921 research outputs found
Constraining the metallicities, ages, star formation histories, and ionizing continua of extragalactic massive star populations
We infer the properties of massive star populations using the far-ultraviolet
stellar continua of 61 star-forming galaxies: 42 at low-z observed with HST and
19 at z~2 from the Megasaura sample. We fit each stellar continuum with a
linear combination of up to 50 single age and single metallicity Starburst99
models. From these fits, we derive light-weighted ages and metallicities, which
agree with stellar wind and photospheric spectral features, and infer the
spectral shapes and strengths of the ionizing continua. Inferred light-weighted
stellar metallicities span 0.05-1.5 Z and are similar to the measured
nebular metallicities. We quantify the ionizing continua using the ratio of the
ionizing flux at 900\AA\ to the non-ionizing flux at 1500\AA\ and demonstrate
the evolution of this ratio with stellar age and metallicity using theoretical
single burst models. These single burst models only match the inferred ionizing
continua of half of the sample, while the other half are described by a mixture
of stellar ages. Mixed age populations produce stronger and harder ionizing
spectra than continuous star formation histories, but, contrary to previous
studies that assume constant star formation, have similar stellar and nebular
metallicities. Stellar population age and metallicity affect the far-UV
continua in different and distinguishable ways; assuming a constant star
formation history diminishes the diagnostic power. Finally, we provide simple
prescriptions to determine the ionizing photon production efficiency
() from the stellar population properties. has a range
of log( Hz erg that depends on stellar age,
metallicity, star formation history, and contributions from binary star
evolution. These stellar population properties must be observationally
determined to determine the number of ionizing photons generated by massive
stars.Comment: 31 pages, 23 figures, resubmitted to ApJ after incorporating the
referee's comments. Comments encourage
Accurately predicting the escape fraction of ionizing photons using restframe ultraviolet absorption lines
The fraction of ionizing photons that escape high-redshift galaxies
sensitively determines whether galaxies reionized the early universe. However,
this escape fraction cannot be measured from high-redshift galaxies because the
opacity of the intergalactic medium is large at high redshifts. Without methods
to indirectly measure the escape fraction of high-redshift galaxies, it is
unlikely that we will know what reionized the universe. Here, we analyze the
far-ultraviolet (UV) H I (Lyman series) and low-ionization metal absorption
lines of nine low-redshift, confirmed Lyman continuum emitting galaxies. We use
the H I covering fractions, column densities, and dust attenuations measured in
a companion paper to predict the escape fraction of ionizing photons. We find
good agreement between the predicted and observed Lyman continuum escape
fractions (within ) using both the H I and ISM absorption lines. The
ionizing photons escape through holes in the H I, but we show that dust
attenuation reduces the fraction of photons that escape galaxies. This means
that the average high-redshift galaxy likely emits more ionizing photons than
low-redshift galaxies. Two other indirect methods accurately predict the escape
fractions: the Ly escape fraction and the optical [O III]/[O II] flux
ratio. We use these indirect methods to predict the escape fraction of a sample
of 21 galaxies with rest-frame UV spectra but without Lyman continuum
observations. Many of these galaxies have low escape fractions (\%), but 11 have escape fractions \%. The methods presented here will
measure the escape fractions of high-redshift galaxies, enabling future
telescopes to determine whether star-forming galaxies reionized the early
universe.Comment: Accepted for publication in A&A. 12 pages, 5 figure
Genetic diversity within and among Atlantic cod (Gadus morhua) farmed in marine cages: a proof-of-concept study for the identification of escapees
This study presents a molecular genetic characterization of Atlantic cod reared in commercial marine farms. Samples consisted of approximately 47 fish collected from nine cages located on four farms throughout Norway. In addition, 28 farmed escapees were recaptured in the sea (443 fish in total). Nine microsatellite loci and the Pan I gene were analysed, revealing a total of 181 alleles. Each sample contained 43–63% of total allelic variation. Comparing variation with published data for wild cod indicates that lower genetic variation exists within single cages than in wild populations. Significant linkage disequilibrium was observed amongst pairs of loci in all samples, suggesting a low number of contributing parental fish. Global FST was 0.049, and the highest pairwise FST value (pooled loci) was 0.085. For single loci, the Pan I gene was the most diagnostic, displaying a global FST of 0.203. Simulations amongst the samples collected on farms revealed an overall correct self-assignment percentage of 75%, demonstrating a high probability of identifying individuals to their farm of origin. Identification of the 28 escapees revealed a single cage as the most likely source of origin for half of the escapees, whilst the remaining fish were assigned to a mixture of samples, suggesting more than one source of escapees
Nucleation and Growth Dynamics of the α-Al / β-Al5FeSi Eutectic in a Complex Al-Si-Cu-Fe Alloy
Secondary-sourced recycled aluminium alloys can exhibit high levels of different impurities. It is well known that the presence of iron, the most common impurity, can lead to the formation of hard and brittle intermetallic phases which are detrimental to the machining properties and the mechanical behaviour of the material in service. The purpose of this work is to study the nucleation and growth of the β-Al5FeSi intermetallic phase in the framework of the eutectic reaction: Liquid → α-Al + β-Al5FeSi. In situ X-ray microtomography has been used to investigate the formation of the irregular eutectic β-phase plates during the solidification at low cooling rate of an Al-8Si-4Cu-0.8Fe alloy. The results show that only a few plates form, nucleating early near the sample surface. Next, growth occurs very rapidly in the principal growth direction and slowly in the thickness direction. The plates are highly branched and appear to form as a divorced eutectic, i.e. not coupled with the α-Al. These features are inconsistent with the common irregular eutectic solidification theory based on the Jackson and Hunt model
The Burst Cluster: Dark Matter in a Cluster Merger Associated with the Short Gamma Ray Burst, GRB 050509B
We have identified a merging galaxy cluster with evidence of two distinct
sub-clusters. The X-ray and optical data suggest that the subclusters are
moving away from each other after closest approach. This cluster merger was
discovered from observations of the well localized short-duration gamma-ray
burst (GRB), GRB 050509B. The Swift/Burst Alert Telescope (BAT) source position
is coincident with a cluster of galaxies ZwCl 1234.0+02916. The subsequent
Swift/X-Ray Telescope (XRT) localization of the X-ray afterglow found the GRB
coincident with 2MASX J12361286+2858580, a giant red elliptical galaxy in the
cluster. Deep multi-epoch optical images were obtained to constrain the
evolution of the GRB afterglow, including a 27480s exposure in the F814W band
with Hubble Space Telescope Advanced Camera for Surveys (ACS), among the
deepest imaging ever obtained towards a known galaxy cluster in a single
passband. We perform a weak gravitational lensing analysis, including mapping
the total mass distribution of the merger system. Combined with Chandra X-ray
Observatory and Swift/XRT observations, we investigate the dynamical state of
the merger to probe the nature of the dark matter component. Our weak
gravitational lensing measurements reveal a separation of the X-ray centroid of
the western subcluster from the center of the mass and galaxy light
distributions, similar to that of the famous "Bullet cluster". We conclude that
the "Burst cluster" is another candidate merger system for determining the
nature of dark matter and for studying the environment of short GRBs. We
discuss connections between the cluster dynamical state and/or matter
composition and compact object mergers, the leading model for the origin of
short GRBs. Finally, we present results from a weak lensing survey based on
archival Very Large Telescope (VLT) images in the areas of 5 other short GRBs.Comment: 17 pages, 7 figures, accepted by Ap
Separating the BL Lac and Cluster X-ray Emissions in Abell 689 with Chandra
We present the results of a Chandra observation of the galaxy cluster Abell
689 (z=0.279). Abell 689 is one of the most luminous clusters detected in the
ROSAT All Sky Survey (RASS), but was flagged as possibly including significant
point source contamination. The small PSF of the Chandra telescope allows us to
confirm this and separate the point source from the extended cluster X-ray
emission. For the cluster we determine a bolometric luminosity of
L_{bol}=(3.3+/-0.3)x10^{44} erg s-1 and a temperature of kT=5.1^{+2.2}_{-1.3}
keV when including a physically motivated background model. We compare our
measured luminosity for A689 to that quoted in the Rosat All Sky Survey (RASS)
and find L_{0.1-2.4,keV}=2.8x10^{44} erg s-1, a value \sim10 times lower than
the ROSAT measurement. Our analysis of the point source shows evidence for
significant pileup, with a pile-up fraction of ~60%. SDSS spectra and HST
images lead us to the conclusion that the point source within Abell 689 is a BL
Lac object. Using radio and optical observations from the VLA and HST archives,
we determine {\alpha}_{ro}=0.50, {\alpha}_{ox}=0.77 and {\alpha}_{rx}=0.58 for
the BL Lac, which would classify it as being of 'High-energy peak BL Lac' (HBL)
type. Spectra extracted of A689 show a hard X-ray excess at energies above 6
keV that we interpret as inverse Compton emission from aged electrons that may
have been transported into the cluster from the BL Lac.Comment: 11 pages, 15 figures, MNRAS in pres
Study of heterogeneous nucleation of eutectic Si in high-purity Al-Si alloys with Sr addition
The official published version can be accessed from the link below - Copyright @ 2010 The Minerals, Metals & Materials Society and ASM InternationalAl-5 wt pct Si master-alloys with controlled Sr and/or P addition/s were produced using super purity Al 99.99 wt pct and Si 99.999 wt pct materials in an arc melter. The master-alloy was melt-spun resulting in the production of thin ribbons. The Al matrix of the ribbons contained entrained Al-Si eutectic droplets that were subsequently investigated. Differential scanning calorimetry, thermodynamic calculations, and transmission electron microscopy techniques were employed to examine the effect of the Sr and P additions on eutectic undercoolings and nucleation phenomenon. Results indicate that, unlike P, Sr does not promote nucleation. Increasing Sr additions depressed the eutectic nucleation temperature. This may be a result of the formation of a Sr phase that could consume or detrimentally affect potent AlP nucleation sites.This work is financially supported by the
Higher Education Commission of Pakistan and managerially supported from the OAD
The evolution of substructure II: linking dynamics to environment
We present results from a series of high-resolution N-body simulations that
focus on the formation and evolution of eight dark matter halos, each of order
a million particles within the virial radius. We follow the time evolution of
hundreds of satellite galaxies with unprecedented time resolution, relating
their physical properties to the differing halo environmental conditions. The
self-consistent cosmological framework in which our analysis was undertaken
allows us to explore satellite disruption within live host potentials, a
natural complement to earlier work conducted within static potentials. Our host
halos were chosen to sample a variety of formation histories, ages, and
triaxialities; despite their obvious differences, we find striking similarities
within the associated substructure populations. Namely, all satellite orbits
follow nearly the same eccentricity distribution with a correlation between
eccentricity and pericentre. We also find that the destruction rate of the
substructure population is nearly independent of the mass, age, and triaxiality
of the host halo. There are, however, subtle differences in the velocity
anisotropy of the satellite distribution. We find that the local velocity bias
at all radii is greater than unity for all halos and this increases as we move
closer to the halo centre, where it varies from 1.1 to 1.4. For the global
velocity bias we find a small but slightly positive bias, although when we
restrict the global velocity bias calculation to satellites that have had at
least one orbit, the bias is essentially removed.Comment: 14 pages, 14 figures, MNRAS in pres
Annihilation of NMSSM neutralinos in the Sun and neutrino telescope limits
We investigate neutralino dark matter in the framework of NMSSM performing a
scan over its parameter space and calculating neutralino capture and
annihilation rates in the Sun. We discuss the prospects of searches for
neutralino dark matter in neutrino experiments depending on neutralino content
and its main annihilation channel. We recalculate the upper limits on
neutralino-proton elastic cross sections directly from neutrino telescopes
upper bounds on annihilation rates in the Sun. This procedure has advantages as
compared with corresponding recalcalations from the limits on muon flux,
namely, it is independent on details of the experiment and the recalculation
coefficients are universal for any kind of WIMP dark matter models. We derive
90% c.l. upper limits on neutralino-proton cross sections from the results of
the Baksan Underground Scintillator Telescope.Comment: 28 pages, 16 figures, accepted for publication in JCAP, references
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