68 research outputs found
Interrupting "Green Capital": transformative political practice at the frontiers of wind energy in Mexico
This thesis examines how contestations over the large-scale production of wind energy are transforming political life in the Isthmus of Tehuantepec, Mexico. In the 1980s, the Mexican state and international actors targeted the Isthmus as a place for investing in green energy development. Today, wind farms occupy between 10,000 and 15,000 hectares, but could extend to between 40,000 and 50,000 hectares if the 'full' wind energy potential of the Isthmus is developed. Indigenous social movements are interrupting this `greening' of the Isthmus, however, sustaining that the expansion of green energy, despite its associations with the greater ecological good, is detrimental to their survival. How is the harnessing of wind in the Isthmus becoming a new frontier of green capitalism? And how are indigenous movements reshaping the practices of green energy in Mexico? I examine these questions through the case study of indigenous-led resistance to the construction of a large-scale wind energy project in the southern Isthmus: the San Dionisio project along the Barra de Santa Teresa. As the Mexican government pursues green energy, the Isthmus is re-signified as an environmentally responsible space of liberal peace. Drawing on participant observation, interviews with NGOs and local residents, and analysis of environmental and energy policy documents, I argue that this is a virtual peace, one that makes invisible the character, agency, and needs of civil society actors. By framing resistance to wind farming as the cause of conflicts to be managed for the greater common good, state-sanctioned `pacification' of the Isthmus entrenches short-term direct violence and structural violence as constitutive of green capitalism.Master of Art
Resonant Thickening of Disks by Small Satellite Galaxies
We study the vertical heating and thickening of galaxy disks due to accretion
of small satellites. Our simulations are restricted to axial symmetry, which
largely eliminates numerical evolution of the target galaxy but requires the
trajectory of the satellite to be along the symmetry axis of the target. We
find that direct heating of disk stars by the satellite is not important
because the satellite's gravitational perturbation has little power at
frequencies resonant with the vertical stellar orbits. The satellite does
little damage to the disk until its decaying orbit resonantly excites
large-scale disk bending waves. Bending waves can damp through dynamical
friction from the halo or internal wave-particle resonances; we find that
wave-particle resonances dominate the damping. The principal vertical heating
mechanism is therefore dissipation of bending waves at resonances with stellar
orbits in the disk. Energy can thus be deposited some distance from the point
of impact of the satellite. The net heating from a tightly bound satellite can
be substantial, but satellites that are tidally disrupted before they are able
to excite bending waves do not thicken the disk.Comment: 13 pages, 8 figures, to appear in ApJ, latex (aaspp4.sty
Dark Matter Densities during the Formation of the First Stars and in Dark Stars
The first stars in the universe form inside dark matter
(DM) haloes whose initial density profiles are laid down by gravitational
collapse in hierarchical structure formation scenarios. During the formation of
the first stars in the universe, the baryonic infall compresses the dark matter
further. The resultant dark matter density is presented here, using an
algorithm originally developed by Young to calculate changes to the profile as
the result of adiabatic infall in a spherical halo model; the Young
prescription takes into account the non-circular motions of halo particles. The
density profiles obtained in this way are found to be within a factor of two of
those obtained using the simple adiabatic contraction prescription of
Blumenthal et al. Our results hold regardless of the nature of the dark matter
or its interactions and rely merely on gravity. If the dark matter consists of
weakly interacting massive particles, which are their own antiparticles, their
densities are high enough that their annihilation in the first protostars can
indeed provide an important heat source and prevent the collapse all the way to
fusion. In short, a ``Dark Star'' phase of stellar evolution, powered by DM
annihilation, may indeed describe the first stars in the universe.Comment: 14 pages, 3 figures, and 1 tabl
Resonant capture, counter-rotating disks, and polar rings
We suggest that polar rings and/or counter-rotating disks in flattened
galaxies can be formed from stars captured at the Binney resonance, where the
rate of precession of the angular momentum vector of a disk star equals the
pattern speed of a triaxial halo. If the halo pattern speed is initially
retrograde and slowly decays to zero, stars can be trapped as the Binney
resonance sweeps past them, and levitated into polar orbits. If the halo
pattern speed is initially retrograde and slowly changes to prograde, trapped
stars can evolve from prograde to retrograde disk orbits. The stellar
components of polar rings formed by this process should consist of two equal,
counter-rotating star streams.Comment: 10 pages, 6 figures, Latex. Submitted to MNRA
The stellar velocity distribution in the solar neighbourhood
We explore the heating of the velocity distribution in the solar
neighbourhood by stochastic spiral waves. Our investigation is based on direct
numerical integration of initially circular test-particle orbits in the sheared
sheet. We confirm the conclusion of other investigators that heating by spiral
structure can explain the principal features of the age-velocity dispersion
relation and other parameters of the velocity distribution in the solar
neighbourhood. In addition, we find that heating by strong transient spirals
can naturally explain the presence of small-scale structure in the velocity
distribution (``moving groups''). Heating by spiral structure also explains why
the stars in a single velocity-space moving group have a wide range of ages, a
result which is difficult to understand in the traditional model that these
structures result from inhomogeneous star formation. Thus we suggest that old
moving groups arise from irregularities in the Galactic potential rather than
irregularities in the star-formation rate.Comment: Accepted by MNRAS for publication; 20 pages, 11 figures, mn2e.cl
Towards optimal softening in 3D N-body codes: I. Minimizing the force error
In N-body simulations of collisionless stellar systems, the forces are
softened to reduce the shot noise. Softening modifies gravity at r=|x-y|
smaller than softening length epsilon and the softened forces are increasingly
biased for ever larger epsilon. There is, thus, some optimum between reducing
the fluctuations and introducing a bias. Here, analytical relations are derived
for the amplitudes of the bias and the fluctuations in the limit of small
epsilon and large N. It is shown that the fluctuations of the force are
generated locally, in contrast to the variations of the potential, which
originate from noise in the whole system. Based on the asymptotic relations and
using numerical experiments, I study the dependence of the resulting force
error on N, epsilon, and on the functional form by which Newtonian gravity is
replaced. The Plummer softening, where each body is replaced by a Plummer
sphere of scale radius epsilon, yields significantly larger force errors than
do methods in which the bodies are replaced by density kernels of finite
extent. I also give special kernels, which reduce the errors even further.
These kernels largely compensate the errors made with too small inter-particle
forces at r<epsilon by exceeding Newtonian forces at r epsilon. Additionally,
the possibilities of locally adapting epsilon and of using unequal weights for
the bodies are investigated. These various techniques allow, without increasing
N, to reduce the rms force error by a factor 2 compared to Plummer softening
with constant epsilon. The results of this study are directly relevant to
N-body simulations using direct summation techniques or the tree code.
(abridged)Comment: 20 pages, 8 figs, accepted for publication in MNRA
Dark Matter Capture in the First Stars: a Power Source and Limit on Stellar Mass
The annihilation of weakly interacting massive particles can provide an
important heat source for the first (Pop. III) stars, potentially leading to a
new phase of stellar evolution known as a "Dark Star". When dark matter (DM)
capture via scattering off of baryons is included, the luminosity from DM
annihilation may dominate over the luminosity due to fusion, depending on the
DM density and scattering cross-section. The influx of DM due to capture may
thus prolong the lifetime of the Dark Stars. Comparison of DM luminosity with
the Eddington luminosity for the star may constrain the stellar mass of zero
metallicity stars; in this case DM will uniquely determine the mass of the
first stars. Alternatively, if sufficiently massive Pop. III stars are found,
they might be used to bound dark matter properties.Comment: 19 pages, 4 figures, 3 Tables updated captions and graphs, corrected
grammer, and added citations revised for submission to JCA
The Star Formation History and Dust Content in the Far Outer Disc of M31
We present a detailed analysis of two fields located 26 kpc (~5 scalelengths)
from the centre of M31. One field samples the major axis populations--the Outer
Disc field--while the other is offset by ~18' and samples the Warp in the
stellar disc. The CMDs based on HST/ACS imaging reach old main-sequence
turn-offs (~12.5 Gyr). We apply the CMD-fitting technique to the Warp field to
reconstruct the star formation history (SFH). We find that after undergoing
roughly constant SF until about 4.5 Gyr ago, there was a rapid decline in
activity and then a ~1.5 Gyr lull, followed by a strong burst lasting 1.5 Gyr
and responsible for 25% of the total stellar mass in this field. This burst
appears to be accompanied by a decline in metallicity which could be a
signature of the inflow of metal-poor gas. The onset of the burst (~3 Gyr ago)
corresponds to the last close passage of M31 and M33 as predicted by detailed
N-body modelling, and may have been triggered by this event. We reprocess the
deep M33 outer disc field data of Barker et al. (2011) in order to compare
consistently-derived SFHs. This reveals a similar duration burst that is
exactly coeval with that seen in the M31 Warp field, lending further support to
the interaction hypothesis. The complex SFHs and the smoothly-varying
age-metallicity relations suggest that the stellar populations observed in the
far outer discs of both galaxies have largely formed in situ rather than
migrated from smaller galactocentric radii. The strong differential reddening
affecting the CMD of the Outer Disc field prevents derivation of the SFH.
Instead, we quantify this reddening and find that the fine-scale distribution
of dust precisely follows that of the HI gas. This indicates that the outer HI
disc of M31 contains a substantial amount of dust and therefore suggests
significant metal enrichment in these parts, consistent with inferences from
our CMD analysis.Comment: Abstract shortened. 17 pages, 12 figures (+ 6 pages & 5 figures in
Appendix). MNRAS, in pres
Impact of baryon physics on dark matter structures: a detailed simulation study of halo density profiles
The back-reaction of baryons on the dark matter halo density profile is of
great interest, not least because it is an important systematic uncertainty
when attempting to detect the dark matter. Here, we draw on a large suite of
high resolution cosmological hydrodynamical simulations, to systematically
investigate this process and its dependence on the baryonic physics associated
with galaxy formation. The inclusion of baryons results in significantly more
concentrated density profiles if radiative cooling is efficient and feedback is
weak. The dark matter halo concentration can in that case increase by as much
as 30 (10) per cent on galaxy (cluster) scales. The most significant effects
occur in galaxies at high redshift, where there is a strong anti-correlation
between the baryon fraction in the halo centre and the inner slope of both the
total and the dark matter density profiles. If feedback is weak, isothermal
inner profiles form, in agreement with observations of massive, early-type
galaxies. However, we find that AGN feedback, or extremely efficient feedback
from massive stars, is necessary to match observed stellar fractions in groups
and clusters, as well as to keep the maximum circular velocity similar to the
virial velocity as observed for disk galaxies. These strong feedback models
reduce the baryon fraction in galaxies by a factor of 3 relative to the case
with no feedback. The AGN is even capable of reducing the baryon fraction by a
factor of 2 in the inner region of group and cluster haloes. This in turn
results in inner density profiles which are typically shallower than isothermal
and the halo concentrations tend to be lower than in the absence of baryons.Comment: 20 pages, 14 figures, 1 table. MNRAS in press. Version 2: added a few
references
3D kinematics through the X-shaped Milky Way bulge
Context. It has recently been discovered that the Galactic bulge is X-shaped, with the two southern arms of the X both crossing the lines of sight at l = 0 and | b| > 4, hence producing a double red clump in the bulge color magnitude diagram. Dynamical models predict the formation of X-shaped bulges as extreme cases of boxy-peanut bulges. However, since X-shaped bulges were known to be present only in external galaxies, models have never been compared to 3D kinematical data for individual stars.
Aims. We study the orbital motion of Galactic bulge stars in the two arms (overdensities) of the X in the southern hemisphere. The goal is to provide observational constraints to bulge formation models that predict the formation of X-shapes through bar dynamical instabilities.
Methods. Radial velocities have been obtained for a sample of 454 bulge giants, roughly equally distributed between the bright and the faint red clump, in a field at (l,b) = (0, â6). Proper motions were derived for all red clump stars in the same field by combining images from two epochs, which were obtained 11 years apart, with WFI at the 2.2âm at La Silla. The observed field contains the globular cluster NGC 6558, whose member stars were used to assess the accuracy of the proper motion measurement. At the same time, as a by-product, we provide the first proper motion measurement of NGC 6558. The proper motions for the spectroscopic subsample are analyzed for a subsample of 352 stars, taking into account the radial velocities and metallicities measured from near-infrared calcium triplet lines.
Results. The radial velocity distribution of stars in the bright red clump, which traces the closer overdensity of bulge stars, shows an excess of stars moving towards the Sun. Similarly, an excess of stars receding from the Sun is seen in the far overdensity, which is traced by faint red clump stars. This is explained by the presence of stars on elongated orbits, which are most likely streaming along the arms of the X-shaped bulge. Proper motions for these stars are consistent with qualitative predictions of dynamical models of peanut-shaped bulges. Surprisingly, stars on elongated orbits have preferentially metal-poor (subsolar) metallicities, while the metal rich ones, in both overdensities, are preferentially found in more axisymmetric orbits. The observed proper motion of NGC 6558 has been measured as (ÎŒlcos â (b),ÎŒb) = (0.30 â ± â 0.14, â0.43 ± 0.13), with a velocity dispersion of (Ïlcos(b),Ïb) = (1.8,1.7) mas/yr. This is the first proper motion measurement for this cluster
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