448 research outputs found
Supermassive black hole pairs in clumpy galaxies at high redshift: delayed binary formation and concurrent mass growth
Massive gas-rich galaxy discs at host massive star-forming
clumps with typical baryonic masses in the range ~M which
can affect the orbital decay and concurrent growth of supermassive black hole
(BH) pairs. Using a set of high-resolution simulations of isolated clumpy
galaxies hosting a pair of unequal-mass BHs, we study the interaction between
massive clumps and a BH pair at kpc scales, during the early phase of the
orbital decay. We find that both the interaction with massive clumps and the
heating of the cold gas layer of the disc by BH feedback tend to delay
significantly the orbital decay of the secondary, which in many cases is
ejected and then hovers for a whole Gyr around a separation of 1--2 kpc. In the
envelope, dynamical friction is weak and there is no contribution of disc
torques: these lead to the fastest decay once the orbit of the secondary BH has
circularised in the disc midplane. In runs with larger eccentricities the delay
is stronger, although there are some exceptions. We also show that, even in
discs with very sporadic transient clump formation, a strong spiral pattern
affects the decay time-scale for BHs on eccentric orbits. We conclude that,
contrary to previous belief, a gas-rich background is not necessarily conducive
to a fast BH decay and binary formation, which prompts more extensive
investigations aimed at calibrating event-rate forecasts for ongoing and future
gravitational-wave searches, such as with Pulsar Timing Arrays and the future
evolved Laser Interferometer Space Antenna.Comment: Accepted by MNRA
Growth and activity of black holes in galaxy mergers with varying mass ratios
We study supermassive black holes (BHs) in merging galaxies, using a suite of
hydrodynamical simulations with very high spatial (~10 pc) and temporal (~1
Myr) resolution, where we vary the initial mass ratio, the orbital
configuration, and the gas fraction. (i) We address the question of when and
why, during a merger, increased BH accretion occurs, quantifying gas inflows
and BH accretion rates. (ii) We also quantify the relative effectiveness in
inducing AGN activity of merger-related versus secular-related causes, by
studying different stages of the encounter: the stochastic (or early) stage,
the (proper) merger stage, and the remnant (or late) stage. (iii) We assess
which galaxy mergers preferentially enhance BH accretion, finding that the
initial mass ratio is the most important factor. (iv) We study the evolution of
the BH masses, finding that the BH mass contrast tends to decrease in minor
mergers and to increase in major mergers. This effect hints at the existence of
a preferential range of mass ratios for BHs in the final pairing stages. (v) In
both merging and dynamically quiescent galaxies, the gas accreted by the BH is
not necessarily the gas with angular momentum, but the gas that
angular momentum.Comment: Accepted for publication in MNRAS, 23 pages, 22 figures, 3 table
The birth of a supermassive black hole binary
We study the dynamical evolution of supermassive black holes, in the late
stage of galaxy mergers, from kpc to pc scales. In particular, we capture the
formation of the binary, a necessary step before the final coalescence, and
trace back the main processes causing the decay of the orbit. We use
hydrodynamical simulations of galaxy mergers with different resolutions, from
down to , in order to study the effects of the
resolution on our results, remove numerical effects, and assess that resolving
the influence radius of the orbiting black hole is a minimum condition to fully
capture the formation of the binary. Our simulations include the relevant
physical processes, namely star formation, supernova feedback, accretion onto
the black holes and the ensuing feedback. We find that, in these mergers,
dynamical friction from the smooth stellar component of the nucleus is the main
process that drives black holes from kpc to pc scales. Gas does not play a
crucial role and even clumps do not induce scattering or perturb the orbits. We
compare the time needed for the formation of the binary to analytical
predictions and suggest how to apply such analytical formalism to obtain
estimates of binary formation times in lower resolution simulations.Comment: 12 pages, 12 Figures, submitted to MNRA
A comparison of black hole growth in galaxy mergers with Gasoline and Ramses
Supermassive black hole dynamics during galaxy mergers is crucial in
determining the rate of black hole mergers and cosmic black hole growth. As
simulations achieve higher resolution, it becomes important to assess whether
the black hole dynamics is influenced by the treatment of the interstellar
medium in different simulation codes. We here compare simulations of black hole
growth in galaxy mergers with two codes: the Smoothed Particle Hydrodynamics
code Gasoline, and the Adaptive Mesh Refinement code Ramses. We seek to
identify predictions of these models that are robust despite differences in
hydrodynamic methods and implementations of sub-grid physics. We find that the
general behavior is consistent between codes. Black hole accretion is minimal
while the galaxies are well-separated (and even as they "fly-by" within 10 kpc
at first pericenter). At late stages, when the galaxies pass within a few kpc,
tidal torques drive nuclear gas inflow that triggers bursts of black hole
accretion accompanied by star formation. We also note quantitative
discrepancies that are model-dependent: our Ramses simulations show less star
formation and black hole growth, and a smoother gas distribution with larger
clumps and filaments, than our Gasoline simulations. We attribute these
differences primarily to the sub-grid models for black hole fueling and
feedback and gas thermodynamics. The main conclusion is that differences exist
quantitatively between codes, and this should be kept in mind when making
comparisons with observations. However, reassuringly, both codes capture the
same dynamical behaviors in terms of triggering of black hole accretion, star
formation, and black hole dynamics.Comment: 11 pages, 7 figures. Submitted to A&A. Comments welcom
Nuclear coups: dynamics of black holes in galaxy mergers
We study the dynamical evolution of supermassive black holes (BHs) in merging
galaxies on scales of hundreds of kpc to 10 pc, to identify the physical
processes that aid or hinder the orbital decay of BHs. We present
hydrodynamical simulations of galaxy mergers with a resolution of 20 pc,
chosen to accurately track the motion of the nuclei and provide a realistic
environment for the evolution of the BHs. We find that, during the late stages
of the merger, tidal shocks inject energy in the nuclei, causing one or both
nuclei to be disrupted and leaving their BH `naked', without any bound gas or
stars. In many cases, the nucleus that is ultimately disrupted is that of the
larger galaxy (`nuclear coup'), as star formation grows a denser nuclear cusp
in the smaller galaxy. We supplement our simulations with an analytical
estimate of the orbital-decay time required for the BHs to form a binary at
unresolved scales, due to dynamical friction. We find that, when a nuclear coup
occurs, the time-scale is much shorter than when the secondary's nucleus is
disrupted, as the infalling BH is more massive, and it also finds itself in a
denser stellar environment.Comment: Accepted for publication in MNRAS, 16 pages, 13 figures, 2 table
Rational design of multi-functional nanomaterials
FCT-MEC project PTDC/QEQMED/2118/2014publishersversionpublishe
Photocatalytic hydrogen production using noble and transition metals surface modified titania [Resumo]
ABSTRACT: A large number of photocatalytic materials have been studied for water splitting since the seminal work of Fujishima and Honda1, showing great potential for solar energy conversion, including H2 production. The irradiation of a suspension of semiconductor oxides, as is the case of TiO2, presents attracting features but also stringent requirements regarding materials properties, including the tailoring of the electronic structure.
Furthermore, efficient charge transport is necessary, as well as effective charge separation and prevention of
electron-hole pair recombination, before the redox reactions can proceed2,3. In this work, the catalytic activity under UV excitation of TiO2-Au photocatalyst for H2 production was undertaken using glycerol and ethanol as sacrificial agents. Furthermore, substitution of Au by transition metal Cu was attempted with good results.
Comparison is made with results obtained using TiO2-rGO-Pt catalyst under analogous loading conditions.N/
Enhancing Pt electrocatalytic activity by surface functionalization of carbon support with aromatic sulphonic groups
Preliminary results are presented for Pt deposited on 4-aminobenzenesulphonic acid-functionalized carbon, Pt/C_ABSA. Vulcan XC-72R was functionalized with the objective of influencing the dispersion of catalyst nanoparticles and to decrease the resistance of three phase boundaries, by introducing sulphonic groups. Electrochemical characterization of the supported catalyst was done in a 0.5 M sulphuric acid solution with added chloride and also using methanol demonstrating well defined features and stable voltammograms after 30 cycles, with apparent higher currents when compared with commercial catalyst. The structure sensitive adsorption of anions on platinum is confirmed by the blocking effect of chloride ions in the hydrogen adsorption-desorption region as well as by the consequent dissolution of platinum, evident in both anodic and cathodic features of the voltammograms regarding surface oxide formation and reduction. The partial reversibility of the effect of chloride ions
is discussed. Research is in progress in order to accomplish a comprenhensive characterization of the synthesized catalyst and to ascertain the effect of the sulphonic
groups
Determination of phytoextraction potential of plant speciesfor toxic elements in soils of abandoned sulphide-mining areas
This study has determined contamination levels in soils and plants from the Sa˜o Domingos mining area, Portugal, by k0-INAA. Total concentrations of As, Sb, Cr, Hg, Cu, Zn and Fe in soils were very high, exceeding the maximum limits in Portuguese legislation. Concentrations of toxic elements like As, Sb and Zn were highest in roots of Erica andevalensis, Juncus acutus, Agrostis castellana and Nicotiana glauca. Additionally, As, Br, Cr, Fe, Sb and Zn in all organs of most plants were above toxicity levels.
Those species that accumulated relatively high concentrations of toxic elements in roots (and tops) may be cultivated for phytostabilisation of similar areas
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