53 research outputs found
Cosmology with galaxy clusters
A number of different ways of using galaxy clusters to provide information concerning fundamental cosmological parameters are considered. Using the observed local cluster X-ray temperature function in conjunction with the Press-Schechter formalism, the normalisation of a CDM power spectrum is found to be σ(_8) = (0.52 ± 0.04)Ω(_o)(^-0.46+0.10Ωo) if Ʌ(_o) = 0 or σ(_8) = (0.52 ± 0.04)Ω(_o)(^-0.52+0.13Ωo) if Ʌ(_o) = 1 — Ω(_0). This result is employed to provide detailed predictions for the abundance of clusters at high redshift, and the differences between predictions for various cosmologies are emphasised. New tests using available high-redshift cluster data are presented. For the adopted power spectrum normalisation, it is found that an Ω(_o) = 0.3, Ʌ(_o) = 0 cosmology vastly overpredicts the number of clusters that were actually found with 0.4 < z < 0.6 in the Extended Medium Sensitivity Survey. The rapid variation in the expected abundance with both σ(_8) and the assumed scatter in the L(_x) – T_x) relation limits the significance of this result, but this model is still ruled out at the ~ 95% confidence level. Order statistics are utilised to calculate the probability of finding extremely massive clusters at high redshifts. With presently available observations, no interesting upper limit can yet be placed on Ω(_o). Systematic variations in the cluster-cluster correlation length calculated using numerical simulations and resulting from the definition of clusters, the chosen σ(_8), the mean intercluster separation and whether or not redshift space distortions are included, are found to exceed the statistical errors on the measurements. Although the uncertainty in ε(_cc) derived from an ensemble of 10 Standard CDM simulations is not sufficient at large separations to remove the discrepancy between this model and results from the APM Cluster Survey, this does suggest that the level at which such a scenario has previously been rejected using ε(_cc) should be significantly reduced. Details and a few tests of a procedure for improving mass and spatial resolution in cosmological simulations are presented. After showing that a coarse-sampling technique can be used to represent the large-scale forces sufficiently accurately, the method is then used to perform ten simulations of clusters forming in an Ω(_o) = 0.3, Ʌ(_o) = 0.7 CDM cosmology. To incorporate non-radiative gas, an SPH code adapted to work on a GRAPEsupercomputer is used. The resulting clusters are found to have virial radii in good agreement with the predictions of the spherical collapse model, dark matter density profiles well described by the 'NFW formula and isothermal central gas components, with temperatures dropping by a factor of ~ 2 near the virial radius. The evolution of these properties is studied as well as that of the bulk quantities describing the clusters, with particular reference to the β parameters relating cluster gas temperatures with virial mass or velocity dispersion. Slightly greater evolution in the luminosity is seen than in previous Ω(_o) = 1 simulations, suggesting that the improved resolution is important. The β parameter relevant to the normalisation of the mass fluctuation spectrum is found to be 0.98 ± 0.07
Using the Evolution of Clusters to Constrain Omega
The population of rich galaxy clusters evolves much more rapidly in a
universe with critical density than one with low density, thus offering the
possibility of determining the cosmological density parameter, Omega_0. We
quantify this evolution using the Press-Schechter formalism which we extend to
flat models with a cosmological constant. Using new large N-body simulations,
we verify that this formalism accurately predicts the abundance of rich
clusters as a function of redshift in various cosmologies. We normalise the
models by comparing them to the local abundance of clusters as a function of
their X-ray temperature which we rederive from data compiled by Henry & Arnaud.
This gives values of the rms density fluctuation in spheres of radius 8 Mpc/h
of sigma_8 = (0.50+/- 0.04) Omega_0^{-0.47+0.10 Omega_0} if Lambda_0=0 and
sigma_8 = (0.50 +/- 0.04) Omega_0^{-0.53+0.13 Omega_0} if Lambda_0=1-Omega_0.
These values depend very weakly on the shape of the power spectrum. We then
examine how the distributions of mass, X-ray temperature and Sunyaev-Zel'dovich
decrement evolve as a function of Omega_0. We present the expected
distributions at z=0.33 and z=0.5 and the predicted number counts of the
largest clusters. We find that even at z=0.33, these distributions depend very
strongly on Omega_0 and only weakly on Lambda_0. For example, at this redshift,
we expect 20 times as many clusters per comoving volume with M>3.5 10^{14}
Msol/h and 5 times as many clusters with kT>5 keV if Omega_0=0.3 than if
Omega_0=1. The splitting in the integrated counts is enhanced by the larger
volume element in low Omega_0 models. There is therefore a real prospect of
estimating Omega_0 from forthcoming surveys of intermediate redshift clusters
that will determine their masses, X-ray temperatures or SZ decrements.Comment: Compressed postscript also available at
ftp://star-ftp.dur.ac.uk/pub/preprints/ecf.ps.g
The Cores of Dwarf Galaxy Halos
We use N-body simulations to examine the effects of mass outflows on the
density profiles of cold dark matter (CDM) halos surrounding dwarf galaxies. In
particular, we investigate the consequences of supernova-driven winds that
expel a large fraction of the baryonic component from a dwarf galaxy disk after
a vigorous episode of star formation. We show that this sudden loss of mass
leads to the formation of a core in the dark matter density profile, although
the original halo is modeled by a coreless (Hernquist) profile. The core radius
thus created is a sensitive function of the mass and radius of the baryonic
disk being blown up. The loss of a disk with mass and size consistent with
primordial nucleosynthesis constraints and angular momentum considerations
imprints a core radius which is only a small fraction of the original
scale-length of the halo. These small perturbations are, however, enough to
reconcile the rotation curves of dwarf irregulars with the density profiles of
haloes formed in the standard CDM scenario.Comment: PS file. 6 figures included. 531072 bytes. Accepted for publication
in MNRAS Letters. Also available from
http://penedes.as.arizona.edu/~jfn/preprints . (File name: rcore.ps.gz
Optimal Constraints on Local Primordial Non-Gaussianity from the Two-Point Statistics of Large-Scale Structure
One of the main signatures of primordial non-Gaussianity of the local type is
a scale-dependent correction to the bias of large-scale structure tracers such
as galaxies or clusters, whose amplitude depends on the bias of the tracers
itself. The dominant source of noise in the power spectrum of the tracers is
caused by sampling variance on large scales (where the non-Gaussian signal is
strongest) and shot noise arising from their discrete nature. Recent work has
argued that one can avoid sampling variance by comparing multiple tracers of
different bias, and suppress shot noise by optimally weighting halos of
different mass. Here we combine these ideas and investigate how well the
signatures of non-Gaussian fluctuations in the primordial potential can be
extracted from the two-point correlations of halos and dark matter. On the
basis of large -body simulations with local non-Gaussian initial conditions
and their halo catalogs we perform a Fisher matrix analysis of the two-point
statistics. Compared to the standard analysis, optimal weighting- and
multiple-tracer techniques applied to halos can yield up to one order of
magnitude improvements in \fnl-constraints, even if the underlying dark
matter density field is not known. We compare our numerical results to the halo
model and find satisfactory agreement. Forecasting the optimal
\fnl-constraints that can be achieved with our methods when applied to
existing and future survey data, we find that a survey of
volume resolving all halos down to 10^{11}\hMsun at
will be able to obtain \sigma_{\fnl}\sim1 (68% cl), a factor of
improvement over the current limits. Decreasing the minimum mass of
resolved halos, increasing the survey volume or obtaining the dark matter maps
can further improve these limits, potentially reaching the level of
\sigma_{\fnl}\sim0.1. (abridged)Comment: V1: 23 pages, 12 figures, submitted to PRD. V2: 24 pages, added
appendix and citations, matched to PRD published versio
The baryon fraction of LambdaCDM haloes
We investigate the baryon fraction in dark matter haloes formed in
non-radiative gas-dynamical simulations of the LambdaCDM cosmogony. By
combining a realisation of the Millennium Simulation (Springel et al.) with a
simulation of a smaller volume focussing on dwarf haloes, our study spans five
decades in halo mass, from 10^10 Msun/h to 10^15 Msun/h. We find that the
baryon fraction within the halo virial radius is typically 90% of the cosmic
mean, with an rms scatter of 6%, independently of redshift and of halo mass
down to the smallest resolved haloes. Our results show that, contrary to the
proposal of Mo et al. (2005), pre-virialisation gravitational heating is unable
to prevent the collapse of gas within galactic and proto-galactic haloes, and
confirm the need for non-gravitational feedback in order to reduce the
efficiency of gas cooling and star formation in dwarf galaxy haloes.
Simulations including a simple photoheating model (where a gas temperature
floor of T_{floor} = 2x10^4 K is imposed from z=11) confirm earlier suggestions
that photoheating can only prevent the collapse of baryons in systems with
virial temperatures T_{200} < ~2.2 T_{floor} ~ 4.4x10^4 K (corresponding to a
virial mass of M_{200} ~ 10^10 Msun/h and a circular velocity of V_{200} ~ 35
km/s). Photoheating may thus help regulate the formation of dwarf spheroidals
and other galaxies at the extreme faint-end of the luminosity function, but it
cannot, on its own, reconcile the abundance of sub-L* galaxies with the vast
number of dwarf haloes expected in the LambdaCDM cosmogony. The lack of
evolution or mass dependence seen in the baryon fraction augurs well for X-ray
cluster studies that assume a universal and non-evolving baryon fraction to
place constraints on cosmological parameters.Comment: 9 pages, 5 figures (Figs 1. and 2 reduced in quality), 1 table,
submitted to MNRAS. Version with high-resolution figures can be obtained from
http://star-www.dur.ac.uk/~rcrain/baryonfractions
Equatorial locations of water on Mars: Improved resolution maps based on Mars Odyssey Neutron Spectrometer data
We present a map of the near subsurface hydrogen distribution on Mars, based on epithermal neutron data from the Mars Odyssey Neutron Spectrometer. The map’s spatial resolution is approximately improved two-fold via a new form of the pixon image reconstruction technique. We discover hydrogen-rich mineralogy far from the poles, including  ∼10 wt.% water equivalent hydrogen (WEH) on the flanks of the Tharsis Montes and  >40 wt.% WEH at the Medusae Fossae Formation (MFF). The high WEH abundance at the MFF implies the presence of bulk water ice. This supports the hypothesis of recent periods of high orbital obliquity during which water ice was stable on the surface. We find the young undivided channel system material in southern Elysium Planitia to be distinct from its surroundings and exceptionally dry; there is no evidence of hydration at the location in Elysium Planitia suggested to contain a buried water ice sea. Finally, we find that the sites of recurring slope lineae (RSL) do not correlate with subsurface hydration. This implies that RSL are not fed by large, near-subsurface aquifers, but are instead the result of either small ( < 120 km diameter) aquifers, deliquescence of perchlorate and chlorate salts or dry, granular flows
Simulations of galaxy formation in a Λ cold dark matter universe : I : dynamical and photometric properties of a simulated disk galaxy.
We present a detailed analysis of the dynamical and photometric properties of a disk galaxy simulated in the cold dark matter (CDM) cosmogony. The galaxy is assembled through a number of high-redshift mergers followed by a period of quiescent accretion after z1 that lead to the formation of two distinct dynamical components: a spheroid of mostly old stars and a rotationally supported disk of younger stars. The surface brightness profile is very well approximated by the superposition of an R1/4 spheroid and an exponential disk. Each photometric component contributes a similar fraction of the total luminosity of the system, although less than a quarter of the stars form after the last merger episode at z1. In the optical bands the surface brightness profile is remarkably similar to that of Sab galaxy UGC 615, but the simulated galaxy rotates significantly faster and has a declining rotation curve dominated by the spheroid near the center. The decline in circular velocity is at odds with observation and results from the high concentration of the dark matter and baryonic components, as well as from the relatively high mass-to-light ratio of the stars in the simulation. The simulated galaxy lies 1 mag off the I-band Tully-Fisher relation of late-type spirals but seems to be in reasonable agreement with Tully-Fisher data on S0 galaxies. In agreement with previous simulation work, the angular momentum of the luminous component is an order of magnitude lower than that of late-type spirals of similar rotation speed. This again reflects the dominance of the slowly rotating, dense spheroidal component, to which most discrepancies with observation may be traced. On its own, the disk component has properties rather similar to those of late-type spirals: its luminosity, its exponential scale length, and its colors are all comparable to those of galaxy disks of similar rotation speed. This suggests that a different form of feedback than adopted here is required to inhibit the efficient collapse and cooling of gas at high redshift that leads to the formation of the spheroid. Reconciling, without fine-tuning, the properties of disk galaxies with the early collapse and high merging rates characteristic of hierarchical scenarios such as CDM remains a challenging, yet so far elusive, proposition
The haloes of bright satellite galaxies in a warm dark matter universe
High-resolution N-body simulations of galactic cold dark matter haloes indicate that we should expect to find a few satellite galaxies around the Milky Way whose haloes have a maximum circular velocity in excess of 40 km s−1. Yet, with the exception of the Magellanic Clouds and the Sagittarius dwarf, which likely reside in subhaloes with significantly larger velocities than this, the bright satellites of the Milky Way all appear to reside in subhaloes with maximum circular velocities below 40 km s−1. As recently highlighted by Boylan-Kolchin et al., this discrepancy implies that the majority of the most massive subhaloes within a cold dark matter galactic halo are too concentrated to be consistent with the kinematic data for the bright Milky Way satellites. Here we show that no such discrepancy exists if haloes are made of warm rather than cold dark matter because these haloes are less concentrated on account of their typically later formation epochs. Warm dark matter is one of several possible explanations for the observed kinematics of the satellite
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