929 research outputs found
Lyalpha heating and its impact on early structure formation
In this paper we have calculated the effect of Lyalpha photons emitted by the
first stars on the evolution of the IGM temperature. We have considered both a
standard Salpeter IMF and a delta-function IMF for very massive stars with mass
300 M_sun. We find that the Lyalpha photons produced by the stellar populations
considered here are able to heat the IGM at z<25, although never above ~100 K.
Stars with a Salpeter IMF are more effective as, due to the contribution from
small-mass long-living stars, they produce a higher Lyalpha background. Lyalpha
heating can affect the subsequent formation of small mass objects by producing
an entropy floor that may limit the amount of gas able to collapse and reduce
the gas clumping.We find that the gas fraction in halos of mass below ~ 5 x
10^6 M_sun is less than 50% (for the smallest masses this fraction drops to 1%
or less) compared to a case without Lyalpha heating. Finally, Lyalpha photons
heat the IGM temperature above the CMB temperature and render the 21cm line
from neutral hydrogen visible in emission at z<15.Comment: 7 pages, 5 figures, to be printed in MNRA
Quantifying the heart of darkness with GHALO - a multi-billion particle simulation of our galactic halo
We perform a series of simulations of a Galactic mass dark matter halo at
different resolutions, our largest uses over three billion particles and has a
mass resolution of 1000 M_sun. We quantify the structural properties of the
inner dark matter distribution and study how they depend on numerical
resolution. We can measure the density profile to a distance of 120 pc (0.05%
of R_vir) where the logarithmic slope is -0.8 and -1.4 at (0.5% of R_vir). We
propose a new two parameter fitting function that has a linearly varying
logarithmic density gradient which fits the GHALO and VL2 density profiles
extremely well. Convergence in the density profile and the halo shape scales as
N^(-1/3), but the shape converges at a radius three times larger at which point
the halo becomes more spherical due to numerical resolution. The six
dimensional phase-space profile is dominated by the presence of the
substructures and does not follow a power law, except in the smooth
under-resolved inner few kpc.Comment: 6 pages, 4 figures, submitted to MNRAS Letters, for full sized
images, see http://www.itp.uzh.ch/news.htm
The Formation and Survival of Discs in a Lambda-CDM Universe
We study the formation of galaxies in a Lambda-CDM Universe using high
resolution hydrodynamical simulations with a multiphase treatment of gas,
cooling and feedback, focusing on the formation of discs. Our simulations
follow eight haloes similar in mass to the Milky Way and extracted from a large
cosmological simulation without restriction on spin parameter or merger
history. This allows us to investigate how the final properties of the
simulated galaxies correlate with the formation histories of their haloes. We
find that, at z = 0, none of our galaxies contain a disc with more than 20 per
cent of its total stellar mass. Four of the eight galaxies nevertheless have
well-formed disc components, three have dominant spheroids and very small
discs, and one is a spheroidal galaxy with no disc at all. The z = 0 spheroids
are made of old stars, while discs are younger and formed from the inside-out.
Neither the existence of a disc at z = 0 nor the final disc-to-total mass ratio
seems to depend on the spin parameter of the halo. Discs are formed in haloes
with spin parameters as low as 0.01 and as high as 0.05; galaxies with little
or no disc component span the same range in spin parameter. Except for one of
the simulated galaxies, all have significant discs at z > ~2, regardless of
their z = 0 morphologies. Major mergers and instabilities which arise when
accreting cold gas is misaligned with the stellar disc trigger a transfer of
mass from the discs to the spheroids. In some cases, discs are destroyed, while
in others, they survive or reform. This suggests that the survival probability
of discs depends on the particular formation history of each galaxy. A
realistic Lambda-CDM model will clearly require weaker star formation at high
redshift and later disc assembly than occurs in our models.Comment: 14 pages, 10 figures, mn2e.cls. MNRAS in press, updated to match
published versio
Mass and pressure constraints on galaxy clusters from interferometric SZ observations
Following on our previous study of an analytic parametric model to describe
the baryonic and dark matter distributions in clusters of galaxies with
spherical symmetry, we perform an SZ analysis of a set of simulated clusters
and present their mass and pressure profiles. The simulated clusters span a
wide range in mass, 2.0 x 10^14 Msun < M200 < 1.0 x 10^15Msun, and observations
with the Arcminute Microkelvin Imager (AMI) are simulated through their
Sunyaev- Zel'dovich (SZ) effect. We assume that the dark matter density follows
a Navarro, Frenk and White (NFW) profile and that the gas pressure is described
by a generalised NFW (GNFW) profile. By numerically exploring the probability
distributions of the cluster parameters given simulated interferometric SZ data
in the context of Bayesian methods, we investigate the capability of this model
and analysis technique to return the simulated clusters input quantities. We
show that considering the mass and redshift dependency of the cluster halo
concentration parameter is crucial in obtaining an unbiased cluster mass
estimate and hence deriving the radial profiles of the enclosed total mass and
the gas pressure out to r200.Comment: 5 pages, 2 tables, 3 figure
Mass models from high-resolution HI data of the dwarf galaxy NGC 1560
We present HI observations performed at the GMRT of the nearby dwarf galaxy
NGC 1560. This Sd galaxy is well-known for a distinct "wiggle" in its rotation
curve. Our new observations have twice the resolution of the previously
published HI data. We derived the rotation curve by taking projection effects
into account, and we verified the derived kinematics by creating model
datacubes. This new rotation curve is similar to the previously published one:
we confirm the presence of a clear wiggle. The main differences are in the
innermost ~100 arcsec of the rotation curve, where we find slightly (<~ 5 km/s)
higher velocities. Mass modelling of the rotation curve results in good fits
using the core-dominated Burkert halo (which however does not reproduce the
wiggle), bad fits using the a Navarro, Frenk & White halo, and good fits using
MOND (Modified Newtonian Dynamics), which also reproduces the wiggle.Comment: Accepted for publication in MNRAS. 11 pages, 13 figures.
High-resolution version available at
http://users.ugent.be/~ggianfra/1560_final.pd
A mass-dependent density profile for dark matter haloes including the influence of galaxy formation
We introduce a mass-dependent density profile to describe the distribution of dark matter within galaxies, which takes into account the stellar-to-halo mass dependence of the response of dark matter to baryonic processes. The study is based on the analysis of hydrodynamically simulated galaxies from dwarf to Milky Way mass, drawn from the Making Galaxies In a Cosmological Context project, which have been shown to match a wide range of disc scaling relationships. We find that the best-fitting parameters of a generic double power-law density profile vary in a systematic manner that depends on the stellar-to-halo mass ratio of each galaxy. Thus, the quantity M⋆/Mhalo constrains the inner (γ) and outer (β) slopes of dark matter density, and the sharpness of transition between the slopes (α), reducing the number of free parameters of the model to two. Due to the tight relation between stellar mass and halo mass, either of these quantities is sufficient to describe the dark matter halo profile including the effects of baryons. The concentration of the haloes in the hydrodynamical simulations is consistent with N-body expectations up to Milky Way-mass galaxies, at which mass the haloes become twice as concentrated as compared with pure dark matter runs. This mass-dependent density profile can be directly applied to rotation curve data of observed galaxies and to semi-analytic galaxy formation models as a significant improvement over the commonly used NFW profile
Effect of halo modelling on WIMP exclusion limits
WIMP direct detection experiments are just reaching the sensitivity required
to detect galactic dark matter in the form of neutralinos. Data from these
experiments are usually analysed under the simplifying assumption that the
Milky Way halo is an isothermal sphere with maxwellian velocity distribution.
Observations and numerical simulations indicate that galaxy halos are in fact
triaxial and anisotropic. Furthermore, in the cold dark matter paradigm
galactic halos form via the merger of smaller subhalos, and at least some
residual substructure survives. We examine the effect of halo modelling on WIMP
exclusion limits, taking into account the detector response. Triaxial and
anisotropic halo models, with parameters motivated by observations and
numerical simulations, lead to significant changes which are different for
different experiments, while if the local WIMP distribution is dominated by
small scale clumps then the exclusion limits are changed dramatically.Comment: 9 pages, 9 figures, version to appear in Phys. Rev. D, minor change
Detecting the dark matter annihilation at the ground EAS detectors
In this paper we study the possibility of detecting gamma rays from dark
matter annihilation in the subhalos of the Milky Way by the ground based EAS
detectors within the frame of the minimal supersymmetric standard model. Based
on the Monte Carlo simulation we also study the properties of two specific EAS
detectors, the ARGO and HAWC, and the sensitivities of these detectors on the
detection of dark matter annihilation. We find the ground EAS detectors have
the possibility to observe such signals. Conversely if no signal observed we
give the constraints on the supersymmetric parameter space, which however
depends on the subhalos properties.Comment: 23 pages, 9 figures, accepted by NP
Inflation, cold dark matter, and the central density problem
A problem with high central densities in dark halos has arisen in the context
of LCDM cosmologies with scale-invariant initial power spectra. Although n=1 is
often justified by appealing to the inflation scenario, inflationary models
with mild deviations from scale-invariance are not uncommon and models with
significant running of the spectral index are plausible. Even mild deviations
from scale-invariance can be important because halo collapse times and
densities depend on the relative amount of small-scale power. We choose several
popular models of inflation and work out the ramifications for galaxy central
densities. For each model, we calculate its COBE-normalized power spectrum and
deduce the implied halo densities using a semi-analytic method calibrated
against N-body simulations. We compare our predictions to a sample of dark
matter-dominated galaxies using a non-parametric measure of the density. While
standard n=1, LCDM halos are overdense by a factor of 6, several of our example
inflation+CDM models predict halo densities well within the range preferred by
observations. We also show how the presence of massive (0.5 eV) neutrinos may
help to alleviate the central density problem even with n=1. We conclude that
galaxy central densities may not be as problematic for the CDM paradigm as is
sometimes assumed: rather than telling us something about the nature of the
dark matter, galaxy rotation curves may be telling us something about inflation
and/or neutrinos. An important test of this idea will be an eventual consensus
on the value of sigma_8, the rms overdensity on the scale 8 h^-1 Mpc. Our
successful models have values of sigma_8 approximately 0.75, which is within
the range of recent determinations. Finally, models with n>1 (or sigma_8 > 1)
are highly disfavored.Comment: 13 pages, 6 figures. Minor changes made to reflect referee's
Comments, error in Eq. (18) corrected, references updated and corrected,
conclusions unchanged. Version accepted for publication in Phys. Rev. D,
scheduled for 15 August 200
Clues on the origin of galactic angular momentum from looking at galaxy pairs
We search for correlations between the spin in pairs of spiral galaxies, to
study if the angular momentum gain for each galaxy was the result of tidal
torques imprint by the same tidal field. To perform our study we made use of a
sample of galaxy pairs identified using the Sloan Digital Sky Survey. We find a
weak, but statistically significant correlation between the spin magnitude of
neighbouring galaxies, but no clear alignment between their orientation. We
show that events such as interactions with close neighbours play an important
role in the value of the spin for the final configuration, as we find these
interactions tend to reduce the value of the spin parameter of
late-type galaxies considerably, with dependence on the morphology of the
neighbour. This implies that the original tidal field for each pair could have
been similar, but the redistribution of angular momentum at later stages of
evolution is important.Comment: 10 pages, 4 figures, 1 table. Replaced to match the version accepted
for publication in MNRA
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