Transcript www.astro.utu.fi

22.1.2008, Tuorla Observatory
Dark Matter Substructure in the
Simulations and Observed Universe
P. Nurmi
1
22.1.2008, Tuorla Observatory
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
2
22.1.2008, Tuorla Observatory
Cosmological N-body Simulations
Louhi: Cray XT4
Our simulations: 6 different simulations with 3 different resolutions and
2 different simulation codes (AMIGA and GADGET-2):
3
22.1.2008, Tuorla Observatory
Subhalo-galaxy connection?
For large halos Mtot≈ 1013 - 1015
MSun/h:
Main halo= massive “elliptical” galaxy
Substructure = normal galaxies
For small halos Mtot≈ 1011 - 1013
MSun/h:
Main halo = typical spiral galaxy
Substructure = dwarf galaxy
5-10% of total mass are in substructures
dN/dm~
m-1.8±0.1
4
22.1.2008, Tuorla Observatory
Substructure in the DM (only) simulations?
Two sets of slides:
1. Z-evolution of all halos in the 40 Mpc/h simulation. An
interesting region is shown with several merger events.
2. Zoom of substructure in the 20 Mpc/h simulation of a
system with 2.4·1013 MSun/h and containing 275 subhalos.
Subhalo masses are between 109 MSun/h and 1011 MSun/h.
5
22.1.2008, Tuorla Observatory
7
22.1.2008, Tuorla Observatory
8
22.1.2008, Tuorla Observatory
Mass accretion history of subhalos:
Zentner & Bullock, ApJ, 598, 2005
(semi-analytic)
Most accreted
subhalos 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
.
9
22.1.2008, Tuorla Observatory
10
22.1.2008, Tuorla Observatory
11
22.1.2008, Tuorla Observatory
Dynamical and physical evolution of
subhalos
Tidal effects:
- mainhalo-subhalo encounters, subhalo-subhalo encounters
Depends on the halo profile and halo masses
- Leads to massloss, profile changes etc.
Dynamical friction:
Dynamical friction arises because of the wake of particles that grow
behind the motion of particle due to gravitational focusing.
- Orbital changes
12
22.1.2008, Tuorla Observatory
Some problems concerning substructure
• Overmerging, a problem related to resolution
(White (1976), van Kampen (1995))
• Abundance of CDM structure match galaxy abundance in
clusters, but not in local group satellites
(Moore at al. (1999))
• Spatial distribution of subhalos: they are too far from the
center
(Diemand (2004))
• Some improvement by selecting subhalos according to
mass (or circular velocity) before accretion:
(Nagai & Kravtsov (2005), Conroy at al. (2006))
13
22.1.2008, Tuorla Observatory
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)-> dots = SDSS, solid line =
ART simulations 512³ in (80 Mpc/h)³
Similar results from Virgo Consortium
simulations in larger scales (Springel et al.
2005, Nature, 435)-> 2160³ in (500 Mpc/h)³
Also the galaxy formation physics incorporated
in the SPH simulation give a good account of
observed galaxy clustering (Weinberg et al.
2005, ApJ 601). [144³ in (50 Mpc/h)³ cube]
14
22.1.2008, Tuorla Observatory
Comparison between SDSS galaxy data and
our simulations ?
ΛCDM simulations
SDSS DR5 data
The 2.5-meter
SDSS survey
telescope
?
Rvir
Typical halo with several subhalos (galaxies)
Abell 2151: The Hercules Galaxy Cluster
15
22.1.2008, Tuorla Observatory
How to populate halos with galaxies
(a major problem to DM-simulations) ?
We can use a simplified
procedure (varying M/L
function) that is based on the
analytical fit that gives
luminosity when halo mass is
given (Vale & 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.
16
22.1.2008, Tuorla Observatory
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; d<100 Mpc/h, d<200 Mpc/h and d<300 Mpc/h; and SDSS completeness
limit mr(lim)=17.5. This gives us three luminosity limits for galaxies that are
included in the analysis.
17
22.1.2008, Tuorla Observatory
Comparison 1: Richness ?
18
22.1.2008, Tuorla Observatory
Comparison 2a: Luminosity ?
(all galaxies that have L > Llim(d) are included, for observations Lgroup is
corrected for invisible galaxies)
19
22.1.2008, Tuorla Observatory
But what about small subhalos around
Milky Way sized halos?
“Classical” Dwarf Galaxy
Problem:
A simple DM halo mass – Luminosity correlation does not work anymore
Too many subhalos if compared with observed dwarf galaxies
(Moore et al. 1999)
20
22.1.2008, Tuorla Observatory
Scientific context: small-scale galaxy
clustering -> 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 (e.g. Via Lactea simulation
Diemand et al. 2007, ApJ 657)->234 million particles in (90 Mpc/h)³ multimass simulation, mp=20900 Msun
Recently discovered (from SDSS data) ultra-faint dwarfs with M/L~1000 help to solve this discrepancy, but not
fully (factor of 4 difference). However, If reionization occurred around redshift 9 − 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, astroph. 0706.0516)
21
22.1.2008, Tuorla Observatory
Can MW dwarfs be used at all for
comparison?
(Kroupa et al. 2005A&A, 431, 517)
“The 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
99.5 per cent. Most of the MW satellites
therefore cannot be relate to dark-matter
dominated satellites.”
If the MW dwarfs do indeed
constitute the shining fraction of DM
sub-structures, then their numberdensity distribution should be
consistent with an isotropic (i.e.
spherical) or oblate power-law radial
parent distribution.
22
22.1.2008, Tuorla Observatory
Can MW dwarfs be used at all for
comparison?
Great Disk (pancake) has thickness ~ 20kpc
~ perpedicular to the MW disk
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)
23
22.1.2008, Tuorla Observatory
Other Groups?
(Karachentsev, AJ, 129, 178,
2005)
- Good targets (M31, M81, M83)
- There is maybe some signal,
but it is much weaker
24
22.1.2008, Tuorla Observatory
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 M31's
satellites, which suggests that
reionization may be an important
factor controlling the observability
of subhalos.
25
22.1.2008, Tuorla Observatory
Observational signature of substructure
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 (“missing satellite
problem”).
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.
26
22.1.2008, Tuorla Observatory
Is it possible to observe substructure by
strong gravitational lensing ?
27
22.1.2008, Tuorla Observatory
Is it possible to observe substructure by
strong gravitational lensing ?
28
22.1.2008, Tuorla Observatory
Observational signature of substructure
3. Dark Matter Annihilation:
Because of their high phase-space densities, subhalos may be detectable
via γ-rays from DM particle annihilation in their cores (Diemand,
Kuhlen, & 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)
29
22.1.2008, Tuorla Observatory
Observational signature of substructure
5. Signatures of long-term dynamical effects of subhalos to galaxies:
Satellite-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)
30
22.1.2008, Tuorla Observatory
Conclusions
Kazantzidis, 2007,
arXiv:0708.1949v1
Cdm models predict several
close encounters of massive
subhalos with the galactic disks
since z<1.
Unless a mechanism (gas accretion?) can
somehow stabilize the disks to these
violent gravitational encounters stellar
disks as old and thin as the Milky Way’s
will have severe difficulties to survive
typical satellite accretion within ΛCDM.
31
22.1.2008, Tuorla Observatory
Summary
The galaxy-halo-subhalo-DM
connection is not yet fully understood !
32