333 research outputs found
The impact of spatial fluctuations in the ultra-violet background on intergalactic carbon and silicon
Spatial inhomogeneities in the spectral shape of the ultra-violet background
(UVB) at the tail-end of HeII reionisation are thought to be the primary cause
of the large fluctuations observed in the HeII to HI Ly-a forest optical depth
ratio, tau_HeII/tau_HI, at z~2-3. These spectral hardness fluctuations will
also influence the ionisation balance of intergalactic metals; we extract
realistic quasar absorption spectra from a large hydrodynamical simulation to
examine their impact on intergalactic SiIV and CIV absorbers. Using a variety
of toy UVB models, we find that while the predicted spatial inhomogeneities in
spectral hardness have a significant impact on tau_HeII/tau_HI, the longer mean
free path for photons with frequencies above and below the HeII ionisation edge
means these fluctuations have less effect on the SiIV and CIV ionisation
balance. Furthermore, UVB models which produce the largest fluctuations in
specific intensity at the HeII ionisation edge also have the softest ionising
spectra, and thus result in photo-ionisation rates which are too low to produce
significant fluctuations in the observed tau_SiIV/tau_CIV. Instead, we find
spatial variations in the IGM metallicity will dominate any scatter in
tau_SiIV/tau_CIV. Our results suggest that observational evidence for
homogeneity in the observed tau_SiIV/tau_CIV distribution does not rule out the
possibility of significant fluctuations in the UVB spectral shape at z~2-3. On
the other hand, the scatter in metallicity inferred from observations of
intergalactic CIV and SiIV absorption at z~2-3 using spatially uniform
ionisation corrections is likely intrinsic, and therefore provides a valuable
constraint on intergalactic metal enrichment scenarios at these redshifts.Comment: 13 pages, 7 figures, accepted to MNRA
Is the Concentration of Dark Matter Halos at Virialization Universal ?
Several recent studies suggest a correlation between dark matter halo mass
and the shape of the density profile. We re-analyze simulations from Ricotti
(2003) in which such a correlation was proposed. We use a standard analysis of
the halo density profiles and compare the old simulations to new ones performed
with Gadget2, including higher resolution runs. We confirm Ricotti's result
that, at virialization, the central log slopes alpha, at 5%-10% of the virial
radius are correlated with the halo mass and that the halo concentration is a
universal constant. Our results do not contradict the majority of published
papers: when using a split power law to fit the density profiles, due to the
alpha-concentration degeneracy, the fits are consistent with halos having a
universal shape with alpha=1 or 1.5 and concentrations that depend on the mass,
in agreement with results published elsewhere.
Recently, several groups have found no evidence for convergence of the inner
halo profile to a constant power law. The choice of a split power law
parameterization used in this letter is motivated by the need to compare our
results to previous ones and is formally valid because we are not able to
resolve regions where the slope of the fitting function reaches its asymptotic
constant value. Using a non-parameterized technique, we also show that the
density profiles of dwarf galaxies at z ~ 10 have a log slope shallower than
0.5 within 5% of the virial radius.Comment: Accepted for Publication in the Astrophysical Journal Lette
The Statistics of Cosmological Lyman-alpha Absorption
We study the effect of the non-Gaussianity induced by gravitational evolution
upon the statistical properties of absorption in quasar (QSO) spectra. Using
the generic hierarchical ansatz and the lognormal approximation we derive the
analytical expressions for the one-point PDF as well as for the joint two-point
probability distribution (2PDF) of transmitted fluxes in two neighbouring QSOs.
These flux PDFs are constructed in 3D as well as in projection (i.e. in 2D).
The PDFs are constructed by relating the lower-order moments, i.e. cumulants
and cumulant correlators, of the fluxes to the 3D neutral hydrogen distribution
which is, in turn, expressed as a function of the underlying dark matter
distribution. The lower-order moments are next modelled using a generating
function formalism in the context of a {\em minimal tree-model} for the
higher-order correlation hierarchy. These different approximations give nearly
identical results for the range of redshifts probed, and we also find a very
good agreement between our predictions and outputs of hydrodynamical
simulations. The formalism developed here for the joint statistics of
flux-decrements concerning two lines of sight can be extended to multiple lines
of sight, which could be particularly important for the 3D reconstruction of
the cosmic web from QSO spectra (e.g. in the BOSS survey). These statistics
probe the underlying projected neutral hydrogen field and are thus linked to
"hot-spots" of absorption. The results for the PDF and the bias presented here
use the same functional forms of scaling functions that have previously been
employed for the modelling of other cosmological observation such as the
Sunyaev-Zel'dovich effect.Comment: 17 pages, 8 figure
The Lyman-alpha forest and WMAP year three
A combined analysis of Cosmic Microwave Background (CMB) and Lyman-a forest
data allows to constrain the matter power spectrum from small scales of about 1
Mpc/h all the way to the horizon scale. The long lever arm and complementarity
provided by such an analysis has previously led to a significant tightening of
the constraints on the shape and the amplitude of the power spectrum of
primordial density fluctuations. We present here a combined analysis of the
WMAP three year results with Lyman-a forest data. The amplitude of the matter
power spectrum sigma_8 and the spectral index ns inferred from the joint
analysis with high resolution Lyman-a forest data and low resolution Lyman-a
forest data as analyzed by Viel & Haehnelt (2006) are consistent with the new
WMAP results to within 1 sigma. The joint analysis with the mainly low
resolution data as analysed by McDonald et al. (2005) suggests a value of
sigma_8 which is ~ 2 sigma higher than that inferred from the WMAP three year
data alone. The joint analysis of the three year WMAP and the Lyman-a forest
data also does not favour a running of the spectral index. The best fit values
for a combined analysis of the three year WMAP data, other CMB data, 2dF and
the Lyman-a forest data are (sigma_8, ns) = (0.78\pm 0.03,0.96 \pm 0.01).Comment: 5 pages, 4 figs, 2 tables. MNRAS letters in pres
Lyman-alpha constraints on warm and on warm-plus-cold dark matter models
We revisit Lyman-alpha bounds on the dark matter mass in Lambda Warm Dark
Matter (Lambda-WDM) models, and derive new bounds in the case of mixed Cold
plus Warm models (Lambda-CWDM), using a set up which is a good approximation
for several theoretically well-motivated dark matter models. We combine WMAP5
results with two different Lyman-alpha data sets, including observations from
the Sloan Digital Sky Survey. We pay a special attention to systematics, test
various possible sources of error, and compare the results of different
statistical approaches. Expressed in terms of the mass of a non-resonantly
produced sterile neutrino, our bounds read m_NRP > 8 keV (frequentist 99.7%
confidence limit) or m_NRP > 12.1 keV (Bayesian 95% credible interval) in the
pure Lambda-WDM limit. For the mixed model, we obtain limits on the mass as a
function of the warm dark matter fraction F_WDM. Within the mass range studied
here (5 keV < m_NRP < infinity), we find that any mass value is allowed when
F_WDM < 0.6 (frequentist 99.7% confidence limit); similarly, the Bayesian joint
probability on (F_WDM, 1/m_NRP) allows any value of the mass at the 95%
confidence level, provided that F_WDM < 0.35.Comment: 55 pages, 14 figures. References added, discussion of the data
analysis expanded. Final version to appear in JCA
Neutrino Signatures on the High Transmission Regions of the Lyman-alpha Forest
We quantify the impact of massive neutrinos on the statistics of low density
regions in the intergalactic medium (IGM) as probed by the Lyman-alpha forest
at redshifts z=2.2--4. Based on mock but realistic quasar (QSO) spectra
extracted from hydrodynamic simulations with cold dark matter, baryons and
neutrinos, we find that the probability distribution of weak Lyman-alpha
absorption features, as sampled by Lyman-alpha flux regions at high
transmissivity, is strongly affected by the presence of massive neutrinos. We
show that systematic errors affecting the Lyman-alpha forest reduce but do not
erase the neutrino signal. Using the Fisher matrix formalism, we conclude that
the sum of the neutrino masses can be measured, using the method proposed in
this paper, with a precision smaller than 0.4 eV using a catalog of 200 high
resolution (S/N~100) QSO spectra. This number reduces to 0.27 eV by making use
of reasonable priors in the other parameters that also affect the statistics of
the high transitivity regions of the Lyman-alpha forest. The constraints
obtained with this method can be combined with independent bounds from the CMB,
large scale structures and measurements of the matter power spectrum from the
Lyman-alpha forest to produce tighter upper limits on the sum of the masses of
the neutrinos.Comment: 9 pages, 6 figures. MNRAS Accepte
The impact of coupled dark energy cosmologies on the high-redshift intergalactic medium
We present an analysis of high-resolution hydrodynamical N-body simulations of coupled dark energy cosmologies which focuses on the statistical properties of the transmitted Lyman alpha flux in the high-redshift intergalactic medium (IGM). In these models the growth of the diffuse cosmic web differs from the standard Lambda CDM case: the density distribution is skewed towards underdense regions and the matter power spectra are typically larger (in a scale-dependent way). These differences are also appreciable in the Lyman alpha flux and are larger than 5 per cent (10 per cent) at z = 2-4 in the flux probability distribution function (pdf) for high-transmissivity regions and for values of the coupling parameter beta = 0.08 (beta = 0.2). The flux power spectrum is also affected at the similar to 2 per cent (similar to 5-10 per cent) level for beta = 0.08 (beta = 0.2) in a redshift-dependent way. We infer the behaviour of flux pdf and flux power for a reasonable range of couplings and present constraints using present high-and low-resolution data sets. We find an upper limit beta less than or similar to 0.15 (at 2 sigma confidence level), which is obtained using only IGM data and is competitive with those inferred from other large-scale structure probes
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