59 research outputs found
The Race Between Stars and Quasars in Reionizing Cosmic Hydrogen
The cosmological background of ionizing radiation has been dominated by
quasars once the Universe aged by ~2 billion years. At earlier times (redshifts
z>3), the observed abundance of bright quasars declined sharply, implying that
cosmic hydrogen was reionized by stars instead. Here, we explain the physical
origin of the transition between the dominance of stars and quasars as a
generic feature of structure formation in the concordance LCDM cosmology. At
early times, the fraction of baryons in galaxies grows faster than the maximum
(Eddington-limited) growth rate possible for quasars. As a result, quasars were
not able to catch up with the rapid early growth of stellar mass in their host
galaxies.Comment: 5 pages, 1 figure, Accepted for publication in JCA
Gamma-Ray Luminosity Function of Blazars and the Cosmic Gamma-Ray Background: Evidence for the Luminosity Dependent Density Evolution
We present a comprehensive study for the gamma-ray luminosity function (GLF)
of blazars and their contribution to the extragalactic diffuse gamma-ray
background (EGRB). Radio and gamma-ray luminosity correlation is introduced to
take into account the radio detectability which is important for the blazar
identification. Previous studies considered only pure luminosity evolution
(PLE) or pure density evolution, but we introduce the luminosity dependent
density evolution (LDDE) model, which is favored from the evolution of X-ray
luminosity function (XLF) of AGNs. The model parameters are constrained by
likelihood analyses about the observed redshift and gamma-ray flux
distributions of the EGRET blazars. We find that the LDDE model gives a better
fit to the observed distributions than the PLE model, indicating that the LDDE
model is also appropriate for gamma-ray blazars, and that the jet activity is
universally correlated with the accretion history of AGNs. The normalization
between the GLF and XLF is consistent with the unified picture of AGNs, when
the beaming and a reasonable duty cycle of jet activity are taken into account.
We then find that only 25--50% of the EGRB can be explained by unresolved
blazars with the best-fit LDDE parameters. Unresolved blazars can account for
all the EGRB only with a steeper index of the faint-end slope of the GLF, which
is marginally consistent with the EGRET data but inconsistent with that of the
XLF. Therefore unresolved AGNs cannot be the dominant source of the EGRB,
unless there is a new population of gamma-ray emitting AGNs that evolves
differently from the XLF of AGNs. Predictions for the GLAST mission are made,
and we find that the best-fit LDDE model predicts about 3000 blazars in the
entire sky, which is considerably fewer than a previous estimate.Comment: 13 pages, 12 figures, accepted by ApJ; minor typos corrected and some
figures revised, main conclusions essentially unchange
Olber's Paradox for Superluminal Neutrinos: Constraining Extreme Neutrino Speeds at TeV-ZeV Energies with the Diffuse Neutrino Background
The only invariant speed in special relativity is c; therefore, if some
neutrinos travel at even tiny speeds above c, normal special relativity is
incomplete and any superluminal speed may be possible. I derive a limit on
superluminal neutrino speeds v >> c at high energies by noting that such speeds
would increase the size of the neutrino horizon. The increased volume of the
Universe visible leads to a brighter astrophysical neutrino background. The
nondetection of "guaranteed" neutrino backgrounds from star-forming galaxies
and ultrahigh energy cosmic rays (UHECRs) constrains v/c at TeV--ZeV energies.
I find that v/c <= 820 at 60 TeV from the nondetection of neutrinos from
star-forming galaxies. The nondetection of neutrinos from UHECRs constrains v/c
to be less than 2500 at 0.1 EeV in a pessimistic model and less than 4.6 at 4
EeV in an optimistic model. The UHECR neutrino background nondetection is
strongly inconsistent with a naive quadratic extrapolation of the OPERA results
to EeV energies. The limits apply subject to some caveats, particularly that
the expected pionic neutrino backgrounds exist and that neutrinos travel faster
than c when they pass the detector. They could be improved substantially as the
expected neutrino backgrounds are better understood and with new experimental
neutrino background limits. I also point out that extremely subluminal speeds
would result in a much smaller neutrino background intensity than expected.Comment: 13 pages, 2 figures, fixed titl
State of the climate in 2013
In 2013, the vast majority of the monitored climate variables reported here maintained trends established in recent decades. ENSO was in a neutral state during the entire year, remaining mostly on the cool side of neutral with modest impacts on regional weather patterns around the world. This follows several years dominated by the effects of either La Niña or El Niño events. According to several independent analyses, 2013 was again among the 10 warmest years on record at the global scale, both at the Earths surface and through the troposphere. Some regions in the Southern Hemisphere had record or near-record high temperatures for the year. Australia observed its hottest year on record, while Argentina and New Zealand reported their second and third hottest years, respectively. In Antarctica, Amundsen-Scott South Pole Station reported its highest annual temperature since records began in 1957. At the opposite pole, the Arctic observed its seventh warmest year since records began in the early 20th century. At 20-m depth, record high temperatures were measured at some permafrost stations on the North Slope of Alaska and in the Brooks Range. In the Northern Hemisphere extratropics, anomalous meridional atmospheric circulation occurred throughout much of the year, leading to marked regional extremes of both temperature and precipitation. Cold temperature anomalies during winter across Eurasia were followed by warm spring temperature anomalies, which were linked to a new record low Eurasian snow cover extent in May. Minimum sea ice extent in the Arctic was the sixth lowest since satellite observations began in 1979. Including 2013, all seven lowest extents on record have occurred in the past seven years. Antarctica, on the other hand, had above-average sea ice extent throughout 2013, with 116 days of new daily high extent records, including a new daily maximum sea ice area of 19.57 million km2 reached on 1 October. ENSO-neutral conditions in the eastern central Pacific Ocean and a negative Pacific decadal oscillation pattern in the North Pacific had the largest impacts on the global sea surface temperature in 2013. The North Pacific reached a historic high temperature in 2013 and on balance the globally-averaged sea surface temperature was among the 10 highest on record. Overall, the salt content in nearsurface ocean waters increased while in intermediate waters it decreased. Global mean sea level continued to rise during 2013, on pace with a trend of 3.2 mm yr-1 over the past two decades. A portion of this trend (0.5 mm yr-1) has been attributed to natural variability associated with the Pacific decadal oscillation as well as to ongoing contributions from the melting of glaciers and ice sheets and ocean warming. Global tropical cyclone frequency during 2013 was slightly above average with a total of 94 storms, although the North Atlantic Basin had its quietest hurricane season since 1994. In the Western North Pacific Basin, Super Typhoon Haiyan, the deadliest tropical cyclone of 2013, had 1-minute sustained winds estimated to be 170 kt (87.5 m s-1) on 7 November, the highest wind speed ever assigned to a tropical cyclone. High storm surge was also associated with Haiyan as it made landfall over the central Philippines, an area where sea level is currently at historic highs, increasing by 200 mm since 1970. In the atmosphere, carbon dioxide, methane, and nitrous oxide all continued to increase in 2013. As in previous years, each of these major greenhouse gases once again reached historic high concentrations. In the Arctic, carbon dioxide and methane increased at the same rate as the global increase. These increases are likely due to export from lower latitudes rather than a consequence of increases in Arctic sources, such as thawing permafrost. At Mauna Loa, Hawaii, for the first time since measurements began in 1958, the daily average mixing ratio of carbon dioxide exceeded 400 ppm on 9 May. The state of these variables, along with dozens of others, and the 2013 climate conditions of regions around the world are discussed in further detail in this 24th edition of the State of the Climate series. © 2014, American Meteorological Society. All rights reserved
Sleutelfactoren en ecosysteemfunctioneren. I. Eerste verkenning in laagveensloten
Dit rapport beschrijft onderzoek dat gedaan is naar mogelijke sleutelfactoren voor aquatisch ecologische kwaliteit in laagveensloten. Het onderzoek is uitgevoerd in drie deelgebieden in de Wieden, die in een gradiënt van sterke beïnvloeding naar een meer natuurlijke situatie lagen. In deze sloot zijn met continuloggers temperatuurkarakteristieken van zes sloten bepaald. Elke sloot bleek zijn eigen karakteristieken te hebben, samenhangend met waterdiepte, kwel en begroeiing. De macrofaunagemeenschappen van de sloten bleken sterk per locatie te verschillen. Binnen de verschillende habitats van deze sloten waren de verschillen kleiner. Verwacht wordt dat sleutelfactoren daarom meer op slootniveau dan op habitatniveau zullen ingrijpen. In vervolgonderzoek zal de rol van de zuurstofconcentratie in deze systemen uitgebreider onderzocht moeten worden
Sleutelfactoren en ecosysteemfunctioneren. I. Eerste verkenning in laagveensloten
Dit rapport beschrijft onderzoek dat gedaan is naar mogelijke sleutelfactoren voor aquatisch ecologische kwaliteit in laagveensloten. Het onderzoek is uitgevoerd in drie deelgebieden in de Wieden, die in een gradiënt van sterke beïnvloeding naar een meer natuurlijke situatie lagen. In deze sloot zijn met continuloggers temperatuurkarakteristieken van zes sloten bepaald. Elke sloot bleek zijn eigen karakteristieken te hebben, samenhangend met waterdiepte, kwel en begroeiing. De macrofaunagemeenschappen van de sloten bleken sterk per locatie te verschillen. Binnen de verschillende habitats van deze sloten waren de verschillen kleiner. Verwacht wordt dat sleutelfactoren daarom meer op slootniveau dan op habitatniveau zullen ingrijpen. In vervolgonderzoek zal de rol van de zuurstofconcentratie in deze systemen uitgebreider onderzocht moeten worden
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