Dark Matter Substructure in the Simulations and Observed Universe

2212008 Tuorla Observatory

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Dark Matter Substructure in the Simulations and Observed Universe

P Nurmi


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Pure N-body vs Hydro

Collisionless N-body (DM only) simulations (accurate solution to an idealized problem)- Ωm is WIMP and is distributed as N particles- problems in the center of galaxies where baryons dominate- only gravity- high resolution- no free parameters (ICs taken from CMB)Hydrodynamical simulations (approximate solution to a more realistic problem)- computationally expensive relatively low resolution- complicated (SPH and grid codes often disagrees)- important physical processes typically act on scales far below resolution and are implemented through uncertain functions and free parameters


3

Cosmological N-body Simulations

Our simulations 6 different simulations with 3 different resolutions and 2 different simulation codes (AMIGA and GADGET-2)

Louhi Cray XT4


4

Subhalo-galaxy connection

For large halos Mtotasymp 1013 - 1015

MSunhMain halo= massive ldquoellipticalrdquo galaxy

Substructure = normal galaxies

For small halos Mtotasymp 1011 - 1013

MSunhMain halo = typical spiral galaxy

Substructure = dwarf galaxy

5-10 of total mass are in substructures dNdm~m-18plusmn01


5

Substructure in the DM (only) simulations

Two sets of slides

1 Z-evolution of all halos in the 40 Mpch simulation An interesting region is shown with several merger events

2 Zoom of substructure in the 20 Mpch simulation of a system with 241013 MSunh and containing 275 subhalos Subhalo masses are between 109 MSunh and 1011 MSunh


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8


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Mass accretion history of subhalosZentner amp Bullock ApJ 598 2005(semi-analytic)

Most accretedsubhalos are destroyed

Some general results confirmed by many studies

1 Most of the mass is accreted in large ~1011Msun subhalos

2 Majority of accreted systems are destroyed before z=0

3 Surviving substructure is generally young


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Dynamical and physical evolution of subhalos

Tidal effects- mainhalo-subhalo encounters subhalo-subhalo encountersDepends on the halo profile and halo masses- Leads to massloss profile changes etc

Dynamical frictionDynamical friction arises because of the wake of particles that grow behind the motion of particle due to gravitational focusing- Orbital changes


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Some problems concerning substructure

bull Overmerging a problem related to resolution

(White (1976) van Kampen (1995))

bull Abundance of CDM structure match galaxy abundance in clusters but not in local group satellites

(Moore at al (1999))

bull Spatial distribution of subhalos they are too far from the center

(Diemand (2004))

bull Some improvement by selecting subhalos according to

mass (or circular velocity) before accretion

(Nagai amp Kravtsov (2005) Conroy at al (2006))


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Large-scale galaxy clustering

Two-point correlation functions calculated from the halos in ΛCDM-simulations and galaxies from SDSS agree very well (Conroy et al 2006 ApJ 647)-gt dots = SDSS solid line = ART simulations 512sup3 in (80 Mpch)sup3

Similar results from Virgo Consortium simulations in larger scales (Springel et al 2005 Nature 435)-gt 2160sup3 in (500 Mpch)sup3

Also the galaxy formation physics incorporated in the SPH simulation give a good account of observed galaxy clustering (Weinberg et al 2005 ApJ 601) [144sup3 in (50 Mpch)sup3 cube]


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Comparison between SDSS galaxy data and our simulations

Abell 2151 The Hercules Galaxy Cluster

SDSS DR5 data ΛCDM simulations

Typical halo with several subhalos (galaxies)

Rvir

The 25-meter SDSS survey telescope


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How to populate halos with galaxies(a major problem to DM-simulations)

We can use a simplified procedure (varying ML function) that is based on the analytical fit that gives luminosity when halo mass is given (Vale amp Ostriker 2004 MNRAS 353)

We test if this is statistically satisfied by using another method in which suitable galaxies that resemble DM halos and subhalos are selected from the Millenium run semi-analytic galaxy catalogue


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SDSS DR5 galaxy group sample

Observational ingredient is based on the galaxy group catalogue by Tago et al (2007)

From this data we select three volume limited samples based on the group distance dlt100 Mpch dlt200 Mpch and dlt300 Mpch and SDSS completeness limit mr(lim)=175 This gives us three luminosity limits for galaxies that are included in the analysis


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Comparison 1 Richness


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Comparison 2a Luminosity (all galaxies that have L gt Llim(d) are included for observations Lgroup is

corrected for invisible galaxies)


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But what about small subhalos around Milky Way sized halos

A simple DM halo mass ndash Luminosity correlation does not work anymore

Too many subhalos if compared with observed dwarf galaxies

ldquoClassicalrdquo Dwarf Galaxy Problem

(Moore et al 1999)


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Scientific context small-scale galaxy clustering -gt missing dwarf problem

Basically all cosmological simulations predict that there are at least one order of magnitude more small subhalos (dwarf galaxies) around Milky Way like galaxies than what is observed (eg Via Lactea simulation Diemand et al 2007 ApJ 657)-gt234 million particles in (90 Mpch)sup3 multimass simulation mp=20900 Msun

Recently discovered (from SDSS data) ultra-faint dwarfs with ML~1000 help to solve this discrepancy but not fully (factor of 4 difference) However If reionization occurred around redshift 9 minus 14 and dwarf galaxy formation was strongly suppressed thereafter the circular velocity function of Milky Way satellite galaxies approximately matches that of CDM subhalos in Via Lactea simulation (Simon and Geha 2007 astro-ph 07060516)


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Can MW dwarfs be used at all for comparison

(Kroupa et al 2005AampA 431 517)

ldquoThe shape of the observed distribution of Milky Way (MW) satellites is inconsistent with their being drawn from a cosmological sub-structure population with a confidence of 995 per cent Most of the MW satellites therefore cannot be relate to dark-matter dominated satellitesrdquo

If the MW dwarfs do indeed constitute the shining fraction of DM sub-structures then their number-density distribution should be consistent with an isotropic (ie spherical) or oblate power-law radial parent distribution


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But also in simulations accretions are an-isotropic and large subhalos tend to be more accreted along the major axis of the host halo

Consistent if the major axis of MW halo is perpendicular to Galactic disk (Kang Mao Jing Gao 2005)

Great Disk (pancake) has thickness ~ 20kpc~ perpedicular to the MW disk



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Other Groups(Karachentsev AJ 129 178 2005)

- Good targets (M31 M81 M83)

- There is maybe some signal but it is much weaker


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Radial distribution of subhalos

(Willman et al MNRAS 353 (2004) 639-646 )

Incompleteness needs to be taken seriously

Radial distribution of the oldest subhalos in a Lambda+CDM simulation of a Milky Way-like galaxy possess a close match to the observed distribution of M31s satellites which suggests that reionization may be an important factor controlling the observability of subhalos


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Observational signature of substructure

2 Gravitational Lensing - Galaxy substructure may explain the flux ratio anomalies observed in multiply-imaged lensed QSOs- Milliarcsecond scale image splitting of quasars that are known to be splitted on arcsecond level (Zackrisson et al 2008)One major problem is the density profile of small subhalos

1 Satellite Galaxies of MW Most massive DM subhalos are associated with luminous dSph satellites Problem most dark matter subhalos appear to have no optically luminous counterparts in the Local Group (ldquomissing satellite problemrdquo)


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Is it possible to observe substructure by strong gravitational lensing


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3 Dark Matter AnnihilationBecause of their high phase-space densities subhalos may be detectablevia γ-rays from DM particle annihilation in their cores (Diemand Kuhlen amp Madau 2006) (GLAST VERITAS)

4 Tidal streams Presence of a population of CDM clumps alters the phase-space structure of a globular cluster tidal stream If the global Galactic potential is nearly spherical this corresponds to a broadening of the stream from a thin great-circle stream into a wide band on the sky (Ibata et al 2002) (GC streams detectable by GAIA)


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5 Signatures of long-term dynamical effects of subhalos to galaxiesSatellite-disk encounters of the kind expected in CDM models can induce morphological features in galactic disks that are similar to those being discovered in the Milky Way M31 and in other nearby and distant disk galaxies (Kazantsidis et al 2007)


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Conclusions

Cdm models predict several close encounters of massive subhalos with the galactic disks since zlt1 Unless a mechanism (gas accretion) can somehow stabilize the disks to these violent gravitational encounters stellar disks as old and thin as the Milky Wayrsquos will have severe difficulties to survive typical satellite accretion within ΛCDM

Kazantzidis 2007 arXiv07081949v1


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Summary

The galaxy-halo-subhalo-DM connection is not yet fully understood

Slide 6


2

Pure N-body vs Hydro

Collisionless N-body (DM only) simulations (accurate solution to an idealized problem)- Ωm is WIMP and is distributed as N particles- problems in the center of galaxies where baryons dominate- only gravity- high resolution- no free parameters (ICs taken from CMB)Hydrodynamical simulations (approximate solution to a more realistic problem)- computationally expensive relatively low resolution- complicated (SPH and grid codes often disagrees)- important physical processes typically act on scales far below resolution and are implemented through uncertain functions and free parameters


3



Louhi Cray XT4


4










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Two sets of slides




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(Diemand (2004))





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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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Louhi Cray XT4


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Two sets of slides




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(Diemand (2004))





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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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Two sets of slides




7


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(Diemand (2004))





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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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Two sets of slides




7


8


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10


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(Diemand (2004))





14






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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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(Diemand (2004))





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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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(Diemand (2004))





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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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(Diemand (2004))





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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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(Diemand (2004))





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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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(Diemand (2004))





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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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(Diemand (2004))





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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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(Diemand (2004))





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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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Rvir



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(Moore et al 1999)


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Conclusions




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Summary


Slide 6


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(Moore et al 1999)


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Summary


Slide 6


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(Moore et al 1999)


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Summary


Slide 6


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(Moore et al 1999)


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Slide 6


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(Moore et al 1999)


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Summary


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(Moore et al 1999)


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Slide 6


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Slide 6


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Slide 6

Dark Matter Substructure in the Simulations and Observed Universe

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Transcript of Dark Matter Substructure in the Simulations and Observed Universe