294 research outputs found
Predicting the hypervelocity star population in Gaia
Hypervelocity stars (HVSs) are amongst the fastest objects in our Milky Way.
These stars are predicted to come from the Galactic center (GC) and travel
along unbound orbits across the Galaxy. In the coming years, the ESA satellite
Gaia will provide the most complete and accurate catalogue of the Milky Way,
with full astrometric parameters for more than billion stars. In this
paper, we present the expected sample size and properties (mass, magnitude,
spatial, velocity distributions) of HVSs in the Gaia stellar catalogue. We
build three Gaia mock catalogues of HVSs anchored to current observations,
exploring different ejection mechanisms and GC stellar population properties.
In all cases, we predict hundreds to thousands of HVSs with precise proper
motion measurements within a few tens of kpc from us. For stars with a relative
error in total proper motion below , the mass range extends to ~ but peaks at ~ . The majority of Gaia HVSs will
therefore probe a different mass and distance range compared to the current
non-Gaia sample. In addition, a subset of a few hundreds to a few thousands of
HVSs with ~ will be bright enough to have a precise
measurement of the three-dimensional velocity from Gaia alone. Finally, we show
that Gaia will provide more precise proper motion measurements for the current
sample of HVS candidates. This will help identifying their birthplace narrowing
down their ejection location, and confirming or rejecting their nature as HVSs.
Overall, our forecasts are extremely encouraging in terms of quantity and
quality of HVS data that can be exploited to constrain both the Milky Way
potential and the GC properties.Comment: 17 pages, 18 figures, accepted for publication in MNRA
Reconstructing the massive black hole cosmic history through gravitational waves
The massive black holes we observe in galaxies today are the natural
end-product of a complex evolutionary path, in which black holes seeded in
proto-galaxies at high redshift grow through cosmic history via a sequence of
mergers and accretion episodes. Electromagnetic observations probe a small
subset of the population of massive black holes (namely, those that are active
or those that are very close to us), but planned space-based gravitational-wave
observatories such as the Laser Interferometer Space Antenna (LISA) can measure
the parameters of ``electromagnetically invisible'' massive black holes out to
high redshift. In this paper we introduce a Bayesian framework to analyze the
information that can be gathered from a set of such measurements. Our goal is
to connect a set of massive black hole binary merger observations to the
underlying model of massive black hole formation. In other words, given a set
of observed massive black hole coalescences, we assess what information can be
extracted about the underlying massive black hole population model. For
concreteness we consider ten specific models of massive black hole formation,
chosen to probe four important (and largely unconstrained) aspects of the input
physics used in structure formation simulations: seed formation, metallicity
``feedback'', accretion efficiency and accretion geometry. For the first time
we allow for the possibility of ``model mixing'', by drawing the observed
population from some combination of the ``pure'' models that have been
simulated. A Bayesian analysis allows us to recover a posterior probability
distribution for the ``mixing parameters'' that characterize the fractions of
each model represented in the observed distribution. Our work shows that LISA
has enormous potential to probe the underlying physics of structure formation.Comment: 24 pages, 16 figures, submitted to Phys. Rev.
The Local Nanohertz Gravitational-Wave Landscape From Supermassive Black Hole Binaries
Supermassive black hole binaries (SMBHBs) in the 10 million to 10 billion
range form in galaxy mergers, and live in galactic nuclei with large
and poorly constrained concentrations of gas and stars. There are currently no
observations of merging SMBHBs--- it is in fact possible that they stall at
their final parsec of separation and never merge. While LIGO has detected high
frequency GWs, SMBHBs emit GWs in the nanohertz to millihertz band. This is
inaccessible to ground-based interferometers, but possible with Pulsar Timing
Arrays (PTAs). Using data from local galaxies in the 2 Micron All-Sky Survey,
together with galaxy merger rates from Illustris, we find that there are on
average sources emitting GWs in the PTA band, and binaries
which will never merge. Local unresolved SMBHBs can contribute to GW background
anisotropy at a level of , and if the GW background can be
successfully isolated, GWs from at least one local SMBHB can be detected in 10
years.Comment: submitted to Nature Astronomy (reformatted for arXiv
Constraining properties of the black hole population using LISA
LISA should detect gravitational waves from tens to hundreds of systems
containing black holes with mass in the range from 10 thousand to 10 million
solar masses. Black holes in this mass range are not well constrained by
current electromagnetic observations, so LISA could significantly enhance our
understanding of the astrophysics of such systems. In this paper, we describe a
framework for combining LISA observations to make statements about massive
black hole populations. We summarise the constraints that LISA observations of
extreme-mass-ratio inspirals might be able to place on the mass function of
black holes in the LISA range. We also describe how LISA observations can be
used to choose between different models for the hierarchical growth of
structure in the early Universe. We consider four models that differ in their
prescription for the initial mass distribution of black hole seeds, and in the
efficiency of accretion onto the black holes. We show that with as little as 3
months of LISA data we can clearly distinguish between these models, even under
relatively pessimistic assumptions about the performance of the detector and
our knowledge of the gravitational waveforms.Comment: 12 pages, 3 figures, submitted to Class. Quantum Grav. for
proceedings of 8th LISA Symposium; v2 minor changes for consistency with
accepted versio
An integrated approach for assessing the vulnerability of World Heritage Sites to climate change impacts
One of the most difficult problem facing those responsible for managing World Heritage Sites (WHS) is climate change, as it poses continuous new challenges to the conservation of cultural heritage. Moreover, as our climate continues to change our cultural heritage will potentially be exposed to diverse pressures and potentially to risks not previously experienced. Thus, management practices will need to be tailored in order to include climate change impacts. For climate change impacts to be incorporated into preservation frameworks and management practices from government policy level down to the practice in the field, data, information and assessment methods need to be available at a scale relevant to decision-makers. This paper presents an integrated vulnerability assessment methodology and applies it to three UNESCO cultural WHS in Europe. Through this process, semi-structured interviews were conducted with academics and experts in the management and conservation of cultural heritage, as well as with the managers and coordinators of WHS. The incorporation of bottom-up knowledge in the assessment process allowed for an understanding of the sensitivity and adaptive capacity of the sites, two components of vulnerability that are not given sufficient attention and ignored, respectively, in typical top-down climate change impact assessments. In particular, the interviews elucidated the determinants that enable or constrain the capacity to adapt, i.e., resources, including technical, economic and human; information and awareness; management capacity; learning capacity; leadership; communication and collaboration; and governance; with the lack of resources most commonly mentioned as the determinant impeding adaptation. ‘Information and awareness’ and ‘management capacity’ are determinants that were not previously identified in the field of cultural heritage. The former stresses the need to disseminate the results of scientific research for their incorporation in the management of heritage sites. Vulnerability assessments such as those performed in this paper can be used to target interventions to protect and strengthen the resilience of cultural heritage to climate change impacts
On the mass radiated by coalescing black-hole binaries
We derive an analytic phenomenological expression that predicts the final
mass of the black-hole remnant resulting from the merger of a generic binary
system of black holes on quasi-circular orbits. Besides recovering the correct
test-particle limit for extreme mass-ratio binaries, our formula reproduces
well the results of all the numerical-relativity simulations published so far,
both when applied at separations of a few gravitational radii, and when applied
at separations of tens of thousands of gravitational radii. These validations
make our formula a useful tool in a variety of contexts ranging from
gravitational-wave physics to cosmology. As representative examples, we first
illustrate how it can be used to decrease the phase error of the
effective-one-body waveforms during the ringdown phase. Second, we show that,
when combined with the recently computed self-force correction to the binding
energy of nonspinning black-hole binaries, it provides an estimate of the
energy emitted during the merger and ringdown. Finally, we use it to calculate
the energy radiated in gravitational waves by massive black-hole binaries as a
function of redshift, using different models for the seeds of the black-hole
population.Comment: 9 pages (emulateapj), 4 figures. Matches version in ApJ but includes
slight changes to fig 4 described in Barausse, et al ApJ 786, 76 (2014)
(doi:10.1088/0004-637X/786/1/76), see also
http://www2.iap.fr/users/barausse/erratum_mass_formula.pd
Probing seed black holes using future gravitational-wave detectors
Identifying the properties of the first generation of seeds of massive black
holes is key to understanding the merger history and growth of galaxies.
Mergers between ~100 solar mass seed black holes generate gravitational waves
in the 0.1-10Hz band that lies between the sensitivity bands of existing
ground-based detectors and the planned space-based gravitational wave detector,
the Laser Interferometer Space Antenna (LISA). However, there are proposals for
more advanced detectors that will bridge this gap, including the third
generation ground-based Einstein Telescope and the space-based detector DECIGO.
In this paper we demonstrate that such future detectors should be able to
detect gravitational waves produced by the coalescence of the first generation
of light seed black-hole binaries and provide information on the evolution of
structure in that era. These observations will be complementary to those that
LISA will make of subsequent mergers between more massive black holes. We
compute the sensitivity of various future detectors to seed black-hole mergers,
and use this to explore the number and properties of the events that each
detector might see in three years of observation. For this calculation, we make
use of galaxy merger trees and two different seed black hole mass distributions
in order to construct the astrophysical population of events. We also consider
the accuracy with which networks of future ground-based detectors will be able
to measure the parameters of seed black hole mergers, in particular the
luminosity distance to the source. We show that distance precisions of ~30% are
achievable, which should be sufficient for us to say with confidence that the
sources are at high redshift.Comment: 14 pages, 6 figures, 2 tables, accepted for proceedings of 13th GWDAW
meetin
About gravitational-wave generation by a three-body system
We highlight some subtleties that affect naive implementations of quadrupolar and octupolar gravitational waveforms from numerically-integrated trajectories of three-body systems. Some of those subtleties arise from the requirement that the source be contained in its "coordinate near zone" when applying the standard PN formulae for gravitational-wave emission, and from the need to use the non-linear Einstein equations to correctly derive the quadrupole emission formula. We show that some of these subtleties were occasionally overlooked in the literature, with consequences for published results. We also provide prescriptions that lead to correct and robust predictions for the waveforms computed from numerically-integrated orbits
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