122 research outputs found
The role of metal ions in the electron transport through azurin-based junctions
We studied the coherent electron transport through metal–protein–metal junctions based on a blue copper azurin, in which the copper ion was replaced by three different metal ions (Co, Ni and Zn). Our results show that neither the protein structure nor the transmission at the Fermi level change significantly upon metal replacement. The discrepancy with previous experimental observations suggests that the transport mechanism taking place in these types of junctions is probably not fully coherent
Linking bacterial and eukaryotic microbiota to litter chemistry: Combining next generation sequencing with 13 C CPMAS NMR spectroscopy
Microbial succession over decomposing litter is controlled by biotic interactions, dispersal limitation, grazing pressure, and substrate chemical changes. Recent evidence suggests that the changes in litter chemistry and microbiome during decomposition are interdependent. However, most previous studies separately addressed the microbial successional dynamics or the molecular changes of decomposing litter. Here, we combined litter chemical characterization by 13 C NMR spectroscopy with next generation sequencing to compare leaf litter chemistry and microbiome dynamics using 30 litter types, either fresh or decomposed for 30 and 180 days. We observed a decrease of cellulose and C/N ratio during decomposition, while lignin content and lignin/N ratio showed the opposite pattern. 13 C NMR revealed significant chemical changes as microbial decomposition was proceeding, with a decrease in O-alkyl C and an increase in alkyl C and methoxyl C relative abundances. Overall, bacterial and eukaryotic taxonomical richness increased with litter age. Among Bacteria, Proteobacteria dominated all undecomposed litters but this group was progressively replaced by members of Actinobacteria, Bacteroidetes, and Firmicutes. Nitrogen-fixing genera such as Beijerinckia and Rhizobium occurred both in undecomposed as well as in aged litters. Among Eukarya, fungi belonging to the Ascomycota phylum were dominant in undecomposed litter with the typical phyllospheric genus Aureobasidium. In aged litters, phyllospheric species were replaced by zygomycetes and other ascomycetous and basidiomycetous fungi. Our analysis of decomposing litter highlighted an unprecedented, widespread occurrence of protists belonging to the Amebozoa and Cercozoa. Correlation network analysis showed that microbial communities are non-randomly structured, showing strikingly distinct composition in relation to litter chemistry. Our data demonstrate that the importance of litter chemistry in shaping microbial community structure increased during the decomposition process, being of little importance for freshly fallen leaves
High-precision multi-band measurements of the angular clustering of X-ray sources
In this paper we present the two-point angular correlation function of the
X-ray source population of 1063 XMM-Newton observations at high Galactic
latitudes, comprising up to ~30000 sources over a sky area of 125.5 sq. deg, in
three energy bands: 0.5-2 (soft), 2-10 (hard), and 4.5-10 (ultrahard) keV. We
have measured the angular clustering of our survey and find significant
positive clustering signals in the soft and hard bands, and a marginal
clustering detection in the ultrahard band. We find dependency of the
clustering strength on the flux limit and no significant differences in the
clustering properties between sources with high hardness ratios and those with
low hardness ratios. Our results show that obscured and unobscured objects
share similar clustering properties and therefore they both reside in similar
environments, in agreement with the unified model of AGN. We deprojected the
angular clustering parameters via Limber's equation to compute their typical
spatial lengths. From that we have inferred the typical mass of the dark matter
haloes in which AGN at redshifts of ~1 are embedded. The short AGN lifetimes
derived suggest that AGN activity might be a transient phase that can be
experienced several times by a large fraction of galaxies throughout their
lives.Comment: 14 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
Power spectrum of the polarized diffuse Galactic radio emission
We have analyzed the available polarization surveys of the Galactic emission
to estimate to what extent it may be a serious hindrance to forthcoming
experiments aimed at detecting the polarized component of Cosmic Microwave
Background (CMB) anisotropies. Regions were identified for which independent
data consistently indicate that depolarization must be small. The power
spectrum of the polarized emission, in terms of antenna temperature, was found
to be described by K, from arcminute to
degree scales. Data on larger angular scales () indicate a steeper
slope . We conclude that polarized Galactic emission is
unlikely to be a serious limitation to CMB polarization measurements at the
highest frequencies of the MAP and {\sc Planck}/LFI instruments, at least for
and standard cosmological models. The weak correlation between
polarization and total power and the low polarization degree of radio emission
close to the Galactic plane, found also in low-depolarization regions, is
interpreted as due to large contributions to the observed intensity from
unpolarized sources, primarily strong HII regions, concentrated on the Galactic
plane. Thus estimates of the power spectrum of total intensity at low Galactic
latitudes are not representative of the spatial distribution of Galactic
emission far from the plane. Both total power and polarized emissions show
highly significant deviations from a Gaussian distribution.Comment: 14 pages, including 15 figures, version accepted for publication on
A&
On the radiative efficiencies, Eddington ratios, and duty cycles of luminous high-redshift quasars
We investigate the characteristic radiative efficiency \epsilon, Eddington
ratio \lambda, and duty cycle P_0 of high-redshift active galactic nuclei
(AGN), drawing on measurements of the AGN luminosity function at z=3-6 and,
especially, on recent measurements of quasar clustering at z=3-4.5 from the
Sloan Digital Sky Survey. The free parameters of our models are \epsilon,
\lambda, and the normalization, scatter, and redshift evolution of the relation
between black hole mass \mbh and halo virial velocity V_vir. We compute the
luminosity function from the implied growth of the black hole mass function and
the quasar correlation length from the bias of the host halos. We test our
adopted formulae for the halo mass function and halo bias against measurements
from the large N-body simulation developed by the MICE collaboration. The
strong clustering of AGNs observed at z=3 and, especially, at z=4 implies that
massive black holes reside in rare, massive dark matter halos. Reproducing the
observed luminosity function then requires high efficiency \epsilon and/or low
Eddington ratio \lambda, with a lower limit (based on 2\sigma agreement with
the measured z=4 correlation length) \epsilon> 0.7\lambda/(1+0.7\lambda),
implying \epsilon > 0.17 for \lambda > 0.25. Successful models predict high
duty cycles, P_0~0.2, 0.5, and 0.9 at z=3.1, 4.5 and 6, respectively, and they
require that the fraction of halo baryons locked in the central black hole is
much larger than the locally observed value. The rapid drop in the abundance of
the massive and rare host halos at z>7 implies a proportionally rapid decline
in the number density of luminous quasars, much stronger than simple
extrapolations of the z=3-6 luminosity function would predict. (abridged)Comment: Replaced with version accepted by ApJ. More detailed analysis
including black hole mergers. Results unchange
Self-Consistent Models of the AGN and Black Hole Populations: Duty Cycles, Accretion Rates, and the Mean Radiative Efficiency
We construct evolutionary models of the populations of AGN and supermassive
black holes, in which the black hole mass function grows at the rate implied by
the observed luminosity function, given assumptions about the radiative
efficiency and the Eddington ratio. We draw on a variety of recent X-ray and
optical measurements to estimate the bolometric AGN luminosity function and
compare to X-ray background data and the independent estimate of Hopkins et al.
(2007) to assess remaining systematic uncertainties. The integrated AGN
emissivity closely tracks the cosmic star formation history, suggesting that
star formation and black hole growth are closely linked at all redshifts.
Observational uncertainties in the local black hole mass function remain
substantial, with estimates of the integrated black hole mass density \rho_BH
spanning the range 3-5.5x10^5 Msun/Mpc^3. We find good agreement with estimates
of the local mass function for a reference model where all active black holes
have efficiency \eps=0.065 and L_bol/L_Edd~0.4. In this model, the duty cycle
of 10^9 Msun black holes declines from 0.07 at z=3 to 0.004 at z=1 and 0.0001
at z=0. The decline is shallower for less massive black holes, a signature of
"downsizing" evolution in which more massive black holes build their mass
earlier. The predicted duty cycles and AGN clustering bias in this model are in
reasonable accord with observational estimates. If the typical Eddington ratio
declines at z<2, then the "downsizing" of black hole growth is less pronounced.
Matching the integrated AGN emissivity to the local black hole mass density
implies \eps=0.075 (\rho_BH/4.5x10^5 Msun/Mpc^3)^{-1} for our standard
luminosity function estimate (25% higher for Hopkins et al.'s), lower than the
values \eps=0.16-0.20 predicted by MHD simulations of disk accretion.Comment: replaced with version accepted by ApJ. Minor revision
Herschel-ATLAS: Far-infrared properties of radio-loud and radio-quiet quasars
This is pre-copyedited, author-produced pdf of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record, E. Kalfountzou, et al., ‘Herschel-ATLAS: Far-infrared properties of radio-loud and radio-quiet quasars’, MNRAS, Vol 42(2): 1181-1196, first published online June 11, 2014, is available online via doi: 10.1093/mnras/stu782 Published by Oxford University Press on behalf of the Royal Astronomical Society.We have constructed a sample of radio-loud and radio-quiet quasars from the Faint Images Radio Sky at Twenty-one centimetres and the Sloan Digital Sky Survey Data Release 7, over the Herschel-ATLAS Phase 1 area (9h, 12h and 14 h . 5 ). Using a stacking analysis, we find a significant correlation between the far-infrared (FIR) luminosity and 1.4-GHz luminosity for radio-loud quasars. Partial correlation analysis confirms the intrinsic correlation after removing the redshift contribution, while for radio-quiet quasars, no partial correlation is found. Using a single-temperature grey-body model, we find a general trend of lower dust temperatures in case of radio-loud quasars compared to radio-quiet quasars. Also, radio-loud quasars are found to have almost constant mean values of dust mass along redshift and optical luminosity bins. In addition, we find that radio-loud quasars at lower optical luminosities tend to have on average higher FIR and 250-μm luminosity with respect to radio-quiet quasars with the same optical luminosites. Even if we use a two-temperature grey-body model to describe the FIR data, the FIR luminosity excess remains at lower optical luminosities. These results suggest that powerful radio jets are associated with star formation especially at lower accretion ratesPeer reviewe
On the co-evolution of supermassive black holes and their host galaxies since z = 3
[Abridged] To investigate the evolution in the relation between galaxy
stellar and central black hole mass we construct a volume limited complete
sample of 85 AGN with host galaxy stellar masses M_{*} > 10^{10.5} M_{sol}, and
specific X-ray luminosities L_{X} > 2.35 x 10^{43} erg s^{-1} at 0.4 < z < 3.
We calculate the Eddington limiting masses of the supermassive black holes
residing at the centre of these galaxies, and observe an increase in the
average Eddington limiting black hole mass with redshift. By assuming that
there is no evolution in the Eddington ratio (\mu) and then that there is
maximum possible evolution to the Eddington limit, we quantify the maximum
possible evolution in the M_{*} / M_{BH} ratio as lying in the range 700 <
M_{*}/M_{BH} < 10000, compared with the local value of M_{*}/M_{BH} ~ 1000. We
furthermore find that the fraction of galaxies which are AGN (with L_{X} > 2.35
x 10^{43} erg s^{-1}) rises with redshift from 1.2 +/- 0.2 % at z = 0.7 to 7.4
+/- 2.0 % at z = 2.5. We use our results to calculate the maximum timescales
for which our sample of AGN can continue to accrete at their observed rates
before surpassing the local galaxy-black hole mass relation. We use these
timescales to calculate the total fraction of massive galaxies which will be
active (with L_{X} > 2.35 x 10^{43} erg s^{-1}) since z = 3, finding that at
least ~ 40% of all massive galaxies will be Seyfert luminosity AGN or brighter
during this epoch. Further, we calculate the energy density due to AGN activity
in the Universe as 1.0 (+/- 0.3) x 10^{57} erg Mpc^{-3} Gyr^{-1}, potentially
providing a significant source of energy for AGN feedback on star formation. We
also use this method to compute the evolution in the X-ray luminosity density
of AGN with redshift, finding that massive galaxy Seyfert luminosity AGN are
the dominant source of X-ray emission in the Universe at z < 3.Comment: 25 pages, 10 figures, accepted for publication in MNRA
The INTEGRAL/IBIS AGN catalogue I: X-ray absorption properties versus optical classification
In this work we present the most comprehensive INTEGRAL AGN sample which
lists 272 objects. Here we mainly use this sample to study the absorption
properties of active galaxies, to probe new AGN classes and to test the AGN
unification scheme. We find that half (48%) of the sample is absorbed while the
fraction of Compton thick AGN is small (~7%). In line with our previous
analysis, we have however shown that when the bias towards heavily absorbed
objects which are lost if weak and at large distance is removed, as it is
possible in the local Universe, the above fractions increase to become 80% and
17%. We also find that absorption is a function of source luminosity, which
implies some evolution in the obscuration properties of AGN. Few peculiar
classes, so far poorly studied in the hard X-ray band, have been detected and
studied for the first time such as 5 XBONG, 5 type 2 QSOs and 11 LINERs. In
terms of optical classification, our sample contains 57% of type 1 and 43% of
type 2 AGN; this subdivision is similar to that found in X-rays if unabsorbed
versus absorbed objects are considered, suggesting that the match between
optical and X-ray classification is overall good. Only a small percentage of
sources (12%) does not fulfill the expectation of the unified theory as we find
22 type 1 AGN which are absorbed and 10 type 2 AGN which are unabsorbed.
Studying in depth these outliers we found that most of the absorbed type 1 AGN
have X-ray spectra characterized by either complex or warm/ionized absorption
more likely due to ionized gas located in an accretion disk wind or in the
biconical structure associated to the central nucleus, therefore unrelated to
the toroidal structure. Among 10 type 2 AGN which resulted to be unabsorbed, at
most 3-4% is still eligible to be classified as a "true" type 2 AGN.Comment: 21 pages, 6 figures, 5 tables. Accepted for publication on MNRAS.
arXiv admin note: text overlap with arXiv:0709.2077 by other author
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