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WHERE ARE THE MISSING GALACTIC SATELLITES?
V(r) = (GM(r)/r)1/2
The LCDM model predicts that thousands of dwarf DM haloes
should exist in the Local Group while only ~50 are observed.
Klypin et al.,1999, Moore et al.,1999, Madau et al., 2008
Currently favored scenario for explanation of the overabundance of the dark matter
subhaloes assumes that dwarf haloes above Vmax ~ 30-50 km/s were forming stars
before they fall into the Milky Way or M31 and that smaller haloes never formed
any substantial amount of stars.
Bullock et al., 2000, Kravtsov et al., 2004
2001
2008
astro-ph/0101127
astro-ph/0804.2475
Anton Tikhonov /SPbSU
Anatoly A. Klypin /NMSU; Stefan Gottloeber /AIP; Gustavo Yepes /UAM
The emptiness of voids in LWDM-model: possibility to
escape LCDM-overabundance?
1. LCDM model faces the same overabundance problem, which it had with the number of satellites in
the LG: the theory predicts a factor of 10 more haloes as compared with the observed number of dwarf
galaxies. (A.V. Tikhonov, A. Klypin, 2008, arXiv:0807.0924)
2. Problems faced by CDM-models on Mpc and sub-Mpc scales motivated reconsideration of WDM
(Warm Dark Matter)-models with typical masses of particles mX around 1keV
which decouple being still ultra-relativistic.
8
Local Volume
Local (Tully) supercluster
6
Z equatorial (Mpc)
4
2
Local
Void
0
-2
-4
-6
-6
-4
-2
0
2
4
Y equatorial (Mpc)
6
8
How small can be a galaxy?
Below some mass the haloes are expected to stop producing galaxies inside them.
There are different arguments for that:
stellar feedback (Dekel, 1986) or
photoionization (Bullock, 2000)
may play a significant role in quenching starformation in too small haloes.
For example, (Loeb, 2008) made asimple estimation of the limiting circular velocity Vlim below
which haloes have essentially no gas infall due to increase of Jeans mass caused by UV
background at the epoch of reionization:
Vlim = 34 В· (TIGM /1.5 В·104 K)1/2 km/s,
where TIGM is thetemperature of intergalactic medium gas ionized by stars.
(Hoeft, 2006) studied formation of dwarf DM haloes in cosmologicalvoid regions using highresolution hydrodynamic simulations andassuming that cosmological UV-background photoevaporates baryons out of haloes of dwarf galaxies, and thereby limits their cooling and
starformation rate.
(Hoeft, 2006) give characteristic mass MC = 6 В· 109 h-1Msun
below which haloes start to fail accreting gas.
We compare the spectrum of void sizes in the Local Volume galaxy sample with the distribution
of voids in high-resolution cosmological simulations. The simulations give us detailed
information (positions, velocities, masses, circular velocities, and so on) for dark matter haloes
and their satellites. Yet, they do not provide luminosities of galaxies..Theoretical predictions of
the luminosity of a galaxy hosted by a halo with given mass, circular velocity and merging
history are quite uncertain and cannot be used for our analysis.
Instead, we ask a more simple question:
What luminosity a halo or subhalo with given circular velocity should
have in order to reproduce the observed spectrum of void sizes?
V(r) = (GM(r)/r)1/2
Vc = 20km/s – Mvir~109 Mo
Vc = 50km/s – Mvir~1010 Mo
When doing this, we assume that haloes with larger circular velocities should host more luminous
galaxies. We will later see that matching of the void spectrum in simulations and with the
observations puts significant constraint on relation of the halo circular velocity and the
luminosity of a galaxy hosted by the halo. If we take too large circular velocity, there are too few
galaxies and sizes of voids become too large. Instead, if very smallhaloes host galaxies, the
number of large voids declines well below what is observed in the Local Volume.
Description of the Local Volume galaxy sample
The first compilation of a Local volume (LV) sample of galaxies situated
within 10Mpc was made by Kraan-Korteweg & Tammann (1979) who published a
list of 179 nearby galaxies with radial velocities VLG < 500 km/s
In his Catalog and Atlas of Nearby Galaxies, Tully (1988) noted the
presence in the Local Supercluster (LSC) that consists of number of intersecting
filaments of the so-called Local void which begins directly from the boundaries of the
Local Group and extends in the direction of North Pole of the LSC by ~ 20 Mpc. The
Local void looks practically free from galaxies.
Later, Karachentsev (1994) published an updated version of the LV list,
which contained 226 galaxies with VLG < 500 km/s. Over the past few years, special
searches for new nearby dwarf galaxies have been undertaken basing on the optical
sky survey POSS-II/ESO/SERC, HI and infrared surveys of the zone of
avoidance, ��blind’’ sky surveys in the 21 cm line, HIPASS and HIJASS.
At the present time, the sample of galaxies with distances less than 10 Mpc
numbers about 550 galaxies. For half of them the distances have been measured to an
accuracy as high as 8-10% (Karachentsev et al., 2004=Catalog of Neighboring
Galaxies). Over the last 5 years, snapshot surveys with Hubble Space Telescope
(HST ) have provided us with the TRGB distances for many nearby galaxies.
The distances are not measured using theredshifts because the
perturbations of the Hubble flow in the LocalVolume are large and significantly
distort the spatial distribution of galaxies.
In spite from the presence of
local voids, the average density
of luminosity within the radius
of 8 Mpc around us exceeds
1.8 - 2.0 times the global
luminosity density. Almost the
same excess is also seen in the
local HI mass density. About
2/3 of the LV galaxies belong
to the known virialized groups
like the LG.
Karachentsev et al., 2007
Void detection algorithm
• 3D grid.
• Empty seed sphere of largest
possible radius Rseed is identified.
• Expansion of seed spheres by
spheres with radius Rsph > 0.9 Rseed
and with centers inside already
fixed part of a void.
• Next seed sphere is determined.
Process continues until Rseed > Rthreshold.
• Voids have flexible but still regular
shapes and are thick enough
throughout their volumes.
• Voids are defined to be completely
inside sample boundaries.
(El-Ad & Piran,1997, Gottloeber,
2003)
Then we considered
Cumulative void
function О”V/V(>Rvoid)
2D-case of point-like distribution. Seed circles and voids growing from
them are shown. The numerals indicate the order of identification of voids
Distribution of six largest minivoids within the LV-sphere of radius 7.5
Mpc.
Вј of the Local Volume is
occupied by Void in Aquila - front
part of the Local (Tully) Void
(Tikhonov & Karachentsev,
2006)
Luminosity functions of LV galaxies
The volume limited sample is complete for galaxies with abs. magnitudes MB < -12 within 8Mpc radius. Another volume
limited sample is MB < -10 within 4Mpc. Karachentsev (2004)noted that luminosity density in LV in B-band with respect to
the mean in the Universe about 1.8 - 2 times higher. Luminosity density is rather unstable characteristic in such a small
volume as LV.
-3
пЃ†пЂ пЂЁ Mpc )
0,1
0,01
LV MB<-12 R<8Mpc
LV MB<-10 R<4Mpc
2dFGRS
-22
-20
-18
-16
-14
-12
-10
MB
This values give us reasonable criteria for selection of "LV-candidates"-8Mpc spheres from
simulations. Since we observe that largest part of number and luminosity overdensity in LV with respect
to the universal values comes from brightest galaxies we used number overdensity criteria for ratio of
number density of halos with circular velocity Vc > 100 km/s in 8Mpc sphere to the mean
density of such halos in the whole simulated box – halos that must contain luminous galaxies.
Some LV-candidates fit geometry of LV-galaxy distribution
RMS Peculiar Velocity – deviations from the HUBBLE FLOW - σH
100
пЃі пЃ€ пЂ пЂЁ km/s)
80
60
40
LV
пЃЊCDM
20
0
3
4
5
6
7
8
R (Mpc)
Apex: min. of
О”2ПѓH =
Results
Cumulative void function О”V/V(>Rvoid) = Vvoids (>Rvoid)/ Vsample, Vvoids - total volume of voids
with Reff > Rvoid, Vsample- total volume of a sample, Reff = (3Vvoid/4ПЂ)-1/3.
Local Volume
Box80S Пѓ8 = 0.9
Box64CR Пѓ8 = 0.75
Box160CR Пѓ8 = 0.75
Density profiles of “dark” halos inside voids
8
Biggest void in Box 20Mpc Vcirc > 45km/s, R = 3.58 Mpc
seed
Slice 4 Mpc thick around center on Z
6
4
Y, Mpc
2
0
-2
Vcirc < 45km/s
-2
0
2
4
6
X, Mpc
8
LCDM- Overabandance
Isolated dIrr
In terms of TF-relation
Dwarf Galaxies
dIrr Camelopardalis B; D=2.2Mpc;
MB=в€’10.9
Vrot ~ 10 km sв€’1
dIrr CGCG 269-049 MB=в€’12.46 D=3.4Mpc
Vrotsin(i) ≤ 8 km s−1
dIrr DDO125 D=2.54Mpc MB=-14.16
Vrot ~ 20 km/s
KKR25
dSph in the field
MB = -9.94
D = 1.86 Mpc (TRGB)
Central SB, ΣV =24m/□”
Karachentsev et al.
2001 A&A, 379, 407
WDM-paradigm mX =1keV
Box 64h-1Mpc, h=0.72 WDM and CDM WMAP3
Initial conditions
Resulting haloes
Mass Functions
WDM
CDM
WDM-like simlations (with P(k)-truncation suffer by fake-haloes:
Appeared via spurious fragmentation of filaments
Wang&White, 2007; Klypin, 2008
Void-functions
WDM
CDM
ПѓH
Vlim = 15-20 km/s
Vlim = 35 km/s
Halos in voids
CDM
WDM
Problem:
The LCDM model faces the same overabundance problem,
which it had with the number of satellites in the LG: the
theory predicts a factor of ten more haloes as compared with
the observed number of dwarf galaxies.
Solutions:
1. Thousands of dSph in the field to find out
2. Halo Vc ~ 2Vrot :dwarf galaxies are hosted by significantly
more massive haloes.
3. Dwarf formation was suppressed by e.g. UV- background
4. LWDM-models (P(k) - truncation) mX ~ 1keV
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