612 research outputs found
Intermedial Maps: The Street as Site of Cultural-Political Regulation in Modern Brazil
Get PDFSince the late 19th century Rio de Janeiro has grown in relation to two primary spatial axes: the planned and regularized cidade (“city”) where most wealth and power are located; and the unregulated morro (“hill”) where most poverty has been concentrated. The history of social order in modern Brazil may not flow from the city per se, but rather from the streets that link cidade and morro the spaces that channel movement between regulated and unregulated areas of the city. One might argue that the modernization of Brazil since 1900 has followed the mapping of Rio de Janeiro’s streets over the same period. Such a claim would require a kind of “alternate” or “intermedial” cartography in which various media are employed to evoke a wide range of sensorial impressions of the city’s streets. This essay therefore attempts an intermedial mapping of Rio’s streets through readings of modernist poetry, modernist architecture, and post-modern television.Depuis la fin du 19e siècle, la croissance de Rio de Janeiro s’est effectuée selon deux axes principaux : la ville (cidade) planifiée et régularisée où se concentrent la richesse et le pouvoir ; et la butte (morro) déréglementée où l’on retrouve la frange la plus défavorisée de la société. L’histoire sociale du Brésil moderne ne découle pas de la ville en soi mais plutôt des rues qui lient morro et cidade – les espaces qui canalisent le mouvement entre les zones qui sont régulées et celles qui ne le sont pas. On pourrait avancer que la modernisation du Brésil, amorcée au début du 20e siècle, suit la cartographie des rues de Rio qui remonte à la même époque. Cette affirmation nécessiterait une cartographie « alternative » ou « intermédiale » constituée de différents médias évoquant un éventail d’impressions suscitées par les rues de la ville. Cet article se propose de produire une cartographie intermédiale des rues de Rio par le biais de la poésie moderne, de l’architecture moderne et de la télévision postmoderne
Measuring the local dark matter density
Full text linkWe examine systematic problems in determining the local matter density from
the vertical motion of stars, i.e. the 'Oort limit'. Using collisionless
simulations and a Monte Carlo Markov Chain technique, we determine the data
quality required to detect local dark matter at its expected density. We find
that systematic errors are more important than observational errors and apply
our technique to Hipparcos data to reassign realistic error bars to the local
dark matter density.Comment: 3 pages, 1 figure, to be published in "Hunting for the Dark: The
Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P.
Debattista & C.C. Popescu, AIP Conf. Se
Fourteen candidate RR Lyrae star streams in the inner Galaxy
Get PDFWe apply the GC3 stream-finding method to RR Lyrae stars (RRLS) in the
Catalina survey. We find two RRLS stream candidates at confidence
and another 12 at confidence over the Galactocentric distance
range . Of these, only two are associated with known
globular clusters (NGC 1261 and Arp2). The remainder are candidate `orphan'
streams, consistent with the idea that globular cluster streams are most
visible close to dissolution. Our detections are likely a lower bound on the
total number of dissolving globulars in the inner galaxy, since many globulars
have few RRLS while only the brightest streams are visible over the Galactic
RRLS background, particularly given the current lack of kinematical
information. We make all of our candidate streams publicly available and
provide a new GALSTREAMS Python library for the footprints of all known streams
and overdensities in the Milky Way.Comment: 18 pages, 4 figures. Accepted for publication at MNRAS. GALSTREAMS
Milky Way Streams Footprint Library are available at
https://github.com/cmateu/galstreams . All RRL data and code used in the
paper are available at
https://cmateu.github.io/Cecilia_Mateu_WebPage/CatalinaGC3_Streams.htm
A Semi-Analytic dynamical friction model that reproduces core stalling
Full text linkWe present a new semi-analytic model for dynamical friction based on
Chandrasekhar's formalism. The key novelty is the introduction of physically
motivated, radially varying, maximum and minimum impact parameters. With these,
our model gives an excellent match to full N-body simulations for isotropic
background density distributions, both cuspy and shallow, without any
fine-tuning of the model parameters. In particular, we are able to reproduce
the dramatic core-stalling effect that occurs in shallow/constant density
cores, for the first time. This gives us new physical insight into the
core-stalling phenomenon. We show that core stalling occurs in the limit in
which the product of the Coulomb logarithm and the local fraction of stars with
velocity lower than the infalling body tends to zero. For cuspy backgrounds,
this occurs when the infalling mass approaches the enclosed background mass.
For cored backgrounds, it occurs at larger distances from the centre, due to a
combination of a rapidly increasing minimum impact parameter and a lack of slow
moving stars in the core. This demonstrates that the physics of core-stalling
is likely the same for both massive infalling objects and low-mass objects
moving in shallow density backgrounds. We implement our prescription for
dynamical friction in the direct summation code NBODY6 as an analytic
correction for stars that remain within the Roche volume of the infalling
object. This approach is computationally efficient, since only stars in the
inspiralling system need to be evolved with direct summation. Our method can be
applied to study a variety of astrophysical systems, including young star
clusters orbiting near the Galactic Centre; globular clusters moving within the
Galaxy; and dwarf galaxies orbiting within dark matter halos.Comment: 16 pages, 21 figures, Accepted for publication in MNRA
A semi-analytic dynamical friction model for cored galaxies
Get PDFWe present a dynamical friction model based on Chandrasekhar's formula that
reproduces the fast inspiral and stalling experienced by satellites orbiting
galaxies with a large constant density core. We show that the fast inspiral
phase does not owe to resonance. Rather, it owes to the background velocity
distribution function for the constant density cores being dissimilar from the
usually-assumed Maxwellian distribution. Using the correct background velocity
distribution function and the semi-analytic model from Petts et al. (2015), we
are able to correctly reproduce the infall rate in both cored and cusped
potentials. However, in the case of large cores, our model is no longer able to
correctly capture core-stalling. We show that this stalling owes to the tidal
radius of the satellite approaching the size of the core. By switching off
dynamical friction when rt(r) = r (where rt is the tidal radius at the
satellite's position) we arrive at a model which reproduces the N-body results
remarkably well. Since the tidal radius can be very large for constant density
background distributions, our model recovers the result that stalling can occur
for Ms/Menc << 1, where Ms and Menc are the mass of the satellite and the
enclosed galaxy mass, respectively. Finally, we include the contribution to
dynamical friction that comes from stars moving faster than the satellite. This
next-to-leading order effect becomes the dominant driver of inspiral near the
core region, prior to stalling.Comment: 13 pages, 12 figures, resubmitted to MNRAS after responding to
feedback from the refere
The influence of Massive Black Hole Binaries on the Morphology of Merger Remnants
Get PDFMassive black hole (MBH) binaries, formed as a result of galaxy mergers, are
expected to harden by dynamical friction and three-body stellar scatterings,
until emission of gravitational waves (GWs) leads to their final coalescence.
According to recent simulations, MBH binaries can efficiently harden via
stellar encounters only when the host geometry is triaxial, even if only
modestly, as angular momentum diffusion allows an efficient repopulation of the
binary loss cone. In this paper, we carry out a suite of N-body simulations of
equal-mass galaxy collisions, varying the initial orbits and density profiles
for the merging galaxies and running simulations both with and without central
MBHs. We find that the presence of an MBH binary in the remnant makes the
system nearly oblate, aligned with the galaxy merger plane, within a radius
enclosing 100 MBH masses. We never find binary hosts to be prolate on any
scale. The decaying MBHs slightly enhance the tangential anisotropy in the
centre of the remnant due to angular momentum injection and the slingshot
ejection of stars on nearly radial orbits. This latter effect results in about
1% of the remnant stars being expelled from the galactic nucleus. Finally, we
do not find any strong connection between the remnant morphology and the binary
hardening rate, which depends only on the inner density slope of the remnant
galaxy. Our results suggest that MBH binaries are able to coalesce within a few
Gyr, even if the binary is found to partially erase the merger-induced
triaxiality from the remnant.Comment: 16 pages, 13 figures, 4 tables; accepted for publication in MNRA
Multiple populations in globular clusters: the distinct kinematic imprints of different formation scenarios
Full text linkSeveral scenarios have been proposed to explain the presence of multiple
stellar populations in globular clusters. Many of them invoke multiple
generations of stars to explain the observed chemical abundance anomalies, but
it has also been suggested that self-enrichment could occur via accretion of
ejecta from massive stars onto the circumstellar disc of low-mass pre-main
sequence stars. These scenarios imply different initial conditions for the
kinematics of the various stellar populations. Given some net angular momentum
initially, models for which a second generation forms from gas that collects in
a cooling flow into the core of the cluster predict an initially larger
rotational amplitude for the polluted stars compared to the pristine stars.
This is opposite to what is expected from the accretion model, where the
polluted stars are the ones crossing the core and are on preferentially radial
(low-angular momentum) orbits, such that their rotational amplitude is lower.
Here we present the results of a suite of -body simulations with initial
conditions chosen to capture the distinct kinematic properties of these
pollution scenarios. We show that initial differences in the kinematics of
polluted and pristine stars can survive to the present epoch in the outer parts
of a large fraction of Galactic globular clusters. The differential rotation of
pristine and polluted stars is identified as a unique kinematic signature that
could allow us to distinguish between various scenarios, while other kinematic
imprints are generally very similar from one scenario to the other.Comment: 22 pages, 16 figures + appendix. Accepted for publication in MNRA
A low pre-infall mass for the Carina dwarf galaxy from disequilibrium modelling
Get PDFDark matter only simulations of galaxy formation predict many more subhalos
around a Milky Way like galaxy than the number of observed satellites. Proposed
solutions require the satellites to inhabit dark matter halos with masses
between one to ten billion solar masses at the time they fell into the Milky
Way. Here we use a modelling approach, independent of cosmological simulations,
to obtain a preinfall mass of 360 (+380,-230) million solar masses for one of
the Milky Way's satellites: Carina. This determination of a low halo mass for
Carina can be accommodated within the standard model only if galaxy formation
becomes stochastic in halos below ten billion solar masses. Otherwise Carina,
the eighth most luminous Milky Way dwarf, would be expected to inhabit a
significantly more massive halo. The implication of this is that a population
of "dark dwarfs" should orbit the Milky Way: halos devoid of stars and yet more
massive than many of their visible counterparts.Comment: 5 pages, 3 figures, 1 table, and supplementary material availabl
The mass distribution of the Fornax dSph: constraints from its globular cluster distribution
Full text linkUniquely among the dwarf spheroidal (dSph) satellite galaxies of the Milky
Way, Fornax hosts globular clusters. It remains a puzzle as to why dynamical
friction has not yet dragged any of Fornax's five globular clusters to the
centre, and also why there is no evidence that any similar star cluster has
been in the past (for Fornax or any other dSph). We set up a suite of 2800
N-body simulations that sample the full range of globular-cluster orbits and
mass models consistent with all existing observational constraints for Fornax.
In agreement with previous work, we find that if Fornax has a large dark-matter
core then its globular clusters remain close to their currently observed
locations for long times. Furthermore, we find previously unreported behaviour
for clusters that start inside the core region. These are pushed out of the
core and gain orbital energy, a process we call 'dynamical buoyancy'. Thus a
cored mass distribution in Fornax will naturally lead to a shell-like globular
cluster distribution near the core radius, independent of the initial
conditions. By contrast, CDM-type cusped mass distributions lead to the rapid
infall of at least one cluster within \Delta t = 1-2Gyr, except when picking
unlikely initial conditions for the cluster orbits (\sim 2% probability), and
almost all clusters within \Delta t = 10Gyr. Alternatively, if Fornax has only
a weakly cusped mass distribution, dynamical friction is much reduced. While
over \Delta t = 10Gyr this still leads to the infall of 1-4 clusters from their
present orbits, the infall of any cluster within \Delta t = 1-2Gyr is much less
likely (with probability 0-70%, depending on \Delta t and the strength of the
cusp). Such a solution to the timing problem requires that in the past the
globular clusters were somewhat further from Fornax than today; they most
likely did not form within Fornax, but were accreted.Comment: 12 pages, 8 figures, 3 tables, submitted to MNRA
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