198 research outputs found
Transitions in non-conserving models of Self-Organized Criticality
We investigate a random--neighbours version of the two dimensional
non-conserving earthquake model of Olami, Feder and Christensen [Phys. Rev.
Lett. {\bf 68}, 1244 (1992)]. We show both analytically and numerically that
criticality can be expected even in the presence of dissipation. As the
critical level of conservation, , is approached, the cut--off of the
avalanche size distribution scales as . The
transition from non-SOC to SOC behaviour is controlled by the average branching
ratio of an avalanche, which can thus be regarded as an order
parameter of the system. The relevance of the results are discussed in
connection to the nearest-neighbours OFC model (in particular we analyse the
relevance of synchronization in the latter).Comment: 8 pages in latex format; 5 figures available upon reques
Boundary effects in a random neighbor model of earthquakes
We introduce spatial inhomogeneities (boundaries) in a random neighbor
version of the Olami, Feder and Christensen model [Phys. Rev. Lett. 68, 1244
(1992)] and study the distributions of avalanches starting both from the bulk
and from the boundaries of the system. Because of their clear geophysical
interpretation, two different boundary conditions have been considered (named
free and open, respectively). In both cases the bulk distribution is described
by the exponent . Boundary distributions are instead
characterized by two different exponents and , for free and open boundary conditions, respectively. These
exponents indicate that the mean-field behavior of this model is correctly
described by a recently proposed inhomogeneous form of critical branching
process.Comment: 6 pages, 2 figures ; to appear on PR
Testing metallicity indicators at z~1.4 with the gravitationally lensed galaxy CASSOWARY 20
We present X-shooter observations of CASSOWARY 20 (CSWA 20), a star-forming
(SFR ~6 Msol/yr) galaxy at z=1.433, magnified by a factor of 11.5 by the
gravitational lensing produced by a massive foreground galaxy at z=0.741. We
analysed the integrated physical properties of the HII regions of CSWA 20 using
temperature- and density-sensitive emission lines. We find the abundance of
oxygen to be ~1/7 of solar, while carbon is ~50 times less abundant than in the
Sun. The unusually low C/O ratio may be an indication of a particularly rapid
timescale of chemical enrichment. The wide wavelength coverage of X-shooter
gives us access to five different methods for determining the metallicity of
CSWA 20, three based on emission lines from HII regions and two on absorption
features formed in the atmospheres of massive stars. All five estimates are in
agreement, within the factor of ~2 uncertainty of each method. The interstellar
medium of CSWA 20 only partially covers the star-forming region as viewed from
our direction; in particular, absorption lines from neutrals and first ions are
exceptionally weak. We find evidence for large-scale outflows of the
interstellar medium (ISM) with speeds of up 750 km/s, similar to the values
measured in other high-z galaxies sustaining much higher rates of star
formation.Comment: 18 pages, 11 figures, accepted for publication in MNRA
A young star-forming galaxy at z = 3.5 with an extended Ly\, halo seen with MUSE
Spatially resolved studies of high redshift galaxies, an essential insight
into galaxy formation processes, have been mostly limited to stacking or
unusually bright objects. We present here the study of a typical (L,
M = 6 ) young lensed galaxy at , observed
with MUSE, for which we obtain 2D resolved spatial information of Ly
and, for the first time, of CIII] emission. The exceptional signal-to-noise of
the data reveals UV emission and absorption lines rarely seen at these
redshifts, allowing us to derive important physical properties (T15600
K, n300 cm, covering fraction f) using multiple
diagnostics. Inferred stellar and gas-phase metallicities point towards a low
metallicity object (Z = 0.07 Z and
Z 0.16 Z). The Ly emission extends over
10 kpc across the galaxy and presents a very uniform spectral profile,
showing only a small velocity shift which is unrelated to the intrinsic
kinematics of the nebular emission. The Ly extension is 4 times
larger than the continuum emission, and makes this object comparable to
low-mass LAEs at low redshift, and more compact than the Lyman-break galaxies
and Ly emitters usually studied at high redshift. We model the
Ly line and surface brightness profile using a radiative transfer code
in an expanding gas shell, finding that this model provides a good description
of both observables.Comment: 19 pages, 15 figures, accepted in MNRA
Distribution of epicenters in the Olami-Feder-Christensen model
We show that the well established Olami-Feder-Christensen (OFC) model for the
dynamics of earthquakes is able to reproduce a new striking property of real
earthquake data. Recently, it has been pointed out by Abe and Suzuki that the
epicenters of earthquakes could be connected in order to generate a graph, with
properties of a scale-free network of the Barabasi-Albert type. However, only
the non conservative version of the Olami-Feder-Christensen model is able to
reproduce this behavior. The conservative version, instead, behaves like a
random graph. Besides indicating the robustness of the model to describe
earthquake dynamics, those findings reinforce that conservative and non
conservative versions of the OFC model are qualitatively different. Also, we
propose a completely new dynamical mechanism that, even without an explicit
rule of preferential attachment, generates a free scale network. The
preferential attachment is in this case a ``by-product'' of the long term
correlations associated with the self-organized critical state. The detailed
study of the properties of this network can reveal new aspects of the dynamics
of the OFC model, contributing to the understanding of self-organized
criticality in non conserving models.Comment: 7 pages, 7 figure
The self-organized critical forest-fire model on large scales
We discuss the scaling behavior of the self-organized critical forest-fire
model on large length scales. As indicated in earlier publications, the
forest-fire model does not show conventional critical scaling, but has two
qualitatively different types of fires that superimpose to give the effective
exponents typically measured in simulations. We show that this explains not
only why the exponent characterizing the fire-size distribution changes with
increasing correlation length, but allows also to predict its asymptotic value.
We support our arguments by computer simulations of a coarse-grained model, by
scaling arguments and by analyzing states that are created artificially by
superimposing the two types of fires.Comment: 26 pages, 7 figure
Emission Line Metallicities From The Faint Infrared Grism Survey and VLT/MUSE
We derive direct measurement gas-phase metallicities of for 14 low-mass Emission Line Galaxies (ELGs) at
identified in the Faint Infrared Grism Survey (FIGS). We use deep slitless G102
grism spectroscopy of the Hubble Ultra Deep Field (HUDF), dispersing light from
all objects in the field at wavelengths between 0.85 and 1.15 microns. We run
an automatic search routine on these spectra to robustly identify 71 emission
line sources, using archival data from VLT/MUSE to measure additional lines and
confirm redshifts. We identify 14 objects with with measurable
O[III]4363 \AA\ emission lines in matching VLT/MUSE spectra. For these
galaxies, we derive direct electron-temperature gas-phase metallicities with a
range of . With matching stellar masses in the
range of , we construct a
mass-metallicity (MZ) relation and find that the relation is offset to lower
metallicities compared to metallicities derived from alternative methods
(e.g.,, O3N2, N2O2) and continuum selected samples. Using star
formation rates (SFR) derived from the emission line, we calculate
our galaxies' position on the Fundamental Metallicity Relation (FMR), where we
also find an offset toward lower metallicities. This demonstrates that this
emission-line-selected sample probes objects of low stellar masses but even
lower metallicities than many comparable surveys. We detect a trend suggesting
galaxies with higher Specific Star Formation (SSFR) are more likely to have
lower metallicity. This could be due to cold accretion of metal-poor gas that
drives star formation, or could be because outflows of metal-rich stellar winds
and SNe ejecta are more common in galaxies with higher SSFR.Comment: 14 pages, 11 figures, accepted in Ap
Into the Lyα jungle: exploring the circumgalactic medium of galaxies at z ∼ 4 − 5 with MUSE
We present a study of the galaxy environment of 9 strong H I+C IV absorption line systems (16.2 < log(N(HI)) < 21.2) spanning a wide range in metallicity at z ∼ 4 − 5, using MUSE integral field and X-Shooter spectroscopic data collected in a z ≈ 5.26 quasar field. We identify galaxies within a 250 kpc and ±1000 km s−1 window for 6 out of the 9 absorption systems, with 2 of the absorption line systems showing multiple associated galaxies within the MUSE field of view. The space density of Lyα emitting galaxies (LAEs) around the H I and C IV systems is ≈10 − 20 times the average sky density of LAEs given the flux limit of our survey, showing a clear correlation between the absorption and galaxy populations. Further, we find that the strongest C IV systems in our sample are those that are most closely aligned with galaxies in velocity space, i.e. within velocities of ±500 km s−1. The two most metal poor systems lie in the most dense galaxy environments, implying we are potentially tracing gas that is infalling for the first time into star-forming groups at high redshift. Finally, we detect an extended Lyα nebula around the z ≈ 5.26 quasar, which extends up to ≈50 kpc at the surface brightness limit of 3.8 × 10−18 erg s−1 cm−2 arcsec−2. After scaling for surface brightness dimming, we find that this nebula is centrally brighter, having a steeper radial profile than the average for nebulae studied at z ∼ 3 and is consistent with the mild redshift evolution seen from z ≈ 2
FIGS -- Faint Infrared Grism Survey: Description and Data Reduction
The Faint Infrared Grism Survey (FIGS) is a deep Hubble Space Telescope (HST)
WFC3/IR (Wide Field Camera 3 Infrared) slitless spectroscopic survey of four
deep fields. Two fields are located in the Great Observatories Origins Deep
Survey-North (GOODS-N) area and two fields are located in the Great
Observatories Origins Deep Survey-South (GOODS-S) area. One of the southern
fields selected is the Hubble Ultra Deep Field. Each of these four fields were
observed using the WFC3/G102 grism (0.8-1.15 continuous coverage)
with a total exposure time of 40 orbits (~ 100 kilo-seconds) per field. This
reaches a 3 sigma continuum depth of ~26 AB magnitudes and probes emission
lines to . This paper details the four
FIGS fields and the overall observational strategy of the project. A detailed
description of the Simulation Based Extraction (SBE) method used to extract and
combine over 10000 spectra of over 2000 distinct sources brighter than
m_F105W=26.5 mag is provided. High fidelity simulations of the observations is
shown to significantly improve the background subtraction process, the spectral
contamination estimates, and the final flux calibration. This allows for the
combination of multiple spectra to produce a final high quality, deep,
1D-spectra for each object in the survey.Comment: 21 Pages. 17 Figures. To appear in Ap
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