485 research outputs found
Simulating IGM Reionization
We have studied the IGM reionization process in its full cosmological context
including structure evolution and a realistic galaxy population. We have used a
combination of high-resolution N-body simulations (to describe the dark matter
and diffuse gas component), a semi-analytic model of galaxy formation (to track
the evolution of the sources of ionizing radiation) and the Monte Carlo
radiative transfer code CRASH (to follow the propagation of ionizing photons
into the IGM). The process has been followed in the largest volume ever used
for this kind of study, a field region of the universe with a comoving length
of L~20/h Mpc, embedded in a much larger cosmological simulation. To assess the
effect of environment on the reionization process, the same radiative transfer
simulations have been performed on a 10/h Mpc comoving box, centered on a
clustered region. We find that, to account for the all ionizing radiation,
objects with total masses of M~10^9 Msun must be resolved. In this case, the
simulated stellar population produces a volume averaged ionization fraction
x_v=0.999 by z~8, consistent with observations without requiring any additional
sources of ionization. We also find that environment substantially affects the
reionization process. In fact, although the simulated proto-cluster occupies a
smaller volume and produces a higher number of ionizing photons, it gets
totally ionized later. This is because high density regions, which are more
common in the proto-cluster, are difficult to ionize because of their high
recombination rates.Comment: 10 pages, 9 figures, minor revisions, in press, MNRAS 343 (2003)
1101-110
The Hubble Legacy Archive NICMOS Grism Data
The Hubble Legacy Archive (HLA) aims to create calibrated science data from
the Hubble Space Telescope archive and make them accessible via user-friendly
and Virtual Observatory (VO) compatible interfaces. It is a collaboration
between the Space Telescope Science Institute (STScI), the Canadian Astronomy
Data Centre (CADC) and the Space Telescope - European Coordinating Facility
(ST-ECF). Data produced by the Hubble Space Telescope (HST) instruments with
slitless spectroscopy modes are among the most difficult to extract and
exploit. As part of the HLA project, the ST-ECF aims to provide calibrated
spectra for objects observed with these HST slitless modes. In this paper, we
present the HLA NICMOS G141 grism spectra. We describe in detail the
calibration, data reduction and spectrum extraction methods used to produce the
extracted spectra. The quality of the extracted spectra and associated direct
images is demonstrated through comparison with near-IR imaging catalogues and
existing near-IR spectroscopy. The output data products and their associated
metadata are publicly available through a web form at http://hla.stecf.org and
via VO interfaces. In total, 2470 spectra of 1923 unique targets are included
in the current release.Comment: 18 pages, 21 figures, accepted for publication in Astronomy &
Astrophysic
The Milky Way's satellite population in a LambdaCDM universe
We compare the structure and kinematics of the 11 known satellites of the
Milky Way with high resolution simulations of the formation of its dark halo in
a LambdaCDM universe. In contrast to earlier work, we find excellent agreement.
The observed kinematics are exactly those predicted for stellar populations
with the observed spatial structure orbiting within the most massive
``satellite'' substructures in our simulations. Less massive substructures have
weaker potential wells than those hosting the observed satellites. If there is
a halo substructure ``problem'', it consists in understanding why halo
substructures have been so inefficient in making stars. Suggested modifications
of dark matter properties (for example, self-interacting or warm dark matter)
may well spoil the good agreement found for standard Cold Dark Matter.Comment: 5 pages, 2 figures, replaced with version accepted by MNRAS (minor
changes
Wavelength dependent ac-Stark shift of the 1S0 - 3P1 transition at 657 nm in Ca
We have measured the ac-Stark shift of the 4s2 1S0 - 4s4p 3P1 line in 40Ca
for perturbing laser wavelengths between 780 nm and 1064 nm with a time domain
Ramsey-Borde atom interferometer. We found a zero crossing of the shift for the
mS = 0 - mP = 0 transition and \sigma polarized perturbation at 800.8(22) nm.
The data was analyzed by a model deriving the energy shift from known
transition wavelengths and strengths. To fit our data, we adjusted the Einstein
A coefficients of the 4s3d 3D - 4s4p 3P and 4s5s 3S - 4s4p 3P fine structure
multiplets. With these we can predict vanishing ac-Stark shifts for the 1S0 m =
0 - 3P1 m = 1 transition and \sigma- light at 983(12) nm and at 735.5(20) nm
for the transition to the 3P0 level.Comment: 8 pages, 5 figures, 2 table
A New Approach to the Study of Stellar Populations in Early-Type Galaxies: K-band Spectral Indices and an Application to the Fornax Cluster
New measurements of K-band spectral features are presented for eleven
early-type galaxies in the nearby Fornax galaxy cluster. Based on these
measurements, the following conclusions have been reached: (1) in galaxies with
no signatures of a young stellar component, the K-band Na I index is highly
correlated with both the optical metallicity indicator [MgFe]' and central
velocity dispersion; (2) in the same galaxies, the K-band Fe features saturate
in galaxies with sigma > 150 km/s while Na I (and [MgFe]') continues to
increase; (3) [Si/Fe] (and possibly [Na/Fe]) is larger in all observed Fornax
galaxies than in Galactic open clusters with near-solar metallicity; (4) in
various near-IR diagnostic diagrams, galaxies with signatures of a young
stellar component (strong Hbeta, weak [MgFe]') are clearly separated from
galaxies with purely old stellar populations; furthermore, this separation is
consistent with the presence of an increased number of M-giant stars (most
likely to be thermally pulsating AGB stars); (5) the near-IR diagrams discussed
here seem as efficient for detecting putatively young stellar components in
early-type galaxies as the more commonly used age/metallicity diagnostic plots
using optical indices (e.g Hbeta vs. [MgFe]').Comment: 47 pages, 16 figures, ApJ accepte
NAD(P)H:quinone oxidoreductase 1 (NQO1) P187S polymorphism and prostate cancer risk in Caucasians
NAD(P)H:quinone oxidoreductase 1 (NQO1) catalyses the reduction of quinoid compounds to hydroquinones, preventing the generation of free radicals and reactive oxygen. A “C” to “T” transversion at position 609 of NQO1, leading to a nonsynonymous amino acid change (Pro187Ser, P187S), results in an altered enzyme activity. No NQO1 protein activity was detected in NQO1 609TT genotype, and low to intermediate activity was detected in NQO1 609CT genotype compared with 609CC genotype. Thus, this polymorphism may result in altered cancer predisposition. For prostate cancer, only sparse data are available. We therefore analyzed the distribution of the NQO1 P187S SNP (single nucleotide polymorphism) in prostate cancer patients and a healthy control group. Allelic variants were determined using RFLP analysis. Overall, 232 patients without any malignancy and 119 consecutive prostate cancer patients were investigated. The genotype distribution in our cohorts followed the Hardy–Weinberg equilibrium in cases and controls. The distribution of the NQO1 codon 187 SNP did not differ significantly between prostate cancer patients and the control group (p = 0.242). There was also no association between the allelic variants and stage or Gleason score of the tumors. The NQO1 P187S SNP was not significantly associated with an increased prostate cancer risk in our cohorts. The SNP has also no influence on histopathological characteristics of the tumors. A combined analysis of all available data from published European studies also showed no significant differences in the genotype distribution between controls and prostate cancer patients. Our data suggest a minor role of the NQO1 nucleotide 609 polymorphism in prostate carcinogenesis
The effect of minihaloes on cosmic reionization
One of the most debated issues in the theoretical modeling of cosmic
reionization is the impact of small-mass gravitationally-bound structures. We
carry out the first numerical investigation of the role of such sterile
`minihaloes', which serve as self-shielding screens of ionizing photons.
Minihaloes are too small to be properly resolved in current large-scale
cosmological simulations, and thus we estimate their effects using a sub-grid
model, considering two cases that bracket their effect within this framework.
In the `extreme suppression' case in which minihalo formation ceases once a
region is partially ionized, their effect on cosmic reionization is modest,
reducing the volume-averaged ionization fraction by an overall factor of less
than 15%. In the other extreme, in which minihalo formation is never
suppressed, they delay complete reionization as much as Delta z~2, in rough
agreement with the results from a previous semi-analytical study by the
authors. Thus, depending on the details of the minihalo formation process,
their effect on the overall progress of reionization can range from modest to
significant, but the minihalo photon consumption is by itself insufficient to
force an extended reionization epoch.Comment: 9 pages, 2 figures, accepted for pubblication in MNRA
Substructures in Cold Dark Matter Haloes
We analyse the properties of substructures within dark matter halos
(subhalos) using a set of high-resolution numerical simulations of the
formation of structure in a Lambda-CDM Universe. Our simulation set includes 11
high-resolution simulations of massive clusters as well as a region of mean
density, allowing us to study the spatial and mass distribution of
substructures down to a mass resolution limit of 10^9 h^(-1)Mo. We also
investigate how the properties of substructures vary as a function of the mass
of the `parent' halo in which they are located. We find that the substructure
mass function depends at most weakly on the mass of the parent halo and is well
described by a power-law. The radial number density profiles of substructures
are steeper in low mass halos than in high mass halos. More massive
substructures tend to avoid the centres of halos and are preferentially located
in the external regions of their parent halos. We also study the mass accretion
and merging histories of substructures, which we find to be largely independent
of environment. We find that a significant fraction of the substructures
residing in clusters at the present day were accreted at redshifts z < 1. This
implies that a significant fraction of present-day `passive' cluster galaxies
should have been still outside the cluster progenitor and more active at z~1.Comment: 13 pages, 15 figure. Accepted to MNRA
The Hubble Legacy Archive ACS Grism Data
A public release of slitless spectra, obtained with ACS/WFC and the G800L
grism, is presented. Spectra were automatically extracted in a uniform way from
153 archival fields (or "associations") distributed across the two Galactic
caps, covering all observations to 2008. The ACS G800L grism provides a
wavelength range of 0.55-1.00 \mu40 \ \AA / pixel\sim 80\ \AA32,149i_{\rm
AB}0.2-4.6$.Comment: Accepted for publication in Astronomy and Astrophysics; 29 pages, 16
Figures, 4 Tables in text and 3Tables in Appendi
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