772 research outputs found
The Two Phases of Galaxy Formation
Cosmological simulations of galaxy formation appear to show a two-phase
character with a rapid early phase at z>2 during which in-situ stars are formed
within the galaxy from infalling cold gas followed by an extended phase since
z<3 during which ex-situ stars are primarily accreted. In the latter phase
massive systems grow considerably in mass and radius by accretion of smaller
satellite stellar systems formed at quite early times (z>3) outside of the
virial radius of the forming central galaxy. These tentative conclusions are
obtained from high resolution re-simulations of 39 individual galaxies in a
full cosmological context with present-day virial halo masses ranging from 7e11
M_sun h^-1 < M_vir < 2.7e13 M_sun h^-1 and central galaxy masses between 4.5e10
M_sun h^-1 < M_* < 3.6e11 M_sun h^-1. The simulations include the effects of a
uniform UV background, radiative cooling, star formation and energetic feedback
from SNII. The importance of stellar accretion increases with galaxy mass and
towards lower redshift. In our simulations lower mass galaxies (M_* > 1.7e11 M_sun h^-1) assembly is dominated by accretion and
merging with about 80 per cent of the stars added by the present-day. In
general the simulated galaxies approximately double their mass since z=1. For
massive systems this mass growth is not accompanied by significant star
formation. The majority of the in-situ created stars is formed at z>2,
primarily out of cold gas flows. We recover the observational result of
archaeological downsizing, where the most massive galaxies harbor the oldest
stars. We find that this is not in contradiction with hierarchical structure
formation. Most stars in the massive galaxies are formed early on in smaller
structures, the galaxies themselves are assembled late.Comment: 13 pages, 13 figures, accepted for publication in Ap
X-Ray Emission from the Warm Hot Intergalactic Medium
The number of detected baryons in the Universe at z<0.5 is much smaller than
predicted by standard big bang nucleosynthesis and by the detailed observation
of the Lyman alpha forest at red-shift z=2. Hydrodynamical simulations indicate
that a large fraction of the baryons today is expected to be in a ``warm-hot''
(10^5-10^7K) filamentary gas, distributed in the intergalactic medium. This
gas, if it exists, should be observable only in the soft X-ray and UV bands.
Using the predictions of a particular hydrodynamic model, we simulated the
expected X-ray flux as a function of energy in the 0.1-2 keV band due to the
Warm-Hot Intergalactic Medium (WHIM), and compared it with the flux from local
and high red-shift diffuse components. Our results show that as much as 20% of
the total diffuse X-ray background (DXB) in the energy range 0.37-0.925keV
could be due to X-ray flux from the WHIM, 70% of which comes from filaments at
redshift z between 0.1 and 0.6. Simulations done using a FOV of 3', comparable
with that of Suzaku and Constellation-X, show that in more than 20% of the
observations we expect the WHIM flux to contribute to more than 20% of the DXB.
These simulations also show that in about 10% of all the observations a single
bright filament in the FOV accounts, alone, for more than 20% of the DXB flux.
Red-shifted oxygen lines should be clearly visible in these observations.Comment: 19 pages, 6 figure
Contribution of Unresolved Point Sources to the Diffuse X-ray Background below 1 keV
We present here the analysis of X-rays point sources detected in several
observations available in the XMM-Newton public archive. We focused, in
particular, on energies below 1 keV, which are of particular relevance to the
understanding of the Diffuse X-ray Background. The average field of all the
exposures is 0.09 deg^-2. We reached an average flux sensitivity of 5.8x10^-16
erg s^-1 cm^-2 in the soft band (0.5-2.0 keV) and 2.5x10^-16 erg s^-1 cm^-2 in
the very soft band (0.4-0.6 keV). In this paper we discuss the logN-logS
results, the contribution to the integrated X-ray sky flux, and the properties
of the cumulative spectrum from all sources. In particular, we found an excess
flux at around 0.5 keV in the composite spectrum of faint sources. The excess
seems to be a general property of all the fields observed suggesting an
additional class of weak sources is contributing to the X-ray emission at these
energies. Combining our results with previous investigations we have also
quantified the contribution of the individual components of the diffuse X-ray
Background in the 3/4 keV band.Comment: Accepted for publication in ApJ; 27 pages, 8 figure
SismicitĂ allâEtna dal 1989 al 2010: evidenze sullâevoluzione spazio-temporale dellâattivitĂ sismica
Il Monte Etna, uno dei piĂč attivi vulcani basaltici tra i piĂč monitorati al mondo, Ăš sede di una notevole attivitĂ sismica e vulcanica. Esso Ăš ubicato in Sicilia orientale in un complesso quadro geodinamico, dove le principali strutture tettoniche regionali giocano un ruolo chiave nei processi dinamici del vulcano. La sismicitĂ dellâEtna si manifesta con un elevato rate di terremoti di bassa e moderata energia che, a volte, a causa dellâestrema superficialitĂ della sorgente, provocano danni ai centri abitati prossimi allâarea epicentrale. Il monitoraggio sistematico dellâattivitĂ sismica etnea Ăš effettuato sin dal 1989, mediante una rete sismica locale permanente che nel tempo Ăš stata oggetto di importanti miglioramenti. La prima configurazione di rete era costituita da circa 10 stazioni analogiche con sensori a corto periodo gestita dallâIstituto Internazionale di Vulcanologia (IIV-CNR). Nel 1994, una rete sismica costituita da circa 40 stazioni (analogiche con sensori a corto periodo) fu installata sullâEtna nellâambito del Progetto Poseidon. Nel 2001, le reti gestite dallâIIV-CNR e dal Progetto Poseidon confluirono nellâIstituto Nazionale di Geofisica e Vulcanologia (INGV); attualmente la rete sismica, costituita da circa 50 stazioni digitali equipaggiate con sismometri broadband a tre componenti, Ăš gestita dalla Sezione di Catania dellâINGV.
Nel periodo 1989-1999, il catalogo dei terremoti risulta costituito da circa 2000 eventi con soglia di completezza per magnitudo pari a 2.0; dal 1999 ad oggi contiene circa 6000 terremoti con soglia di completezza per magnitudo 1.5. La capacitĂ di detezione della rete Ăš migliorata nel tempo permettendo di registrare e localizzare anche gli eventi meno energetici (Mâ„1.0).
In questo lavoro, vengono presentati i caratteri predominanti della sismicitĂ etnea negli ultimi 20 anni, con un maggiore dettaglio della distribuzione spazio-temporale della sismicitĂ verificatasi dal 1999. Lâanalisi della attivitĂ sismica rappresenta un utile strumento per lâinterpretazione delle dinamiche che hanno contraddistinto numerose ed importanti eruzioni (2001, 2002-03, 2004, 2006, 2008-09). In particolare, la variazione del rilascio energetico della sismicitĂ ha contribuito in maniera significativa ad identificare i probabili processi geodinamici legati alla ricarica del sistema magmatico del vulcano.
La distribuzione spaziale della sismicitĂ ha consentito di evidenziare inoltre lâesistenza di diverse aree sismogenetiche caratterizzate da un differente rate sismico, profonditĂ focali e cinematica delle strutture associate. Infine, osservando le caratteristiche della sismicitĂ nel lungo periodo, differenti settori del vulcano sono risultati maggiormente attivi in relazione ai piĂč importanti recenti eventi eruttivi
Crustal deformation, active tectonics and seismic potential in the Sicily Channel (Central Mediterranean), along the NubiaâEurasia plate boundary
Based on multidisciplinary data, including seismological and geodetic observations, as well as seismic reflection profiles and gravity maps, we analysed the pattern of crustal deformation and active tectonics in the Sicily Channel, a key observation point to unravel the complex interaction between two major plates, Nubia and Eurasia, in the Mediterranean Sea. Our data highlight the presence of an active ~ 220-km-long complex lithospheric fault system (here named the Lampedusa-Sciacca Shear Zone), approximately oriented NâS, crossing the study area with left-lateral strike-slip deformations, active volcanism and high heat flow. We suggest that this shear zone represents the most active tectonic domain in the area, while the NWâSE elongated rifting pattern, considered the first order tectonic feature, appears currently inactive and sealed by undeformed recent (Lower Pleistocene?) deposits. Estimates of seismological and geodetic moment-rates, 6.58 Ă 1015 Nm/year and 7.24 Ă 1017 Nm/year, respectively, suggests that seismicity accounts only for ~ 0.9% of crustal deformation, while the anomalous thermal state and the low thickness of the crust would significantly inhibit frictional sliding in favour of creeping and aseismic deformation. We therefore conclude that a significant amount of the estimated crustal deformation-rate occurs aseismically, opening new scenarios for seismic risk assessments in the region
Causality estimates among brain cortical areas by Partial Directed Coherence: simulations and application to real data
The problem of the definition and evaluation of brain connectivity has become a central one in neuroscience during the latest years, as a way to understand the organization and interaction of cortical areas during the execution of cognitive or motor tasks. Among various methods established during the years, the Partial Directed Coherence (PDC) is a frequency-domain approach to this problem, based on a multivariate autoregressive modeling of time series and on the concept of Granger causality. In this paper we propose the use of the PDC method on cortical signals estimated from high resolution EEG recordings, a non invasive method which exhibits a higher spatial resolution than conventional cerebral electromagnetic measures. The principle contributions of this work are the results of a simulation study, testing the performances of PDC, and a statistical analysis (via the ANOVA, analysis of variance) of the influence of different levels of Signal to Noise Ratio and temporal length, as they have been systematically imposed on simulated signals. An application to high resolution EEG recordings during a foot movement is also presented
Studying the Warm-Hot Intergalactic Medium in Emission
We assess the possibility to detect the warm-hot intergalactic medium (WHIM)
in emission and to characterize its physical conditions and spatial
distribution through spatially resolved X-ray spectroscopy, in the framework of
the recently proposed DIOS, EDGE, Xenia, and ORIGIN missions, all of which are
equipped with microcalorimeter-based detectors. For this purpose we analyze a
large set of mock emission spectra, extracted from a cosmological
hydrodynamical simulation. These mock X-ray spectra are searched for emission
features showing both the OVII K alpha triplet and OVIII Ly alpha line, which
constitute a typical signature of the warm hot gas. Our analysis shows that 1
Ms long exposures and energy resolution of 2.5 eV will allow us to detect about
400 such features per deg^2 with a significance >5 sigma and reveals that these
emission systems are typically associated with density ~100 above the mean. The
temperature can be estimated from the line ratio with a precision of ~20%. The
combined effect of contamination from other lines, variation in the level of
the continuum, and degradation of the energy resolution reduces these
estimates. Yet, with an energy resolution of 7 eV and all these effects taken
into account, one still expects about 160 detections per deg^2. These line
systems are sufficient to trace the spatial distribution of the line-emitting
gas, which constitute an additional information, independent from line
statistics, to constrain the poorly known cosmic chemical enrichment history
and the stellar feedback processes.Comment: 19 pages, 10 figures, ApJ in press; revised version according to
revie
A proximity-based method to identify genomic regions correlated with a continuously varying environmental variable
Knowledge of markers in the human genome which show spatial patterns and display extreme correlation with different environmental determinants play an important role in understanding the factors which affect the biological evolution of our species. We used the genotype data of more than half a million single nucleotide polymorphisms (SNPs) from the data set Human Genome Diversity Panel (HGDP-CEPH -CEPH) and we calculated Spearman's correlation between absolute latitude and one of the two allele frequen- cies of each SNP. We selected SNPs with a correlation coefficient within the upper 1% tail of the distribution. We then used a criterion of proximity between significant variants to focus on DNA regions showing a continuous signal over a portion of the genome. Based on external information and genome annotations, we demonstrated that most regions with the strongest signals also have biological relevance. We believe this proximity requirement adds an edge to our novel method compared to the existing literature, highlighting several genes (for example DTNB, DOT1L, TPCN2, RELN, MSRA, NRG3) related to body size or shape, human height, hair color, and schizophrenia. Our approach can be applied generally to any measure of association between polymorphic frequencies and continuously varying environmental variable
Structural features of the Pernicana Fault (M. Etna, Sicily, Italy) inferred by high precise location of the microseismicity
The north-eastern ïŹank of Mt. Etna is crossed by an important and active tectonic structure, the Pernicana Fault
having a mean strike WNWâESE. It links westward to the active NE Rift and seems to have an important
role in controlling instability processes affecting the eastern ïŹank of the volcano. Recent studies suggest that
Pernicana Fault is very active through sinistral, oblique-slip movements and is also characterised by frequent
shallow seismicity (depth < 2 km bsl) on the uphill western segment and by remarkable creeping on the downhill
eastern one. The Pernicana Fault earthquakes, which can reach magnitudes up to 4.2, sometimes with coseismic
surface faulting, caused severe damages to tourist resorts and villages along or close this structure. In the last
years, a strong increase of seismicity, also characterized by swarms, was recorded by INGV-CT permanent local
seismic network close the Pernicana Fault. A three-step procedure was applied to calculate precise hypocentre
locations. In a ïŹrst step, we chose to apply cross-correlation analysis, in order to easily evaluate the similarity
of waveforms useful to identify earthquakes families. In a second step, we calculate probabilistic earthquake
locations using the software package NONLINLOC, which includes systematic, complete grid search and global,
non-linear search methods. Subsequently, we perform relative relocation of correlated event pairs using the
double-difference earthquake algorithm and the program HypoDD. The double-difference algorithm minimizes
the residuals between observed and calculated travel time difference for pairs of earthquakes at common stations
by iteratively adjusting the vector difference between the hypocenters.
We show the recognized spatial seismic clusters identifying the most active and hazarding sectors of the structure,
their geometry and depth.
Finally, in order to clarify the geodynamic framework of the area, we associate these results with calculated focal
mechanisms for the most energetic earthquakes
- âŠ