2,085 research outputs found
Getting Over the Hump: A Theory of Crime, Credit and Accumulation
We explore the implications of endogenous credit market imperfections for the relationship between property crime and the process of economic development. In the initial stages of development, property crime rises as the opportunities to gain from illegal activities expand. In later stages, however, crime falls as capital market imperfections are overcome and legal activities become more profitable. We detail the forces which determine whether the crime generated by an economy's early growth will choke off the development process. We also find that endogenous credit constraints reduce the effectiveness of public expenditures that raise expected criminal sanctions, and that spending on ex ante crime prevention may be more cost-effective. Nous construisons un modèle dans lequel la capacité de l'économie à se développer est déterminée de façon endogène par l'interaction entre les ratés du marché du capital et le taux de crime contre la propriété. Dans les premiers stades du développement économique, les crimes contre la propriété croissent en même temps que la richesse s'accumule, les gains que procure l'activité criminelle étant alors d'autant plus élevés. Mais dans les stades de développement ultérieurs, ces crimes diminuent au fur et à mesure que la richesse s'accumule car les ratés du marché du capital sont de moins en moins importants. Dans certaines économies, les crimes contre la propriété, observables dans les premiers stades du développement, ralentissent et éventuellement arrêtent la croissance économique, alors que dans d'autres, l'économie poursuit sa croissance et atteint la phase de déclin de l'activité criminelle. Insistant sur l'importance des ratés endogènes du marché du capital, nous analysons également l'impact de politiques publiques de prévention et de dissuasion du crime.Crime, property rights economic development, credit markets
Probing Cosmic Dawn: Modelling the Assembly History, SEDs, and Dust Content of Selected Galaxies
The presence of spectroscopically confirmed Balmer breaks in galaxy spectral
energy distributions (SEDs) at provides one of the best probes of the
assembly history of the first generations of stars in our Universe. Recent
observations of the gravitationally lensed source, MACS 1149_JD1 (JD1),
indicate that significant amounts of star formation likely occurred at
redshifts as high as . The inferred stellar mass, dust mass, and
assembly history of JD1, or any other galaxy at these redshifts that exhibits a
strong Balmer break, can provide a strong test of our best theoretical models
from high-resolution cosmological simulations. In this work, we present the
results from a cosmological radiation-hydrodynamics simulation of the region
surrounding a massive Lyman-break galaxy. For two of our most massive systems,
we show that dust preferentially resides in the vicinity of the young stars
thereby increasing the strength of the measured Balmer break such that the
simulated SEDs are consistent with the photometry of JD1 and two other
systems (GN-z10-3 and GN-z9-1) that have proposed Balmer breaks at high
redshift. We find strong variations in the shape and luminosity of the SEDs of
galaxies with nearly identical stellar and halo masses, indicating the
importance of morphology, assembly history, and dust distribution in making
inferences on the properties of individual galaxies at high redshifts. Our
results stress the importance that dust may play in modulating the observable
properties of galaxies, even at the extreme redshifts of .Comment: 16 pages, 13 Figures, Accepted to MNRA
Fluorescent C II* 1335A emission spectroscopically resolved in a galaxy at z = 5.754
We report the discovery of the first spectroscopically resolved C II /C II*
1334, 1335A doublet in the Lyman-break galaxy J0215-0555 at z = 5.754. The
separation of the resonant and fluorescent emission channels was possible
thanks to the large redshift of the source and long integration time, as well
as the small velocity width of the feature, 0.6 +- 0.2A. We model this emission
and find that at least two components are required to reproduce the combination
of morphologies of C II* emission, C II absorption and emission, and
Lyman-alpha emission from the object. We suggest that the close alignment
between the fluorescence and Lyman-alpha emission could indicate an ionisation
escape channel within the object. While the faintness of such a C II /C II*
doublet makes it prohibitively difficult to pursue for similar systems with
current facilities, we suggest it can become a valuable porosity diagnostic in
the era of JWST and the upcoming generations of ELTs.Comment: 5 pages, 3 figures; accepted for publication in MNRAS Letter
A spectroscopic search for AGN activity in the reionization era
The ubiquity of Lyman alpha (Ly) emission in a sample of four bright
[O III]-strong star-forming galaxies with redshifts above 7 has led to the
suggestion that such luminous sources represent a distinct population compared
to their fainter, more numerous, counterparts. The presence of Ly
emission within the reionization era could indicate that these sources created
early ionized bubbles due to their unusually strong radiation, possibly due to
the presence of active galactic nuclei. To test this hypothesis we have secured
long integration spectra with XSHOOTER on the VLT for three 7 sources
selected to have similar luminosities and prominent excess fluxes in the IRAC
3.6 or 4.5m band, usually attributed to strong [O III] emission. We
secured additional spectroscopy for one of these galaxies at =7.15 using
MOSFIRE at the Keck telescope. For this, the most well-studied source in our
sample with the strongest IRAC excess, we detect prominent nebular emission
from He II and NV indicative of a non-thermal source. For the other two sources
at =6.81 and =6.85, for which no previous optical/near infrared
spectroscopy was initially available, Ly is seen in one and CIII]
emission in the other. Although a modest sample, our results further support
the hypothesis that the phenomenon of intense [O III] emission is associated
preferentially with sources lying in early ionized bubbles. However, even
though one of our sources at =7.15 clearly indicates the presence of
non-thermal radiation, such ionized bubbles may not uniquely arise in this
manner. We discuss the unique advantages of extending such challenging
diagnostic studies with JWST.Comment: 11 pages, 7 figures, submitted to Ap
Spectroscopic Constraints on UV Metal Line Emission at z~6-9: The Nature of Lyman-alpha Emitting Galaxies in the Reionization-Era
Recent studies have revealed intense UV metal emission lines in a modest
sample of z>7 Lyman-alpha emitters, indicating a hard ionizing spectrum is
present. If such high ionization features are shown to be common, it may
indicate that extreme radiation fields play a role in regulating the visibility
of Lyman-alpha in the reionization era. Here we present deep near-infrared
spectra of seven galaxies with Lyman-alpha emission at 5.4<z<8.7 (including a
newly-confirmed lensed galaxy at z=6.031) and three bright z>7 photometric
targets. In nine sources we do not detect UV metal lines. However in the
z=8.683 galaxy EGSY8p7, we detect a 4.6 sigma emission line in the narrow
spectral window expected for NV 1243. The feature is unresolved (FWHM<90 km/s)
and is likely nebular in origin. A deep H-band spectrum of EGSY8p7 reveals
non-detections of CIV, He II, and OIII]. The presence of NV requires a
substantial flux of photons above 77 eV, pointing to a hard ionizing spectrum
powered by an AGN or fast radiative shocks. Regardless of its origin, the
intense radiation field of EGSY8p7 may aid the transmission of Lyman-alpha
through what is likely a partially neutral IGM. With this new detection, five
of thirteen known Lyman-alpha emitters at z>7 have now been shown to have
intense UV line emission, suggesting that extreme radiation fields are
commonplace among the Lyman-alpha population. Future observations with JWST
will eventually clarify the origin of these features and explain their role in
the visibility of Lyman-alpha in the reionization era.Comment: 15 pages, 3 tables and 7 figures. Accepted for publication in MNRA
The Mean Absorption Line Spectra of a Selection of Luminous z~6 Lyman Break Galaxies
We examine the absorption line spectra of a sample of 31 luminous (M_UV=-23)
Lyman break galaxies at redshift z~6 using data taken with the FOCAS and OSIRIS
spectrographs on the Subaru and GTC telescopes. For two of these sources we
present longer exposure data taken at higher spectral resolution from ESO's
X-shooter spectrograph. Using these data, we demonstrate the practicality of
stacking our lower resolution data to measure the depth of various interstellar
and stellar absorption lines to probe the covering fraction of low ionization
gas and the gas-phase and stellar metallicities near the end of the era of
cosmic reionization. From maximum absorption line depths of SiII1260 and
CII1334, we infer a mean covering fraction of >0.85+/-0.16 for our sample. This
is larger than that determined using similar methods for lower luminosity
galaxies at slightly lower redshifts, suggesting that the most luminous
galaxies appear to have a lower escape fraction than fainter galaxies, and
therefore may not play a prominent role in concluding reionization. Using
various interstellar absorption lines we deduce gas-phase metallicities close
to solar indicative of substantial early enrichment. Using selected stellar
absorption lines, we model our spectra with a range of metallicities using
techniques successfully employed at lower redshift and deduce a stellar
metallicity of 0.4 +0.3/-0.1 solar, consistent with the stellar mass - stellar
metallicity relation recently found at z~3-5. We discuss the implications of
these metallicity estimates for the typical ages of our luminous galaxies and
conclude our results imply initial star formation at redshifts z~10, consistent
with independent analyses of earlier objects.Comment: 13 pages, 10 figures, 3 tables, Accepted for Publication in Ap
Dust in the reionization era: ALMA observations of a =8.38 Galaxy
We report on the detailed analysis of a gravitationally-lensed Y-band
dropout, A2744_YD4, selected from deep Hubble Space Telescope imaging in the
Frontier Field cluster Abell 2744. Band 7 observations with the Atacama Large
Millimeter Array (ALMA) indicate the proximate detection of a significant 1mm
continuum flux suggesting the presence of dust for a star-forming galaxy with a
photometric redshift of . Deep X-SHOOTER spectra confirms the high
redshift identity of A2744_YD4 via the detection of Lyman emission at
a redshift =8.38. The association with the ALMA detection is confirmed by
the presence of [OIII] 88m emission at the same redshift. Although both
emission features are only significant at the 4 level, we argue their
joint detection and the positional coincidence with a high redshift dropout in
the HST images confirms the physical association. Analysis of the available
photometric data and the modest gravitational magnification ()
indicates A2744_YD4 has a stellar mass of 210 M, a
star formation rate of M/yr and a dust mass of
610 M. We discuss the implications of the
formation of such a dust mass only 200 Myr after the onset of cosmic
reionisation.Comment: Accepted for publication in ApJ
Gravitational wave constraints on Einstein-\ae{}ther theory with LIGO/Virgo data
Lorentz symmetry is a fundamental property of Einstein's theory of general
relativity that one may wish to test with gravitational wave observations.
Einstein-aether theory is a model that introduces Lorentz-symmetry breaking in
the gravitational sector through an aether vector field, while still leading to
second-order field equations. This well-posed theory passes particle physics
constraints because it modifies directly only the gravitational sector, yet it
predicts deviations in the inspiral and coalescence of compact objects. We
here, for the first time, put this theory to the test by comparing its
gravitational wave predictions directly against LIGO/Virgo gravitational wave
data. We construct a waveform model for Einstein-aether theory,
EA_IMRPhenomD_NRT, through modifications of the general relativity
IMRPhenomD_NRTidalv2 model (used by the LIGO/VIRGO collaboration). This model
constructs a reponse function that not only contains the transverse-traceless
polarization, but also additional Einstein-aether (scalar and vectorial)
polarizations simultaneously. We then use the many current constraints on the
theory to construct non-trivial priors for the Einstein-aether coupling
constants. After testing the waveform model, we conduct parameter estimation
studies on two gravitational wave events: GW170817 and GW190425. We find that
these data are not sufficiently informative to place constraints on the theory
that are stronger than current bounds from binary pulsar, solar system and
cosmological observations. This is because, although Einstein-aether
modifications include additional polarizations and have been computed beyond
leading post-Newtonian order, these modifications are dominated by
(already-constrained) dipole effects. These difficulties make it unclear
whether future gravitational wave observations will be able to improve on
current constraints on Einstein-aether theory.Comment: 32 pages, 24 figures; replaced with accepted versio
Preparation and characterization of multiwalled carbon nanotube (MWCNT)/polymer nanostructured materials
http://spiedigitallibrary.org/journals/doc/SPIEDL-home/proc
Preparation and characterisation of vertically aligned single-walled carbon nanotube arrays on porous silicon
http://spiedigitallibrary.org/journals/doc/SPIEDL-home/proc
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