50 research outputs found
Wandering Black Holes in Bright Disk Galaxy Halos
We perform SPH+N-body cosmological simulations of massive disk galaxies,
including a formalism for black hole seed formation and growth, and find that
satellite galaxies containing supermassive black hole seeds are often stripped
as they merge with the primary galaxy. These events naturally create a
population of "wandering" black holes that are the remnants of stripped
satellite cores; galaxies like the Milky Way may host 5 -- 15 of these objects
within their halos. The satellites that harbor black hole seeds are comparable
to Local Group dwarf galaxies such as the Small and Large Magellanic Clouds;
these galaxies are promising candidates to host nearby intermediate mass black
holes. Provided that these wandering black holes retain a gaseous accretion
disk from their host dwarf galaxy, they give a physical explanation for the
origin and observed properties of some recently discovered off-nuclear
ultraluminous X-ray sources such as HLX-1.Comment: Accepted for publication in ApJ Letter
Gravitational recoils of supermassive black holes in hydrodynamical simulations of gas rich galaxies
We study the evolution of gravitationally recoiled supermassive black holes
(BHs) in massive gas-rich galaxies by means of high-resolution hydrodynamical
simulations. We find that the presence of a massive gaseous disc allows
recoiled BHs to return to the centre on a much shorter timescale than for
purely stellar discs. Also, BH accretion and feedback can strongly modify the
orbit of recoiled BHs and hence their return timescale, besides affecting the
distribution of gas and stars in the galactic centre. However, the dynamical
interaction of kicked BHs with the surrounding medium is in general complex and
can facilitate both a fast return to the centre as well as a significant delay.
The Bondi-Hoyle-Lyttleton accretion rates of the recoiling BHs in our simulated
galaxies are favourably high for the detection of off-centred AGN if kicked
within gas-rich discs -- up to a few per cent of the Eddington accretion rate
-- and are highly variable on timescales of a few 10^7 yrs. In major merger
simulations of gas-rich galaxies, we find that gravitational recoils increase
the scatter in the BH mass -- host galaxy relationships compared to simulations
without kicks, with the BH mass being more sensitive to recoil kicks than the
bulge mass. A generic result of our numerical models is that the clumpy massive
discs suggested by recent high-redshift observations, as well as the remnants
of gas-rich mergers, exhibit a gravitational potential that falls steeply in
the central regions, due to the dissipative concentration of baryons. As a
result, supermassive BHs should only rarely be able to escape from massive
galaxies at high redshifts, which is the epoch where the bulk of BH recoils is
expected to occur.[Abridged]Comment: 16 pages, 13 figures, minor revisions, MNRAS accepte
Journey to the M_BH -sigma relation: the fate of low mass black holes in the Universe
In this paper, we explore the establishment and evolution of the empirical
correlation between black hole mass and velocity dispersion with redshift. We
track the growth and accretion history of massive black holes starting from
high redshift using two seeding models:(i) Population III remnants, and (ii)
massive seeds from direct gas collapse. Although the seeds do not initially
satisfy the relation, the correlation is established and
maintained at all times if self-regulating accretion episodes are associated
with major mergers. The massive end of the relation is
established early, and lower mass MBHs migrate over time. How MBHs migrate
toward the relation, the slope and the scatter of the relation all depend
critically on the seeding model as well as the adopted self-regulation
prescription. We expect flux limited AGN surveys and LISA to select accreting
and merging MBHs respectively that have already migrated onto the \msigma
relation. This is a consequence of major mergers being more common at high
redshift for the most massive, biased, galaxies that anchor the \msigma
relation early. We also predict the existence of a large population of low mass
`hidden' MBHs at high redshift which can easily escape detection. Additionally,
we find that if MBH seeds are massive, , the low-mass end
of the \msigma flattens towards this asymptotic value, creating a
characteristic `plume'.Comment: 8 pages, 5 figures, MNRAS in pres
Lunar Propellant Factory Mission Design To Sustain Future Human Exploration
The International Space Exploration Coordination Group (ISECG) Global Exploration Roadmap (GER) is the
standard document reflecting the current focus of the leading space agencies that envision space exploration missions beyond Low Earth Orbit (LEO), returning to the Moon and going to Mars in the upcoming years. The roadmap showcases the Moon as a stepping-stone for further human space exploration, by setting up a sustainable space infrastructure on its surface an orbit. Inspired from this vision, we present the result of a phase A study about a lunar propellant factory near the Shackleton south-pole crater relying on In-Situ Resources Utilization (ISRU) to produce and sell Liquid Oxygen (LOX) on the moon surface and in orbit. The overall timeline of the mission is in line with the ISECG exploration roadmap Moon phase, based on realistic technologies of advanced-enough Technology Readiness Levels (TRL). It is a second iteration on the Lunar Propellant Outpost (LUPO) mission architecture, presented during IAC 2018. We preserved and reviewed the original building blocks (Habitats, Crew Mobility Elements, ISRU Facilities, and Lunar Spaceport) of the LUPO mission architecture, and further improved the mission design, supported by trade-off analysis on different mission scenarios. An extensive analysis and optimisation have been performed on ISRU processes and surface electrical power management, the core of our infrastructure. The mission architecture also includes crew on the lunar surface, so life support systems and habitat, as well as operations concepts, have been studied in-depth, and a synthesis of all results is presented. The main aim of this iteration was to improve and refine the baseline infrastructural and technological design architecture of LUPO and reflect on missions going beyond the Moon by providing refuelling services, with sustainability and economic viability in mind
Dual black holes in merger remnants. II: spin evolution and gravitational recoil
Using high resolution hydrodynamical simulations, we explore the spin
evolution of massive dual black holes orbiting inside a circumnuclear disc,
relic of a gas-rich galaxy merger. The black holes spiral inwards from
initially eccentric co or counter-rotating coplanar orbits relative to the
disc's rotation, and accrete gas that is carrying a net angular momentum. As
the black hole mass grows, its spin changes in strength and direction due to
its gravito-magnetic coupling with the small-scale accretion disc. We find that
the black hole spins loose memory of their initial orientation, as accretion
torques suffice to align the spins with the angular momentum of their orbit on
a short timescale (<1-2 Myr). A residual off-set in the spin direction relative
to the orbital angular momentum remains, at the level of <10 degrees for the
case of a cold disc, and <30 degrees for a warmer disc. Alignment in a cooler
disc is more effective due to the higher coherence of the accretion flow near
each black hole that reflects the large-scale coherence of the disc's rotation.
If the massive black holes coalesce preserving the spin directions set after
formation of a Keplerian binary, the relic black hole resulting from their
coalescence receives a relatively small gravitational recoil. The distribution
of recoil velocities inferred from a simulated sample of massive black hole
binaries has median <70 km/s much smaller than the median resulting from an
isotropic distribution of spins.Comment: 11 pages, 3 figures. Accepted for publication in MNRA
X-ray emission from high-redshift miniquasars: self-regulating the population of massive black holes through global warming
Observations of high-redshift quasars at z>6 imply that supermassive black
holes (SMBHs) with masses over a billion solar masses were in place less than 1
Gyr after the Big Bang. If these SMBHs assembled from "seed" BHs left behind by
the first stars, then they must have accreted gas at close to the Eddington
limit during a large fraction (>50%) of the time. A generic problem with this
scenario, however, is that the mass density in million-solar-mass SMBHs at z=6
already exceeds the locally observed SMBH mass density by several orders of
magnitude; in order to avoid this overproduction, BH seed formation and growth
must become significantly less efficient in less massive protogalaxies, while
proceeding uninterrupted in the most massive galaxies that formed first. Using
Monte-Carlo realizations of the merger and growth history of BHs, we show that
X-rays from the earliest accreting BHs can provide such a feedback mechanism.
Our calculations paint a self-consistent picture of black-hole-made climate
change, in which the first miniquasars---among them the ancestors of the z>6
quasar SMBHs---globally warm the IGM and suppress the formation and growth of
subsequent generations of BHs. We present two specific models with global
miniquasar feedback that provide excellent agreement with recent estimates of
the z=6 SMBH mass function. For each of these models, we estimate the rate of
BH mergers at z>6 that could be detected by the proposed gravitational-wave
observatory eLISA/NGO.Comment: 15 pages, 6 figures, accepted to MNRAS; v2 includes minor changes,
mostly to references, to match version to be publishe
Accretion onto black holes formed by direct collapse
One possible scenario for the formation of massive black holes (BHs) in the
early Universe is from the direct collapse of primordial gas in atomic-cooling
dark matter haloes in which the gas is unable to cool efficiently via molecular
transitions. We study the formation of such BHs, as well as the accretion of
gas onto these objects and the high energy radiation emitted in the accretion
process, by carrying out cosmological radiation hydrodynamics simulations. In
the absence of radiative feedback, we find an upper limit to the accretion rate
onto the central object which forms from the initial collapse of hot (~ 10^4 K)
gas of the order of 0.1 MSun per year. This is high enough for the formation of
a supermassive star, the immediate precursor of a BH, with a mass of the order
of 10^5 MSun. Assuming that a fraction of this mass goes into a BH, we track
the subsequent accretion of gas onto the BH self-consistently with the high
energy radiation emitted from the accretion disk. Using a ray-tracing algorithm
to follow the propagation of ionizing radiation, we model in detail the
evolution of the photoionized region which forms around the accreting BH. We
find that BHs with masses of the order of 10^4 MSun initially accrete at close
to the Eddington limit, but that the accretion rate drops to of order 1 percent
of the Eddington limit after ~ 10^6 yr, due to the expansion of the gas near
the BH in response to strong photoheating and radiation pressure. One signature
of the accretion of gas onto BHs formed by direct collapse, as opposed to
massive Pop III star formation, is an extremely high ratio of the luminosity
emitted in He II 1640 to that emitted in H_alpha (or Ly_alpha); this could be
detected by the James Webb Space Telescope. Finally, we briefly discuss
implications for the coevolution of BHs and their host galaxies.Comment: 16 pages; 17 figures, slightly reduced quality; MNRAS in pres
Bitter Is Better: Wild Greens Used in the Blue Zone of Ikaria, Greece
The current study reports an ethnobotanical field investigation of traditionally gathered and consumed wild greens (Chorta) in one of the five so-called Blue Zones in the world: Ikaria Isle, Greece. Through 31 semi-structured interviews, a total of 56 wild green plants were documented along with their culinary uses, linguistic labels, and locally perceived tastes. Most of the gathered greens were described as bitter and associated with members of Asteraceae and Brassicaceae botanical families (31%), while among the top-quoted wild greens, species belonging to these two plant families accounted for 50% of the wild vegetables, which were consumed mostly cooked. Cross-cultural comparison with foraging in other areas of the central-eastern Mediterranean and the Near East demonstrated a remarkable overlapping of Ikarian greens with Cretan and Sicilian, as well as in the prevalence of bitter-tasting botanical genera. Important differences with other wild greens-related food heritage were found, most notably with the Armenian and Kurdish ones, which do not commonly feature many bitter greens. The proven role of extra-oral bitter taste receptors in the modulation of gastric emptying, glucose absorption and crosstalk with microbiota opens new ways of looking at these differences, in particular with regard to possible health implications. The present study is also an important attempt to preserve and document the bio-cultural gastronomic heritage of Chorta as a quintessential part of the Mediterranean diet. The study recommends that nutritionists, food scientists, and historians, as well as policymakers and practitioners, pay the required attention to traditional rural dietary systems as models of sustainable health