279 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
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
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
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
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
GRB 071028B, a burst behind large amounts of dust in an unabsorbed galaxy
We report on the discovery and properties of the fading afterglow and
underlying host galaxy of GRB 071028B, thereby facilitating a detailed
comparison between these two. Observations were performed with the Gamma-ray
Burst Optical and Near-infrared Detector at the 2.2 m telescope on the La Silla
Paranal Observatory in Chile. We conducted five observations from 1.9 d to
227.2 d after the trigger and obtained deep images in the g'r'i'z' and JHKs
bands. Based on accurate seven-channel photometry covering the optical to
near-infrared wavelength range, we derive a photometric redshift of z = 0.94
+0.05 -0.10 for the unabsorbed host galaxy of GRB 071028B. In contrast, we show
that the afterglow with an intrinsic extinction of AV(SB) = (0.70 +/- 0.11) mag
is moderately absorbed and requires a relatively flat extinction curve.
According to the reported Swift/BAT observations, the energetics yield an
isotropic energy release of E(gamma,iso.,rest) = (1.4 +2.4 -0.7) x 10^51 erg.Comment: 8 pages, 5 figures, accepted for publication in A&
UVES/VLT high resolution absorption spectroscopy of the GRB080330 afterglow: a study of the GRB host galaxy and intervening absorbers
We study the Gamma Ray Burst (GRB) environment and intervening absorbers by
analyzing the optical absorption features produced by gas surrounding the GRB
or along its line of sight. We analyzed high resolution spectroscopic
observations (R=40000, S/N=3 - 6) of the optical afterglow of GRB080330, taken
with UVES at the VLT ~ 1.5 hours after the GRB trigger. The spectrum
illustrates the complexity of the ISM of the GRB host galaxy at z = 1.51 which
has at least four components in the main absorption system. We detect strong
FeII, SiII, and NiII excited absorption lines associated with the bluemost
component only. In addition to the host galaxy, at least two more absorbers
lying along the line of sight to the afterglow have been detected in the
redshift range 0.8 < z < 1.1, each exhibiting MgII absorption. For the bluemost
component in the host galaxy, we derive information about its distance from the
site of the GRB explosion. We do so by assuming that the excited absorption
lines are produced by indirect UV pumping, and compare the data with a time
dependent photo-excitation code. The distance of this component is found to be
280+40-50 pc, which is lower than found for other GRBs (1 - 6 kpc). We identify
two additional MgII absorbers, one of them with a rest frame equivalent width
larger than 1A. The distance between the GRB and the absorber measured in this
paper confirms that the power of the GRB radiation can influence the conditions
of the interstellar medium up to a distance of at least several hundred pc. For
the intervening absorbers, we confirm the trend that on average one strong
intervening system is found per afterglow, as has been noted in studies
exhibiting an excess of strong MgII absorbers along GRB sightlines compared to
quasars.Comment: 8 Pages, 7 ps figures, A&A in pres
The Gemini Deep Deep Survey: II. Metals in Star-Forming Galaxies at Redshift 1.3<z<2
The goal of the Gemini Deep Deep Survey (GDDS) is to study an unbiased sample
of K<20.6 galaxies in the redshift range 0.8<z<2.0. Here we determine the
statistical properties of the heavy element enrichment in the interstellar
medium (ISM) of a subsample of 13 galaxies with 1.34<z<1.97 and UV absolute
magnitude M_2000 < -19.65. The sample contains 38% of the total number of
identified galaxies in the first two fields of the survey with z>1.3. The
selected objects have colors typical of irregular and Sbc galaxies. Strong
[OII] emission indicates high star formation activity in the HII regions
(SFR~13-106 M_sun/yr). The high S/N composite spectrum shows strong ISM MgII
and FeII absorption, together with weak MnII and MgI lines. The FeII column
density, derived using the curve of growth analysis, is logN_FeII =
15.54^{+0.23}_{-0.13}. This is considerably larger than typical values found in
damped Ly-alpha systems (DLAs) along QSO sight lines, where only 10 out of 87
(~11%) have logN_FeII > 15.2. High FeII column densities are observed in the
z=2.72 Lyman break galaxy cB58 (logN_FeII ~ 15.25) and in gamma-ray burst host
galaxies (logN_FeII ~ 14.8-15.9). Given our measured FeII column density and
assuming a moderate iron dust depletion (delta_Fe ~ 1 dex), we derive an
optical dust extinction A_V ~ 0.6. If the HI column density is log N(HI)<21.7
(as in 98% of DLAs), then the mean metallicity is Z/Z_sun > 0.2. The high
completeness of the GDDS sample implies that these results are typical of
star-forming galaxies in the 1<z<2 redshift range, an epoch which has
heretofore been particularly challenging for observational programs.Comment: ApJ in press, corrected HI column density estimat
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