosmological constant ( = vacuum energy = additional constant term in Einstein’s field equation) some still unknown (scalar) field similar to the (still unknown) field which drives inflation something else Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Cosmic parameters 2009 (WMAP5+BAO+SN)

dark matter density ΩDM = 0.214 cosmological constant (dark energy) ΩΛ = 0.742 baryon density Ωb = 0.044 ΩΛ + ΩDM + Ωb = 1 (spatially flat universe) Hubble parameter h = 0.70 slope of the initial power spectrum n = 0.96 normalization σ8 = 0.817 =⇒ Precision cosmology

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Ingredients 21 % Dark Matter 74 % Dark Energy 5 % Baryons Computational very expensive ”sub-grid” physics not (yet) well modeled gas cooling star formation, evolution and feedback on gas chemical evolution formation of super-massive black holes and their influence on galaxies magnetic fields ...

Post-processing of DM simulations (semianalytical models)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Expansion and comoving coordinates

Background expansion according to General Relativity (Friedman equation) Constant vacuum energy leads to accelerated expansion about 7 Gigayears after Big Bang Periodic boundary conditions Physical coordinates vs. comoving coordinates (movies do not show expansion)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Structure formation on all scales

Large-scale structure Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

Individual haloes The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Power spectrum of the density fluctuations

Linearly evolved power spectrum Possible representation in boxes with 10243 particles. High resolution with 40963 particles.

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Dark matter only simulations

advantages large simulation volumes possible high resolution possible straightforward parallel algorithm

disadvantages no gas, no stars, no light

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

20483 DM particles 250h−1 Mpc WMAP5 5 million CPU hours

B o l s h o i Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

Observational background What are the ingredients of numerical simulations? Dark matter simulations

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Bolshoi - mass function

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational background What are the ingredients of numerical simulations? Dark matter simulations

Limitations of present day simulations

Representative volume (> 500h−1 Mpc) ⇐⇒ desired resolution (< 0.1h−1 kpc) =⇒ impossible on present computers Mass range (108 ...1015 h−1 M ) ⇐⇒ mass resolution (> 1000 particles) =⇒ impossible on present computers there are also limits of present day observations =⇒ simulate a smaller volume representative for the neighborhood of Milky Way

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

CLUES Constrained Local Universe Simulations

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations

The CLUES LocalConstrained Group simulations

The Local Group simulations Small scale structure and Warm Dark Matter

Milky Way

Andromeda

M33

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Why are we interested in constrained simulations?

The local neighbourhood of the Milky Way is the most well known piece of the universe. Thus it is an ideal place to test on small scales models of structure formation against observations. However, the local universe is not a representative part of the universe. Hudson (1993)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Observational data and constraints constraining Gaussian random fields (Hoffman & Ribak, 1991) radial velocity field (MARK III, Willick et al., 1997, Tonry 2001, Karachentsev 2004) nearby cluster positions (Reiprich & B¨ohringer, 2002)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

160h−1 Mpc

64h−1 Mpc

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

The Local Group simulations

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Local Group simulations

box 64h−1 Mpc constrained simulation r = 2h−1 Mpc sphere, 40963 mass resolution with Local Group (MW, M33, M31) mass resolution DM: 2.1 × 105 h−1 M mass resolution gas: 4.4 × 104 h−1 M force resolution: 0.15h−1 kpc

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Gas distribution in the local group

flying to M33 gas evolution in the LG

Kristin Riebe

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Evolution of M31 and MW

infall of M31 satellites

mass accretion histories of the dark matter halos hosting the Milky Way and M31 infall of MW satellites

Steffen Knollmann

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Tidal streams of a satellite (infall at z = 0.845)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

Kristin Riebe The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Baryons vs. Dark Matter

density profile

satellite distribution

Libeskind et al. (2009)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Preferential infall

Libeskind et al. (2010)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Small Scale Structure

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Missing satellites The number of observed satellites is an order of magnitude smaller than the predicted number of subhalos. possible solutions: DM subhalos are more massive than assumed suppression of star formation no scale invariance of the power spectrum Warm Dark Matter (small scale power erased) a different inflationary model Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Cold vs. Warm Dark Matter WMAP3 h = 0.73 Ωm = 0.24 Ωbar = 0.042 σ8 = 0.73 n = 0.95 mWDM = 1keV lower limit kpeak = 3.7hMpc−1 less small scale power =⇒ less small scale structure

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

CDM

WDM

How does the nearby universe look like for an observer situated at the simulated MW? Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Simulated sky map (Virgo/Fornax best fit)

dots: halos with M > 5 × 109 h−1 M squares: Virgo and Fornax, circles: their simulated counterparts Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Arecibo Legacy Fast ALFA (ALFALFA) survey

blind HI survey, started February 4, 2005 (6-7 years expected) detection of 20,000 galaxies expected within 200 Mpc gas rich galaxies with only a few or no stars (”dark”) two arrays (Virgo and anti-Virgo)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

ALFALFA observations in Virgo direction velocity function squares with error bars: galaxies taken from the ALFALFA catalog with distances lower than 20h−1 Mpc predictions from the constrained simulation

Zavala et al. (2009)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

ΛCDM: dashed red area ΛWDM: dotted red area dashed/dotted line: disk baryon fraction as function of halo mass (SN feedback) The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Spectrum of mini-voids in the local volume R < 8h−1 M

Warm Dark Matter

Cold Dark Matter

Tikhonov et al. (2009)

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

The end

Numerical simulations are an important tool to study the formation of structure in the universe. Satellites tend to enter our galaxy from a preferred direction. The matter stripped from these subhalos retains a memory of that direction. The velocity function of nearby ALFALFA galaxies as well as the spectrum of mini-voids in the Local Volume point to a possible problem of the ΛCDM model on small scales. Warm Dark Matter could be one solution to this long standing problem of overabundance of small scale structure.

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe

Cosmological simulations Constrained simulations The Local Group simulations Small scale structure and Warm Dark Matter

Acknowledgement

The numerical simulations shown in this talk have been performed and analysed at BSC Barcelona NIC Juelich LRZ Munich NAS Ames We thank for the support.

Stefan Gottl¨ ober (Astrophysikalisches Institut Potsdam)

The small scale structure of the Universe