324 research outputs found
Metallicities, dust and molecular content of a QSO-Damped Lyman-{\alpha} system reaching log N (H i) = 22: An analog to GRB-DLAs
We present the elemental abundance and H2 content measurements of a Damped
Lyman-{\alpha} (DLA) system with an extremely large H i column density, log N(H
i) (cm-2) = 22.0+/-0.10, at zabs = 3.287 towards the QSO SDSS J 081634+144612.
We measure column densities of H2, C i, C i^*, Zn ii, Fe ii, Cr ii, Ni ii and
Si ii from a high signal-to-noise and high spectral resolution VLT-UVES
spectrum. The overall metallicity of the system is [Zn/H] = -1.10 +/- 0.10
relative to solar. Two molecular hydrogen absorption components are seen at z =
3.28667 and 3.28742 (a velocity separation of \approx 52 km s-1) in rotational
levels up to J = 3. We derive a total H2 column density of log N(H2) (cm-2) =
18.66 and a mean molecular fraction of f = 2N(H2)/[2N(H2) + N(H i)] =
10-3.04+/-0.37, typical of known H2-bearing DLA systems. From the observed
abundance ratios we conclude that dust is present in the Interstellar Medium
(ISM) of this galaxy, with a enhanced abundance in the H2-bearing clouds.
However, the total amount of dust along the line of sight is not large and does
not produce any significant reddening of the background QSO. The physical
conditions in the H2-bearing clouds are constrained directly from the column
densities of H2 in different rotational levels, C i and C i^* . The kinetic
temperature is found to be T = 75 K and the particle density lies in the range
nH = 50-80 cm-3 . The neutral hydrogen column density of this DLA is similar to
the mean H i column density of DLAs observed at the redshift of {\gamma}-ray
bursts (GRBs). We explore the relationship between GRB-DLAs and high column
density end of QSO-DLAs finding that the properties (metallicity and depletion)
of DLAs with log N(H i) > 21.5 in the two populations do not appear to be
significantly different
ISM studies of GRB 030329 with high resolution spectroscopy
We present a series of early UVES/VLT high resolution spectra of the
afterglow of GRB 030329 at redshift z=0.16867+-0.00001. In contrast to other
spectra from this burst, both emission and absorption lines were detected. None
of them showed any temporal evolution. From the emission lines, we determine
the properties of the host galaxy which has a star formation rate (SFR) of
0.198 M_solar yr^-1 and a low metallicity of 1/7 Z_solar. Given the low total
stellar host mass M_star=10^7.75+-0.15 M_solar and an absolute luminosity
m_V=-16.37, we derive specific SFRs (SSFR) of log SFR/M = -8.5 yr^-1 and SFR/L
= 14.1 M_solar yr^-1 L_*^-1. This fits well into the picture of GRB hosts as
being low mass, low metallicity, actively star forming galaxies. The MgII and
MgI absorption lines from the host show multiple narrow (Doppler width b=5-10
km/s) components spanning a range of v about 260 km/s, mainly blueshifted
compared to the redshift from the emission lines. These components are likely
probing outflowing material of the host galaxy, which could arise from former
galactic superwinds, driven by supernovae from star forming regions. Similar
features have been observed in QSO spectra. The outflowing material is mainly
neutral with high column densities of log N(MgII)=14.0+-0.1 cm^-2 and log
N(MgI)=12.3+-0.1 cm^-2.Comment: 11 pages, 4 figures, submitted to Ap
A High Signal-to-Noise Ratio Composite Spectrum of Gamma-ray Burst Afterglows
We present a composite spectrum of 60 long duration gamma-ray burst (GRB)
afterglows with redshifts in the range 0.35<z<6.7 observed with low resolution
optical spectra. The composite spectrum covers the wavelength range 700-6600 A
in the rest frame and has a mean signal-to-noise ratio of 150 per 1 A pixel and
reaches a maximum of ~300 in the range 2500-3500 A. Equivalent widths are
measured from metal absorption lines from the Lya line to ~5200 A, and
associated metal and hydrogen lines are identified between the Lyman break and
Lya line. The average transmission within the Lyman forest is consistent with
that found along quasar lines of sight. We find a temporal variation in fine
structure lines when dividing the sample into bursts observed within 2 hours
from their trigger and those observed later. Other lines in the predominantly
neutral gas show variations too, but this is most likely a random effect caused
by weighting of individual strong absorption lines and which mimics a temporal
variation. Bursts characterized with high or low prompt GRB energy release
produce afterglows with similar absorption line strengths, and likewise for
bursts with bright or faint optical afterglows. Bursts defined as dark from
their optical to X-ray spectral index have stronger absorption lines relative
to the optically bright bursts. The composite spectrum has strong CaII and MgII
absorption lines as commonly found in dusty galaxies, however, we find no
evidence for dust or a significant molecular content based on the non-detection
of diffuse interstellar bands. Compared to starburst galaxy spectra, the GRB
composite has much stronger fine structure lines, while metal absorption lines
are weaker.Comment: Accepted for publication in ApJ, 24 page
The metallicity properties of simulated long-GRB galaxy hosts and the Fundamental Metallicity Relation
We study the implication of the collapsar model for Long Gamma-Ray Bursts
(LGRBs) on the metallicity properties of the host galaxies, by combining
high-resolution N-body simulations with semi-analytic models of galaxy
formation. The cosmological model that we use reproduces the Fundamental
Metallicity Relation recently discovered for the SDSS galaxies, whereby the
metallicity decreases with increasing Star Formation Rate for galaxies of a
given stellar mass. We select host galaxies housing pockets of gas-particles,
young and with different thresholds in metallicities, that can be sites of LRGB
events, according to the collapsar model. The simulated samples are compared
with 18 observed LGRB hosts in the aim at discriminating whether the
metallicity is a primary parameter. We find that a threshold in metallicity for
the LGRB progenitors, within the model galaxies, is not necessary in order to
reproduce the observed distribution of host metallicities. The low
metallicities of observed LGRB hosts is a consequence of the high star
formation environment. The star formation rate appears to be the primary
parameter to generate a burst event. Finally, we show that only a few LGRBs are
observed in massive, highly extincted galaxies, while these galaxies are
expected to produce many such events. We identify these missing events with the
fraction of dark LGRBs.Comment: 9 pages, 5 figures, submitted MNRA
The Redshift Evolution of the Metagalactic Ionizing Flux Inferred from Metal Line Ratios in the Lyman Forest
Metal line ratios in a sample of 13 quasar spectra obtained with the HIRES
spectrograph on the KeckI telescope have been analyzed to characterize the
evolution of the metagalactic ionizing flux near a redshift of 3. The evolution
of SiIV/CIV has been determined using three different techniques: using total
column densities of absorption line complexes, as in Songaila & Cowie (1996);
using the column densities of individual Voigt profile components within
complexes; and using direct optical depth ratios. All three methods show that
SiIV/CIV changes abruptly at a redshift near 3, requiring a jump in value of
about a factor of 3.4, and indicating a significant change in the ionizing
spectrum that occurs rapidly between z = 2.9 and z = 3, just above the redshift
at which Reimers et al. (1997) detected patchy HeII Lyman alpha absorption. At
lower redshifts, the ionization balance is consistent with a pure power law
ionizing spectrum but at higher redshifts the spectrum must be very soft, with
a large break at the He+ edge. An optical depth ratio technique is used to
measure the abundances of ions whose transitions lie within the forest and
CIII, SiIII and OVI are detected in this way. The presence of a significant
amount of OVI at z > 3 suggests either a considerable volume of HeIII bubbles
embedded in the more general region where the ionizing flux is heavily broken,
or the addition of collisional ionization to the simple photoionization models.Comment: 51 pages including 21 encapsulated postscript figures. Full version,
including complete Figure 5, available at
http://www.ifa.hawaii.edu/~acowie/meta_flux.html To be published in the June,
1998 Astronomical Journal (accepted February 18, 1998
Dust-to-metal ratios in damped Lyman-alpha absorbers: Fresh clues to the origins of dust and optical extinction towards gamma-ray bursts
Motivated by the anomalous dust-to-metal ratios derived in the literature for
gamma-ray burst (GRB) damped Lyman-alpha absorbers (DLAs), we measure these
ratios using the dust-depletion pattern observed in UV/optical afterglow
spectra associated with the ISM at the GRB host-galaxy redshifts. Our sample
consists of 20 GRB absorbers and a comparison sample of 72 QSO-DLAs with
redshift 1.2 < z < 4.0 and down to Z = 0.002 Z_Sol metallicities. The
dust-to-metal ratio in QSO- and GRB-DLAs increases both with metallicity and
metal column density, spanning ~10--110% of the Galactic value and pointing to
a non universal dust-to-metal ratio. The low values of dust-to-metal ratio
suggest that low-metallicity systems have lower dust fractions than typical
spiral galaxies and perhaps that the dust in these systems is produced
inefficiently, i.e. by grain growth in the low-metallicity regime with
negligible contribution from supernovae (SNe) and asymptotic giant branch (AGB)
stars. On the other hand, some GRB- and QSO-DLAs show high dust-to-metal ratio
values out to z ~ 4, requiring rapid dust production, such as in SN ejecta, but
also in AGB winds and via grain growth for the highest metallicity systems.
GRB-DLAs overall follow the dust-to-metal-ratio properties of QSO-DLAs, GRBs
probing up to larger column and volume densities. For comparison, the
dust-to-metal ratio that we derive for the SMC and LMC are ~82--100% and ~98%
of the Galactic value, respectively. The literature dust-to-metal ratio of the
low-metallicity galaxy I Zw 18 (< 37%) is consistent with the distribution that
we find. The dust extinction Av increases steeply with the column density of
iron in dust, N(Fe)dust, calculated from relative metal abundances, confirming
that dust extinction is mostly occurring in the host galaxy ISM. Most GRB-DLAs
display log N(Fe)dust > 14.7, above which several QSO-DLAs reveal H2
(abridged).Comment: 14 pages, 9 figures. A&A, in pres
A Methodology and Simulation-Based Toolchain for Estimating Deployment Performance of Smart Collective Services at the Edge
Research trends are pushing artificial intelligence (AI) across the Internet of Things (IoT)-edge-fog-cloud continuum to enable effective data analytics, decision making, as well as the efficient use of resources for QoS targets. Approaches for collective adaptive systems (CASs) engineering, such as aggregate computing, provide declarative programming models and tools for dealing with the uncertainty and the complexity that may arise from scale, heterogeneity, and dynamicity. Crucially, aggregate computing architecture allows for 'pulverization': applications can be decomposed into many deployable micromodules that can be spread across the ICT infrastructure, thus allowing multiple potential deployment configurations for the same application logic. This article studies the deployment architecture of aggregate-based edge services and its implications in terms of performance and cost. The goal is to provide methodological guidelines and a model-based toolchain for the generation and simulation-based evaluation of potential deployments. First, we address this subject methodologically by proposing an approach based on deployment code generators and a simulation phase whose obtained solutions are assessed with respect to their performance and costs. We then tailor this approach to aggregate computing applications deployed onto an IoT-edge-fog-cloud infrastructure, and we develop a corresponding toolchain based on Protelis and EdgeCloudSim. Finally, we evaluate the approach and tools through a case study of edge multimedia streaming, where the edge ecosystem exhibits intelligence by self-organizing into clusters to promote load balancing in large-scale dynamic settings
The X-ray absorbing column densities of Swift Gamma-ray bursts
Long gamma-ray bursts (GRBs) are associated with the explosion of massive
stars in star forming regions. A large fraction of GRBs show intrinsic
absorption as detected in optical spectra but absorption signatures are also
detectable in afterglow X-ray spectra. We present here a comprehensive analysis
the full sample of 93 GRBs with known redshift promptly observed by Swift XRT
up to June 2009. The distribution of X-ray column densities clearly shows that
GRBs are heavily absorbed indicating that they indeed occur in dense
environments. Furthermore, there is a lack of heavily absorbed GRBs at low
redshift (z<1-2) that might therefore be candidates for the missing `dark' GRB
population. However, there is no statistically significant correlation between
the amount of X-ray absorption and the `darkness' of a GRB. Finally, we compare
the hydrogen column densities derived in the optical with those derived from
X-ray absorption. The two distributions are different, with the optical column
densities being lower than the X-ray ones, which is even more apparent when
correcting for metallicity effects. The most likely explanation is
photoionization of hydrogen in the circumburst material caused by the radiation
field of the burst.Comment: Accepted for publication in MNRAS (7 pages, 4 figures
GRBs as Cosmological Probes - Cosmic Chemical Evolution
Long-duration gamma-ray bursts (GRBs) are associated with the death of
metal-poor massive stars. Even though they are highly transient events very
hard to localize, they are so bright that they can be detected in the most
difficult environments. GRB observations are unveiling a surprising view of the
chemical state of the distant universe (redshifts z > 2). Contrary to what is
expected for a high-z metal-poor star, the neutral interstellar medium (ISM)
around GRBs is not metal poor (metallicities vary from ~1/10 solar at z = 6.3
to about solar at z = 2) and is enriched with dust (90-99% of iron is in solid
form). If these metallicities are combined with those measured in the warm ISM
of GRB host galaxies at z < 1, a redshift evolution is observed. Such an
evolution predicts that the stellar masses of the hosts are in the range M* =
10^(8.6-9.8) Msun. This prediction makes use of the mass-metallicity relation
(and its redshift evolution) observed in normal star-forming galaxies.
Independent measurements coming from the optical-NIR photometry of GRB hosts
indicate the same range of stellar masses, with a typical value similar to that
of the Large Magellanic Cloud. This newly detected population of
intermediate-mass galaxies is very hard to find at high redshift using
conventional astronomy. However, it offers a compelling and relatively
inexpensive opportunity to explore galaxy formation and cosmic chemical
evolution beyond known borders, from the primordial universe to the present.Comment: Review article to be published in New Journal of Physics
(http://www.njp.org), Focus Issue on Gamma Ray Burst
- …