382 research outputs found
Damped Lyman alpha Absorbing Galaxies At Low Redshifts z<1 From Hierarchical Galaxy Formation Models
We investigate Damped Ly-alpha absorbing galaxies (DLA galaxies) at low
redshifts z<1 in the hierarchical structure formation scenario to clarify the
nature of DLA galaxies because observational data of such galaxies mainly at
low redshifts are currently available. We find that our model well reproduces
distributions of fundamental properties of DLA galaxies such as luminosities,
column densities, impact parameters obtained by optical and near-infrared
imagings. Our results suggest that DLA systems primarily consist of low
luminosity galaxies with small impact parameters (typical radius about 3 kpc,
surface brightness from 22 to 27 mag arcsec^{-2}) similar to low surface
brightness (LSB) galaxies. In addition, we investigate selection biases arising
from the faintness and from the masking effect which prevents us from
identifying a DLA galaxy hidden or contaminated by a point spread function of a
background quasar. We find that the latter affects the distributions of DLA
properties more seriously rather than the former, and that the observational
data are well reproduced only when taking into account the masking effect. The
missing rate of DLA galaxies by the masking effect attains 60-90 % in the
sample at redshift 0<z<1 when an angular size limit is as small as 1 arcsec.
Furthermore we find a tight correlation between HI mass and cross section of
DLA galaxies, and also find that HI-rich galaxies with M(HI) \sim 10^{9} M_sun
dominate DLA systems. These features are entirely consistent with those from
the Arecibo Dual-Beam Survey which is a blind 21 cm survey. Finally we discuss
star formation rates, and find that they are typically about 10^{-2} M_sun
yr^{-1} as low as those in LSB galaxies.Comment: 21 pages, 13 figures, Accepted for publication in Astrophsical
Journa
Formation of Sub-galactic Clouds under UV Background Radiation
The effects of the UV background radiation on the formation of sub-galactic
clouds are studied by means of one-dimensional hydrodynamical simulations. The
radiative transfer of the ionizing photons due to the absorption by HI, HeI and
HeII, neglecting the emission, is explicitly taken into account. We find that
the complete suppression of collapse occurs for the clouds with circular
velocities typically in the range V_c \sim 15-40 km/s and the 50% reduction in
the cooled gas mass with V_c \sim 20-55 km/s. These values depend most
sensitively on the collapse epoch of the cloud, the shape of the UV spectrum,
and the evolution of the UV intensity. Compared to the optically thin case,
previously investigated by Thoul & Weinberg (1996), the absorption of the
external UV photon by the intervening medium systematically lowers the above
threshold values by \Delta V_c \sim 5 km/s. Whether the gas can contract or
keeps expanding is roughly determined by the balance between the gravitational
force and the thermal pressure gradient when it is maximally exposed to the
external UV flux. Based on our simulation results, we discuss a number of
implications on galaxy formation, cosmic star formation history, and the
observations of quasar absorption lines. In Appendix, we derive analytical
formulae for the photoionization coefficients and heating rates, which
incorporate the frequency/direction-dependent transfer of external photons.Comment: 38 pages, 16 figures, accepted for publication in Ap
Análise do mercado de serviços de regulação de frequência secundária e terciária no sistema eléctrico português
Dissertação para obtenção do grau de Mestre em Engenharia Electrotécnica Ramo EnergiaOs operadores das redes de transporte (ORT) enfrentam desafios cada vez maiores e mais imprevisíveis. A forte penetração de energias renováveis e a liberalização do mercado de energia eléctrica dão origem a uma maior volatilidade na produção de energia eléctrica e a desencontros entre produção e consumo, passíveis de acontecer no decorrer da operação do sistema eléctrico. Os serviços de sistema são um complemento aos mercados de energia eléctrica e caracterizam-se como sendo uma segurança necessária ao bom funcionamento do sistema pois é através destes que é feito o equilíbrio entre geração e consumo em tempo real e assim feita a regulação de frequência da rede eléctrica.
Com o presente trabalho pretende-se fazer uma análise aprofundada do que são os serviços de sistema para regulação de frequência, nomeadamente sobre a regulação secundária e a regulação terciária bem como abordar a regulamentação que as fundamenta. Será também aprofundado o funcionamento do mercado de serviços de sistema para regulação de frequência em Portugal e oencadeamento cronológico deste com o mercado diário e com as diversas sessões intradiárias do Mercado Ibérico de Electricidade (MIBEL). Serão ainda referidos diversos serviços de sistema noutros países europeus em comparação com Portugal.
Para apoio a esta análise desenvolveu-se uma ferramenta em Matlab®que permita visualizar sob a
forma de gráficos as ofertas dos agentes de mercado participantes através das suas unidades físicas (grupos geradores), o custo marginal dessas mesmas centrais e ainda visualizar o preço de fecho de mercado na respectiva hora.
Os resultados obtidos apontam para uma dependência do perfil de licitação relativamente à tecnologia da central. Foram ainda analisadas as correlações entre centrais e a dependência do preço das ofertas e da banda contratada relativamente à existência de meses mais ou menos chuvosos, ou seja com maior ou menos índice de produtibilidade hidroeléctrica (IPH). Denota-se ainda uma elevada participação das centrais hídricas neste mercado de banda secundária.Abstract: The transmission system operators (TSO) face increasing and more unpredictable challenges. The high penetration of renewable energy and the liberalization of the electricity market rises the volatility in electricity generation and bigger differences between production and consumption, in the course of the electrical system operation. Ancillary services are a complement to electricity markets and are characterized as a necessary service that allows a proper and reliable functioning of the system as it is the mechanism that allows the TSO to balance generation and consumption in real time operation and control the frequency of the system.
The present work aims to make a thorough analysis of what are the ancillary services for frequency regulation, particularly on secondary regulation and tertiary regulation and to address the laws that allow to develop the frequency regulation in Portugal. It will also be studied the operation of the ancillary services market for frequency regulation in Portugal and the chronological connection with the daily market and the various intraday market sessions of Iberian Electricity Market (MIBEL). Itwill be studied also various ancillary services in other European countries compared to Portugal.
To support this data analysis a Matlab® tool was developed allowing the user to view in the graphical form the offers of market players participating with their physical units (power plants), the marginal cost of these same power plants and the market clearing price, all for the same selected period of time.
The results show a dependence of the bidding profile relative to the power plant technology. It will be also studied the correlations between different power plants and the price variation of the market due to the variation of the hydroelectric capability factor (HCF) variation. It is also relevant the high share of hydro power plants participating in the ancillary services market for secondary frequency control
Implications of a non-universal IMF from C, N, and O abundances in very metal-poor Galactic stars and damped Lyman-alpha absorbers
Recently revealed C, N, and O abundances in the most metal-poor damped
Lyman-alpha (DLA) absorbers are compared with those of extremely metal-poor
stars in the Galactic halo, as well as extragalactic H II regions, to decipher
nucleosynthesis and chemical enrichment in the early Universe. These
comparisons surprisingly identify a relatively high C/O ratio and a low N/O
ratio in DLA systems, which is hard to explain theoretically. We propose that
if these features are confirmed by future studies, this effect occurs because
the initial mass function in metal-poor DLA systems has a cut-off at the upper
mass end at around 20-25 Msun, thus lacks the massive stars that provide the
nucleosynthesis products leading to the low C/O and high N/O ratios. This
finding is a reasonable explanation of the nature of DLA systems in which a
sufficient amount of cold H I gas remains intact because of the suppression of
ionization by massive stars. In addition, our claim strongly supports a high
production rate of N in very massive stars, which might be acceptable in light
of the recent nucleosynthesis calculations with fast rotation models. The
updates of both abundance data and nucleosynthesis results will strengthen our
novel proposition that the C/O and N/O abundances are a powerful tool for
inferring the form of the initial mass function.Comment: 9 pages including 5 figures, accepted for publication in A&
Multifaceted anti-amyloidogenic and pro-amyloidogenic effects of C-reactive protein and serum amyloid P component in vitro
C-reactive protein (CRP) and serum amyloid P component (SAP), two major classical pentraxins in humans, are soluble pattern recognition molecules that regulate the innate immune system, but their chaperone activities remain poorly understood. Here, we examined their effects on the amyloid fibril formation from Alzheimer’s amyloid β (Aβ) (1-40) and on that from D76N β2-microglobulin (β2-m) which is related to hereditary systemic amyloidosis. CRP and SAP dose-dependently and substoichiometrically inhibited both Aβ(1-40) and D76N β2-m fibril formation in a Ca2+-independent manner. CRP and SAP interacted with fresh and aggregated Aβ(1-40) and D76N β2-m on the fibril-forming pathway. Interestingly, in the presence of Ca2+, SAP first inhibited, then significantly accelerated D76N β2-m fibril formation. Electron microscopically, the surface of the D76N β2-m fibril was coated with pentameric SAP. These data suggest that SAP first exhibits anti-amyloidogenic activity possibly via A face, followed by pro-amyloidogenic activity via B face, proposing a model that the pro- and anti-amyloidogenic activities of SAP are not mutually exclusive, but reflect two sides of the same coin, i.e., the B and A faces, respectively. Finally, SAP inhibits the heat-induced amorphous aggregation of human glutathione S-transferase. A possible role of pentraxins to maintain extracellular proteostasis is discussed
Do Damped and Sub-damped Lyman-alpha Absorbers Arise in Galaxies of Different Masses?
We consider the questions of whether the damped Lyman-alpha (DLA) and sub-DLA
absorbers in quasar spectra differ intrinsically in metallicity, and whether
they could arise in galaxies of different masses. Using the recent measurements
of the robust metallicity indicators Zn and S in DLAs and sub-DLAs, we confirm
that sub-DLAs have higher mean metallicities than DLAs, especially at . We find that the intercept of the metallicity-redshift relation
derived from Zn and S is higher than that derived from Fe by 0.5-0.6 dex. We
also show that, while there is a correlation between the metallicity and the
rest equivalent width of Mg II or Fe II for DLAs,
no correlation is seen for sub-DLAs. Given this, and the similar Mg II or Fe II
selection criteria employed in the discovery of both types of systems at lower
redshifts, the difference between metallicities of DLAs and sub-DLAs appears to
be real and not an artefact of selection. This conclusion is supported by our
simulations of Mg II and Fe II lines for a wide
range of physical conditions. On examining the velocity spreads of the
absorbers, we find that sub-DLAs show somewhat higher mean and median velocity
spreads (), and an excess of systems with km
s, than DLAs. Compared to DLAs, the [Mn/Fe] vs. [Zn/H] trend for
sub-DLAs appears to be steeper and closer to the trend for Galactic bulge and
thick disk stars, possibly suggesting different stellar populations. The
absorber data appear to be consistent with galaxy down-sizing. The data are
also consistent with the relative number densities of low-mass and high-mass
galaxies. It is thus plausible that sub-DLAs arise in more massive galaxies on
average than DLAs.Comment: 27 pages, 5 figures, 4 tables. Accepted for publication in New
Astronom
Two-Photon Dichroic Atomic Vapor Laser Lock Using Electromagnetically Induced Transparency and Absorption
We demonstrate a technique to lock the frequency of a laser to a transition
between two excited states in Rb vapor in the presence of a weak magnetic
field. We use a ladder configuration from specific hyperfine sublevels of the
5S 1/2, 5P 3/2, and 5D 5/2 levels. This atomic configuration can show
Electromagnetically Induced Transparency and Absorption processes. The error
signal comes from the difference in the transparency or absorption felt by the
two orthogonal polarizations of the probe beam. A simplified model is in good
quantitative agreement with the observed signals for the experimental
parameters. We have used this technique to lock the frequency of the laser up
to 1.5 GHz off atomic resonance.Comment: 6 pages, 8 figures. Last version for publication in J. Opt. Soc. Am.
First-line treatment and outcome of elderly patients with primary central nervous system lymphoma (PCNSL)—a systematic review and individual patient data meta-analysis
Evidence for prognosis and treatment of elderly patient with primary central nervous system is limited. High-dose methotrexate should be applied whenever possible, especially combination with oral alkylating agents is a promising approach. Further combinations with other intravenous drugs do not seem to improve outcome. More prospective trials designed for elderly PCNSL patients are warrante
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