Anton Tikhonov /Saint-Petersburg State University

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Anton Tikhonov /Saint-Petersburg State
University Anatoly A. Klypin /New Mexico State
University Properties of voids in the Local
Volume and the Limit of appearance of a galaxy in
DM halo
The observational discovery ((Gregory
Thompson 1978 Joeveer et al. 1978 Kirshner et
al. 1981) was soon followed by the theoretical
understanding that voids constitute a natural
outcome of structure formation via gravitational
instability (Peebles 1982 Hoffman Shaham
1982). Emptiness of voids do we have a
problem? Cosmological simulations predict many
small DM halos in voids and it seems that
observations are failing to find a substantial
number of dwarf galaxies inside voids. Statistics
of voids in catalogs of galaxies formation of DM
structures in voids physics of galaxies in voids.
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Description of the Local Volume galaxy sample
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 lt 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.,
2004Catalog 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
absence of the effect of God's fingers' in the
Local Volume because of the virial motions of
galaxies simplifies the analysis of the shape and
orientation of nearby voids.
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After recent systematic optical, IR, and HI
surveys, the total number of known galaxies
within 10 Mpc has increased from 179 to 550.
About half this Local Volume (LV) sample is now
been imaged with HST, yielding the galaxy
distances with an accuracy of about 8. For the
majority of the LV galaxies we currently have H
fluxes that allow us to reconstruct the star
formation history of our neighbourhood. For the
late-type LV galaxies their HI masses and angular
momentum follow the linear relation in the range
of 4 orders, which is expected for rotating
gaseous disks being near the gravitational
instability threshold. The data obtained on the
LV galaxies imply important cosmological
parameters, in particular, the mean local matter
density and HI mass density, as well as SFR
density. LV parameters important for
cosmology. In spite from the presence of local
voids, the average density of luminosity within
the radius of 8 Mpc around us exceeds 1.5 - 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. Because
the average virial mass-to-luminosity ratio for
them is 40 Msun/Lsun, the mean local mass density
within 8 Mpc turns out to be 0.10 in units of the
global critical density. This quantity is 2 - 3
times as low as the global density of matter, Om
0.27. To remove the discrepancy between the
global and local quantities of Om, we assume that
the essential amount of dark matter ( 70) exists
outside the virial radius of the groups.
I.D. Karachentsev, 2007
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2D-case of point-like distribution. Seed circles
and voids growing from them are shown. The
numerals indicate the order of identification of
voids

• Void detection algorithm

• 3D grid (to refer the nodes of this grid to this
  or that void).
• Empty seed sphere of largest possible radius is
  identified.
• Expansion of seed spheres by spheres with radius
• Rsph gt 0.9 Rseed
• and with centers inside already
• fixed part of a void.
• Next seed sphere is determined. Process
  continues until Rseed gt threshold.
• Voids are thick enough throughout their volumes.
• Voids are divided into lying completely inside
  boundaries
• and voids touching when constructed the sample
  boundaries.

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Distribution of six large minivoids within the
LV-sphere of radius 7.5 Mpc.
¼ of Local Volume is occupied by Void N1 in
Aquila. It is the front part of the Local (Tully)
void
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The peak in luminosity function, isolated
galaxies. Tidal Index TIi max log(Mk/D3ik)
C For every galaxy i, we found its main
disturber, producing the highest tidal action or
a maximum density enhancement ??k Mk/D3ik
Inside 8 Mpc around MW there are about 420
galaxies (known today). Galaxies in the peak
(Babs -14) have morphological de Vaucouleur
type 7-10 (dwarf Irregulars). Large M(HI)/Mtotal.
Isolated galaxies with Babs -14 have an order
higher HI/LB and SFR/LB with the respect to
galaxies in more dense environment.
M25/L 3.8 Msun/Lsun, total galaxy mass MT 2.5
M25
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Cumulative Void Functions (CVF) of galaxy and
simulated samples. We look for the limit on halo
circular velocity (Vcirc) that points on halos
which generally do not contain galaxies.

• Simulations
• The simulations were done by A. Klypin (NMSU)
  using the Adaptive Refinement
• Tree (ART) code (Kravtsov, Klypin Khokhlov,
  1997). Spatially flat model with a
• cosmological constant ?CDM 1) Om 1 - O?
  0.3 s8 0.9 h 0.7 (WMAP1)
• 2) Om 0.24 s8 0.75 h 0.73 (WMAP3)
• The total mass of a halo depends on its radius
  which is difficult to define. More robust DM halo
  parameter is maximum circular velocity Vcirc
  (GM/R)1/2. This is the quantity which is more
  meaningful observationally. Numerically Vcirc can
  be measured more easily and more accurately than
  mass.
• 1. Box 160LG 16Mpc (wmap3) ART
• 2. Box 80S 14Mpc (wmap1) mass resolution
• 1.0108 h-1 Msun ,complete for halos with Vcirc
  gt 10 km/s
• Min. halo consists of 20 particles.
• 3. Box 64Mpc (wmap3) constrained simulation,
• 10243 particles, GADGET2,
• mass resolution 1.6 107 h-1 Msun.

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Cumulative void-function. CVF(R) Vcumul
(R)/ Vsample, Vcumul (R) - total volume of voids
with Reff gt R, Vsample- total volume of sample,
Reff (3Vvoid/4p)-1/3. We tried to fit
observational CVF by that of ?CDM simulations
changing limit on Vcirc of haloes that define
voids.

• SDSS galaxies, Mr lt -19.7 vs.
• box120 Mpc, DM haloes, Vcirc gt 150 km/s
• LEDA galaxies, MB lt -16 vs.
• box80 Mpc, DM haloes, Vcirc gt 80 km/s

Good agreement of CVF of distribution of galaxies
above certain luminosity with distribution of
simulated haloes with Vcirc above certain limit
?CDM is consistent with observations with respect
to voids.
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• Limit of galaxy appearance.
• CVF of Local Volume sample 1. All galaxies in
  sphere 7.5Mpc.
• 
  2. complete sample MB lt -12 in sphere 8Mpc.
• ?CDM LV-candidates 8 Mpc sphere
• Selection criteria 1) no haloes with Mass gt
• 1013Msun inside sphere 8Mpc (no clusters in
  sample)
• 2) sphere centered on halo with
• 150 lt Vcirc lt 250 km/s (Milky Way analog).
• 3) Overdensity criteria In 7 Mpc of Local Volume
  
• luminosty density in B-band 2.72 108 Lsun/Mpc3
  
• luminosty density in K-band 6.60 108 Lsun/Mpc3
  .
• Overdensity in LV ?LV / ?mean 1.5-1.8.
• Different overdensity ratio in model 8Mpc LVs
• with respect to mean density in simulated box
• (wmap1 and wmap3) of halos with Vcircgt100km/s.

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Box64CR (wmap3) (Gustavo Yepes, Yehuda
Hoffman, Stefan Gottloeber, Anatoly Klypin) CVF
of 20 realizations of 8Mpc sphere from 64/h Mpc
box with different limits on Vcirc of halos that
define voids. ?100 / ?mean1.5-1.8. Mean CVF with
rms. Voids in distribution of haloes with Vcirc gt
35-40 km/s are the best fit of CVF of Local
Volume galaxy distribution.
Vcirc 40 5km/s limit of appearance
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Largest voids in 8Mpc-sphere. According to ?CDM
simulations totally empty front part of Tully
void is probable. 10 realizations of 8Mpc-sphere
CVF of Vcirc gt 40km/s are plotted on CVFs of LV
(Mlt-12 - complete sample (red)) and all LV-galaxy
sample (blue)). There are voids in ?CDM that
comparable to the largest voids in LV if we
consider entire LV sample.
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Density profiles of haloes in voids Density
profiles grows toward borders of voids voids
are physical. Here are density profiles of halo
(Vcirc lt 40 km/s) number density in shells 0.3Mpc
thick inside 10 largest voids defined by haloes
with Vcirc gt 40 km/s in 20Mpc-sphere simulation.
Profiles are not regular (not flat in deeper part
of voids) - even bumps in inner part of voids
possible. Rseed approximately defines thickness
of voids.
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• Distribution of small haloes inside voids
• We can see inside voids in distribution of
  voids defined by Vcirc gt 40km/s haloes
• totally empty regions. Here's the picture of
  slice of 4Mpc on Z around center of
• biggest void ( Reff 4.1Mpc, Rseed
  3.2Mpc ) in 20Mpc-sphere simulation
• defined by haloes with Vcirc gt 40 km/s.
  There are sufficient in volume holes at
• least of haloes with Vcirc gt 10km/s.
• Vcirc vs. distance to void border of haloes
  with Vc lt 45 that fall inside this void.
• Haloes with Vcirc gt 30 km/s are close to the
  void border.

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Conclusion

• ?CDM is consistent with volume functions of
  voids in distribution of galaxies above some
  luminosity in a large luminosity range. There are
  significant (up to few Mpc) holes in ?CDM that
  are free from haloes with Vcirc gt 10km/s any
  haloes of astronomical interest.
• Voids in distribution of halos (wmap3
  cosmplogy) with Vcirc gt 40 5 km/s reproduce
  Cumulative Void Function of Local Volume galaxy
  sample. We can treat this value as a limit of
  appearance of a galaxy in a DM halo.
• MB -14 isolated galaxies (having Vcirc
  35-45 km/s) may be on the limit of appearance.
  Then the bump in Luminosity Function is natural.
• Dark galaxies are probably located close to
  borders of voids.
• According to ?CDM large empty voids in Local
  Volume like front part of Tully Void are
  probable.