58 research outputs found

### Dwarf Galaxies in Voids: Dark Matter Halos and Gas Cooling

Galaxy surveys have shown that luminous galaxies are mainly distributed in
large filaments and galaxy clusters. The remaining large volumes are virtually
devoid of luminous galaxies. This is in concordance with the formation of the
large-scale structure in Universe as derived from cosmological simulations.
However, the numerical results indicate that cosmological voids are abundantly
populated with dark matter haloes which may in principle host dwarf galaxies.
Observational efforts have in contrast revealed, that voids are apparently
devoid of dwarf galaxies. We investigate the formation of dwarf galaxies in
voids by hydrodynamical cosmological simulations. Due to the cosmic
ultra-violet background radiation low-mass haloes show generally are reduced
baryon fraction. We determine the characteristic mass below which dwarf
galaxies are baryon deficient. We show that the circular velocity below which
the accretion of baryons is suppressed is approximately 40 km/s. The suppressed
baryon accretion is caused by the photo-heating due to the UV-background. We
set up a spherical halo model and show that the effective equation of state of
the gas in the periphery of dwarf galaxies determines the characteristic mass.
This implies that any process which heats the gas around dwarf galaxies
increases the characteristic mass and thus reduces the number of observable
dwarf galaxies.Comment: 23 pages. To appear in Advances in Astronomy, Dwarf-Galaxy Cosmology
issu

### Too small to form a galaxy: How the UV background determines the baryon fraction

The cosmic ultraviolet background (UVB) heats the intergalactic medium (IGM),
as a result the gas in dark matter halos below a certain mass is too hot to
cool within a Hubble time. The UVB effectively suppresses the formation of
dwarf galaxies. Using high resolution cosmological hydrodynamical simulations
we show that photo heating leads to small baryon fractions in halos below ~
6x10^9 h^{-1}M_sun, independent of the cosmic environment. The simulations are
carried out assuming a homogeneous UVB with flux densities as given by Haardt &
Madau (1996). A halo may stop to condense gas significantly after the universe
is reionised, namely when its mass falls below the characteristic mass scale
set by the photo heating. Assuming a spherical halo model we derive this
characteristic mass analytically and identify the main mechanisms that prevent
the gas from cooling in small halos. The theoretically derived characteristic
mass is smaller than the one obtained from observations. Increasing the energy
per ionising photon by a factor between four and eight would be sufficient to
reconcile both. This is equivalent to an average temperature of the IGM of ~
10^4 K. In this sense the faint end of the luminosity function may serve as a
calorimeter for the IGM.Comment: To appear in Proceedings of IAU Symp #244, "Dark Galaxies and Lost
Baryons", June 2007, 5 pages including 3 figure

### Confrontation of a Double Inflationary Cosmological Model with Observations

CDM models with non-scale-free step-like spectra of adiabatic perturbations
produced in a realistic double inflationary model are compared with recent
observational data. The model contains two additional free parameters
relatively to the standard CDM model with the flat ($n=1$) initial spectrum.
Results of the COBE experiment are used for the determination of a free overall
spectrum normalization. Then predictions for the galaxy biasing parameter, the
variance for "counts in cells", the galaxy angular correlation function, bulk
flow peculiar velocities and the Mach number test are obtained. Also considered
are conditions for galaxy and quasar formation. Observational data strongly
restricts allowed values for the two remaining model parameters. However, a
non-empty region for them satisfying all considered tests is found.Comment: 17 pages and 4 figures obtainable by request, LaTex, AIP 93-1

### The accuracy of parameters determined with the core-sampling method: application to Voronoi tessellations

The large-scale matter distribution represents a complex network of structure
elements such as voids, clusters, filaments, and sheets. This network is
spanned by a point distribution. The global properties of the point process can
be measured by different statistical methods, which, however, do not describe
directly the structure elements. The morphology of structure elements is an
important property of the point distribution. Here we apply the core-sampling
method to various Voronoi tessellations. Using the core-sampling method we
identify one- and two-dimensional structure elements (filaments and sheets) in
these Voronoi tessellations and reconstruct their mean separation along random
straight lines. We compare the results of the core-sampling method with the a
priori known structure elements of the Voronoi tessellations under
consideration and find good agreement between the expected and found structure
parameters, even in the presence of substantial noise. We conclude that the
core-sampling method is a potentially powerful tool to investigate the
distribution of such structure elements like filaments and walls of galaxies.Comment: 14 pages (Latex) with 6 figures, the complete paper with 8 figures is
available at http://kosmos.aip.de/~got/projects.html {Characteristical scales
in point distributions}, Astronomy and Astrophysics Supplement Series,
accepte

### Reconstructing cosmological initial conditions from galaxy peculiar velocities. I. Reverse Zeldovich Approximation

We propose a new method to recover the cosmological initial conditions of the
presently observed galaxy distribution, which can serve to run constrained
simulations of the Local Universe. Our method, the Reverse Zeldovich
Approximation (RZA), can be applied to radial galaxy peculiar velocity data and
extends the previously used Constrained Realizations (CR) method by adding a
Lagrangian reconstruction step. The RZA method consists of applying the
Zeldovich approximation in reverse to galaxy peculiar velocities to estimate
the cosmic displacement field and the initial linear matter distribution from
which the present-day Local Universe evolved.We test our method with a mock
survey taken from a cosmological simulation. We show that the halo peculiar
velocities at z = 0 are close to the linear prediction of the Zeldovich
approximation, if a grouping is applied to the data to remove virial motions.
We find that the addition of RZA to the CR method significantly improves the
reconstruction of the initial conditions. The RZA is able to recover the
correct initial positions of the velocity tracers with a median error of only
1.36 Mpc/h in our test simulation. For realistic sparse and noisy data, this
median increases to 5 Mpc/h. This is a significant improvement over the
previous approach of neglecting the displacement field, which introduces errors
on a scale of 10 Mpc/h or even higher. Applying the RZA method to the upcoming
high-quality observational peculiar velocity catalogues will generate much more
precise constrained simulations of the Local Universe.Comment: Accepted for MNRAS 2012 December 1

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