662 research outputs found
Constraints on the Dark Matter Particle Mass from the Number of Milky Way Satellites
We have conducted N-body simulations of the growth of Milky Way-sized halos
in cold and warm dark matter cosmologies. The number of dark matter satellites
in our simulated Milky Ways decreases with decreasing mass of the dark matter
particle. Assuming that the number of dark matter satellites exceeds or equals
the number of observed satellites of the Milky Way we derive lower limits on
the dark matter particle mass. We find with 95% confidence m_s > 13.3 keV for a
sterile neutrino produced by the Dodelson and Widrow mechanism, m_s > 8.9 keV
for the Shi and Fuller mechanism, m_s > 3.0 keV for the Higgs decay mechanism,
and m_{WDM} > 2.3 keV for a thermal dark matter particle. The recent discovery
of many new dark matter dominated satellites of the Milky Way in the Sloan
Digital Sky Survey allows us to set lower limits comparable to constraints from
the complementary methods of Lyman-alpha forest modeling and X-ray observations
of the unresolved cosmic X-ray background and of dark matter halos from dwarf
galaxy to cluster scales. Future surveys like LSST, DES, PanSTARRS, and
SkyMapper have the potential to discover many more satellites and further
improve constraints on the dark matter particle mass.Comment: 17 pages, 13 figures, replaced with final version published in
Physical Review
IL RUOLO DELLA TRANSFERRINA NELL'ASSORBIMENTO INTESTINALE DI Fe59 DEL RATTO NORMALE E SIDEOSICO. EUR 1871.i = THE ROLE OF THE TRANSFERRIN IN THE INTESTINAL ABSORPTION OF Fe59 OF THE NORMAL AND SIDEOUS RAT. EUR 1871.i
Load following with Small Modular Reactors (SMR): A real options analysis
Load following is the potential for a power plant to adjust its power output as demand and price for electricity fluctuates throughout the day. In nuclear power plants, this is done by inserting control rods into the reactor pressure vessel. This operation is very inefficient as nuclear power generation is composed almost entirely of fixed and sunk costs; therefore, lowering the power output doesn't significantly reduce generating costs and the plant is thermo-mechanical stressed. A more efficient solution is to maintain the primary circuit at full power and to use the excess power for cogeneration.
This paper assesses the technical-economic feasibility of this approach when applied to Small Modular Reactors (SMR) with two cogeneration technologies: algae-biofuel and desalinisation. Multiple SMR are of particular interest due to the fractional nature of their power output. The result shows that the power required by an algae-biofuel plant is not sufficient to justify the load following approach, whereas it is in the case of desalination.
The successive economic analysis, based on the real options approach, demonstrates the economic viability of the desalination in several scenarios. In conclusion, the coupling of SMR with a desalination plant is a realistic solution to perform efficient load following
Is Double Reionization Physically Plausible?
Recent observations of z~6 quasars and the cosmic microwave background imply
a complex history to cosmic reionization. Such a history requires some form of
feedback to extend reionization over a long time interval, but the nature of
the feedback and how rapidly it operates remain highly uncertain. Here we focus
on one aspect of this complexity: which physical processes can cause the global
ionized fraction to evolve non-monotonically with cosmic time? We consider a
range of mechanisms and conclude that double reionization is much less likely
than a long, but still monotonic, ionization history. We first examine how
galactic winds affect the transition from metal-free to normal star formation.
Because the transition is actually spatially inhomogeneous and temporally
extended, this mechanism cannot be responsible for double reionization given
plausible parameters for the winds. We next consider photoheating, which causes
the cosmological Jeans mass to increase in ionized regions and hence suppresses
galaxy formation there. In this case, double reionization requires that small
halos form stars efficiently, that the suppression from photoheating is strong
relative to current expectations, and that ionizing photons are preferentially
produced outside of previously ionized regions. Finally, we consider H_2
photodissociation, in which the buildup of a soft ultraviolet background
suppresses star formation in small halos. This can in principle cause the
ionized fraction to temporarily decrease, but only during the earliest stages
of reionization. Finally, we briefly consider the effects of some of these
feedback mechanisms on the topology of reionization.Comment: 13 pages, 5 figures, in press at ApJ (reorganized significantly but
major conclusions unchanged
Cosmic Microwave Background Polarization and reionization: constraining models with a double reionization
Neutral hydrogen around high-z QSO and an optical depth tau ~ 0.17 can be
reconciled if reionization is more complex than a single transition at z ~ 6-8.
Tracing its details could shed a new light on the first sources of radiation.
Here we discuss how far such details can be inspected through planned
experiments on CMB large-scale anisotropy and polarization, by simulating an
actual data analysis. By considering a set of double reionization histories of
Cen (2003) type, a relevant class of models not yet considered by previous
works, we confirm that large angle experiments rival high resolution ones in
reconstructing the reionization history. We also confirm that reionization
histories, studied with the prior of a single and sharp reionization, yield a
biased tau, showing that this bias is generic. We further find a monotonic
trend in the bias for the models that we consider, and propose an explanation
of the trend, as well as the overall bias. We also show that in long-lived
experiments such a trend can be used to discriminate between single and double
reionization patterns.Comment: 8 pages, 11 figures. Substantial rewriting, replaced with accepted
version. To be published in A&
The Evolution of the Effective Equation of State of the IGM
We develop a method to extract the "effective equation of state" of the
intergalactic medium from the doppler b parameter distribution of the
low-density Lyman-alpha forest. We test the method on numerical simulations and
then apply it to published observations of the Lyman-alpha forest at redshifts
z from 0 to 4. We find that the effective equation of state is close to
isothermal at redshift z=3, indicating that a second reheating of the IGM took
place at z=3. This reheating can plausibly be identified with the reionization
of HeII observed to occur at z about 3.Comment: Revised version accepted for publication in ApJ. Vol. 534 May 1, 2000
(in press); 32 pages, 13 figures, 7 table
The Contribution of z < or Approx. 6 Sources to the Spatial Coherence in the Unresolved Cosmic Near-Infrared and X-Ray Backgrounds
A spatial clustering signal has been established in Spitzer/IRAC measurements of the unresolved cosmic near-infrared background (CIB) out to large angular scales, approx. 1deg. This CIB signal, while significantly exceeding the contribution from the remaining known galaxies, was further found to be coherent at a highly statistically significant level with the unresolved soft cosmic X-ray background (CXB). This measurement probes the unresolved CXB to very faint source levels using deep near-IR source subtraction.We study contributions from extragalactic populations at low to intermediate redshifts to the measured positive cross-power signal of the CIB fluctuations with the CXB. We model the X-ray emission from active galactic nuclei (AGNs), normal galaxies, and hot gas residing in virialized structures, calculating their CXB contribution including their spatial coherence with all infrared emitting counterparts. We use a halo model framework to calculate the auto and cross-power spectra of the unresolved fluctuations based on the latest constraints of the halo occupation distribution and the biasing of AGNs, galaxies, and diffuse emission. At small angular scales (1), the 4.5microns versus 0.5-2 keV coherence can be explained by shot noise from galaxies and AGNs. However, at large angular scales (approx.10), we find that the net contribution from the modeled populations is only able to account for approx. 3% of the measured CIBCXB cross-power. The discrepancy suggests that the CIBCXB signal originates from the same unknown source population producing the CIB clustering signal out to approx. 1deg
Machine learning in clinical and epidemiological research: Isn't it time for biostatisticians to work on it?
- …