5,695 research outputs found
Prohibitions on Punishments in Private Contracts
In most contemporary economies loan contracts that mandate exclusionary penalties such as imprisonment or other non-pecuniary punishments for defaulting debtors are illegal, despite the fact that in some cases contracting parties might gain by being able to use them. A possible rationale for contracting restrictions of this type is that exclusion imposes negative externalities on individuals not party to the original loan contract. We explore the ability of such externalities to account for these restrictions. We contrast exclusion with enforceable collateral seizure, a widespread feature of developed financial systems. We also consider “behavioral” agents who underestimate their chances of being punished, and show that overconfidence of this type is a less compelling justification for restrictions on exclusionary punishments than is often argued
Reconciling dwarf galaxies with LCDM cosmology: Simulating a realistic population of satellites around a Milky Way-mass galaxy
Low-mass "dwarf" galaxies represent the most significant challenges to the
cold dark matter (CDM) model of cosmological structure formation. Because these
faint galaxies are (best) observed within the Local Group (LG) of the Milky Way
(MW) and Andromeda (M31), understanding their formation in such an environment
is critical. We present first results from the Latte Project: the Milky Way on
FIRE (Feedback in Realistic Environments). This simulation models the formation
of a MW-mass galaxy to z = 0 within LCDM cosmology, including dark matter, gas,
and stars at unprecedented resolution: baryon particle mass of 7070 Msun with
gas kernel/softening that adapts down to 1 pc (with a median of 25 - 60 pc at z
= 0). Latte was simulated using the GIZMO code with a mesh-free method for
accurate hydrodynamics and the FIRE-2 model for star formation and explicit
feedback within a multi-phase interstellar medium. For the first time, Latte
self-consistently resolves the spatial scales corresponding to half-light radii
of dwarf galaxies that form around a MW-mass host down to Mstar > 10^5 Msun.
Latte's population of dwarf galaxies agrees with the LG across a broad range of
properties: (1) distributions of stellar masses and stellar velocity
dispersions (dynamical masses), including their joint relation; (2) the
mass-metallicity relation; and (3) a diverse range of star-formation histories,
including their mass dependence. Thus, Latte produces a realistic population of
dwarf galaxies at Mstar > 10^5 Msun that does not suffer from the "missing
satellites" or "too big to fail" problems of small-scale structure formation.
We conclude that baryonic physics can reconcile observed dwarf galaxies with
standard LCDM cosmology.Comment: 7 pages, 5 figures. Accepted for publication in ApJ Letters. Several
updates, including: (1) fixed a bug in halo finder, now identifies 13
satellite galaxies and more subhalos in the baryonic simulation; (2) fixed a
minor bug in the feedback coupling and reran the simulation, resulting in a
somewhat lower-mass host galaxy; (3) Fig 2 now shows stellar velocity
dispersion profiles of satellite
Primary sand-dune plant community and soil properties during the west-coast India monsoon
A seven-station interrupted belt transect was established that followed a previously observed plant zonation pattern across an aggrading primary coastal dune system in the dry tropical region of west-coast India. The dominant weather pattern is monsoon from June to November, followed by hot and dry winter months when rainfall is scarce. Physical and chemical soil characteristics in each of the stations were analysed on five separate occasions, the first before the onset of monsoon, three during and the last post-monsoon. The plant community pattern was confirmed by quadrat survey. A pH gradient decreased with distance from the shoreline. Nutrient concentrations were deficient, increasing only in small amounts until the furthest station inland. At that location, there was a distinct and abrupt pedological transition zone from psammite to humic soils. There was a significant increase over previous stations in mean organic matter, ammonium nitrate and soil-water retention, although the increase in real terms was small. ANOVA showed significant variation in electrical conductivity, phosphorus, calcium, magnesium and sodium concentrations over time. There was no relationship between soil chemistry characteristics and plant community structure over the transect. Ipomoea pes-caprae and Spinifex littoreus were restricted to the foredunes, the leguminous forb Alysicarpus vaginalis and Perotis indica to the two stations furthest from the strand. Ischaemum indicum, a C4 perennial grass species adopting an ephemeral strategy was, in contrast, ubiquitous to all stations
Black Holes on FIRE: Stellar Feedback Limits Early Feeding of Galactic Nuclei
We introduce massive black holes (BHs) in the Feedback In Realistic
Environments project and perform high-resolution cosmological hydrodynamic
simulations of quasar-mass halos () down to . These simulations model stellar
feedback by supernovae, stellar winds, and radiation, and BH growth using a
gravitational torque-based prescription tied to resolved properties of galactic
nuclei. We do not include BH feedback. We show that early BH growth occurs
through short (Myr) accretion episodes that can reach or even
exceed the Eddington rate. In this regime, BH growth is limited by bursty
stellar feedback continuously evacuating gas from galactic nuclei, and BHs
remain under-massive relative to the local -
relation. BH growth is more efficient at later times, when the nuclear stellar
potential retains a significant gas reservoir, star formation becomes less
bursty, and galaxies settle into a more ordered state, with BHs rapidly
converging onto the scaling relation when the host reaches . Our results are not sensitive to the details of the
accretion model so long as BH growth is tied to the gas content within pc of the BH. Our simulations imply that bursty stellar feedback has
strong implications for BH and AGN demographics, especially in the early
Universe and for low-mass galaxies.Comment: 5 pages, 3 figures, submitted to MNRA
Numerical optimization of integrating cavities for diffraction-limited millimeter-wave bolometer arrays
Far-infrared to millimeter-wave bolometers designed to make astronomical observations are typically encased in integrating cavities at the termination of feedhorns or Winston cones. This photometer combination maximizes absorption of radiation, enables the absorber area to be minimized, and controls the directivity of absorption, thereby reducing susceptibility to stray light. In the next decade, arrays of hundreds of silicon nitride micromesh bolometers with planar architectures will be used in ground-based, suborbital, and orbital platforms for astronomy. The optimization of integrating cavity designs is required for achieving the highest possible sensitivity for these arrays. We report numerical simulations of the electromagnetic fields in integrating cavities with an infinite plane-parallel geometry formed by a solid reflecting backshort and the back surface of a feedhorn array block. Performance of this architecture for the bolometer array camera (Bolocam) for cosmology at a frequency of 214 GHz is investigated. We explore the sensitivity of absorption efficiency to absorber impedance and backshort location and the magnitude of leakage from cavities. The simulations are compared with experimental data from a room-temperature scale model and with the performance of Bolocam at a temperature of 300 mK. The main results of the simulations for Bolocam-type cavities are that (1) monochromatic absorptions as high as 95% are achievable with <1% cross talk between neighboring cavities, (2) the optimum absorber impedances are 400 Ω/sq, but with a broad maximum from ~150 to ~700 Ω/sq, and (3) maximum absorption is achieved with absorber diameters ≥1.5λ. Good general agreement between the simulations and the experiments was found
Breathing FIRE: How Stellar Feedback Drives Radial Migration, Rapid Size Fluctuations, and Population Gradients in Low-Mass Galaxies
We examine the effects of stellar feedback and bursty star formation on
low-mass galaxies ()
using the FIRE (Feedback in Realistic Environments) simulations. While previous
studies emphasized the impact of feedback on dark matter profiles, we
investigate the impact on the stellar component: kinematics, radial migration,
size evolution, and population gradients. Feedback-driven outflows/inflows
drive significant radial stellar migration over both short and long timescales
via two processes: (1) outflowing/infalling gas can remain star-forming,
producing young stars that migrate within their first , and (2) gas outflows/inflows drive strong fluctuations in the
global potential, transferring energy to all stars. These processes produce
several dramatic effects. First, galaxies' effective radii can fluctuate by
factors of over , and these rapid size fluctuations
can account for much of the observed scatter in radius at fixed
Second, the cumulative effects of many outflow/infall episodes steadily heat
stellar orbits, causing old stars to migrate outward most strongly. This
age-dependent radial migration mixes---and even inverts---intrinsic age and
metallicity gradients. Thus, the galactic-archaeology approach of calculating
radial star-formation histories from stellar populations at can be
severely biased. These effects are strongest at , the same regime where feedback most
efficiently cores galaxies. Thus, detailed measurements of stellar kinematics
in low-mass galaxies can strongly constrain feedback models and test baryonic
solutions to small-scale problems in CDM.Comment: Accepted to ApJ (820, 131) with minor revisions from v1. Figure 4 now
includes dark matter. Main results in Figures 7 and 1
Under the Firelight: Stellar Tracers of the Local Dark Matter Velocity Distribution in the Milky Way
The Gaia era opens new possibilities for discovering the remnants of
disrupted satellite galaxies in the Solar neighborhood. If the population of
local accreted stars is correlated with the dark matter sourced by the same
mergers, one can then map the dark matter distribution directly. Using two
cosmological zoom-in hydrodynamic simulations of Milky Way-mass galaxies from
the Latte suite of Fire-2 simulations, we find a strong correlation between the
velocity distribution of stars and dark matter at the solar circle that were
accreted from luminous satellites. This correspondence holds for dark matter
that is either relaxed or in kinematic substructure called debris flow, and is
consistent between two simulated hosts with different merger histories. The
correspondence is more problematic for streams because of possible spatial
offsets between the dark matter and stars. We demonstrate how to reconstruct
the dark matter velocity distribution from the observed properties of the
accreted stellar population by properly accounting for the ratio of stars to
dark matter contributed by individual mergers. After demonstrating this method
using the Fire-2 simulations, we apply it to the Milky Way and use it to
recover the dark matter velocity distribution associated with the recently
discovered stellar debris field in the Solar neighborhood. Based on results
from Gaia, we estimate that of the local dark matter that
is accreted from luminous mergers is in debris flow.Comment: 18+5 pages, 12+5 figures. Supplementary Data can be found here
https://linoush.github.io/DM_Velocity_Distribution
- …