760 research outputs found
Time Delay Measurements for the Cluster-lensed Sextuple Quasar SDSS J2222+2745
We report first results from an ongoing monitoring campaign to measure time
delays between the six images of the quasar SDSS\,J22222745, gravitationally
lensed by a galaxy cluster. The time delay between A and B, the two most highly
magnified images, is measured to be days (95\%
confidence interval), consistent with previous model predictions for this lens
system. The strong intrinsic variability of the quasar also allows us to derive
a time delay value of days between image C and A,
in spite of modest overlap between their light curves in the current data set.
Image C, which is predicted to lead all the other lensed quasar images, has
undergone a sharp, monotonic flux increase of 60-75\% during 2014. A
corresponding brightening is firmly predicted to occur in images A and B during
2016. The amplitude of this rise indicates that time delays involving all six
known images in this system, including those of the demagnified central images
D-F, will be obtainable from further ground-based monitoring of this system
during the next few years.Comment: 9 pages, 9 figures, Version accepted for publication in Ap
On the lack of correlation between Mg II 2796, 2803 Angstrom and Lyman alpha emission in lensed star-forming galaxies
We examine the Mg II 2796, 2803 Angstrom, Lyman alpha, and nebular line
emission in five bright star-forming galaxies at 1.66<z<1.91 that have been
gravitationally lensed by foreground galaxy clusters. All five galaxies show
prominent Mg II emission and absorption in a P Cygni profile. We find no
correlation between the equivalent widths of Mg II and Lyman alpha emission.
The Mg II emission has a broader range of velocities than do the nebular
emission line profiles; the Mg II emission is redshifted with respect to
systemic by 100 to 200 km/s. When present, Lyman alpha is even more redshifted.
The reddest components of Mg II and Lyman alpha emission have tails to 500-600
km/s, implying a strong outflow. The lack of correlation in the Mg II and Lyman
alpha equivalent widths, the differing velocity profiles, and the high ratios
of Mg II to nebular line fluxes together suggest that the bulk of Mg II
emission does not ultimately arise as nebular line emission, but may instead be
reprocessed stellar continuum emission.Comment: The Astrophysical Journal, in press. 6 pages, 2 figure
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
In-situ X-ray Video Microscopy Studies of Al-Si Eutectic Solidification
In-situ studies of Al-Si eutectic growth has been carried out for the first time by X-ray video microscopy during directional solidification of Al-Si-Cu alloys with and without Sr-addtions. The unmodified eutectics showed distinctive non-isothermal growth dynamics, where Si-crystals attained needle-like tip morphologies and progressed under significantly higher undercooling than Al, leading to formation of an irregular eutectic with Si as the leading phase and subsequent nucleation of Al on the Si-surfaces. In the Sr-modified alloys, the eutectic reaction was found to be strongly suppressed, occurring with low nucleation frequencies at undercoolings in the range 10-18 K. In the Cu-enriched melt, the eutectic front was found to attain meso-scale interface perturbations evolving into equiaxed cellular rosettes in order to accommodate to the long-range redistribution of Cu from the composite eutectic interface. The eutectic front also attained short-range microscale interface perturbations consistent with characteristics of a fibrous Si growth, however further improvements in spatial resolution is required in order to study microscale structure formation in greater detail. Evidence was found in support of Si-nucleation occurring on potent particles suspended in the melt. Yet, both with Sr- modified and unmodified alloys, Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form on the Si-particles. To what extent nucleation mechanisms in the Cu-enriched systems are transferable to binary or commercial Al-Si alloys remains uncertain
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
The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas (MEGaSaURA) I: The Sample and the Spectra
We introduce Project MEGaSaURA: The Magellan Evolution of Galaxies
Spectroscopic and Ultraviolet Reference Atlas. MEGaSaURA comprises
medium-resolution, rest-frame ultraviolet spectroscopy of N=15 bright
gravitationally lensed galaxies at redshifts of 1.68z3.6, obtained with
the MagE spectrograph on the Magellan telescopes. The spectra cover the
observed-frame wavelength range \AA ; the average
spectral resolving power is R=3300. The median spectrum has a signal-to-noise
ratio of per resolution element at 5000 \AA . As such, the MEGaSaURA
spectra have superior signal-to-noise-ratio and wavelength coverage compared to
what COS/HST provides for starburst galaxies in the local universe. This paper
describes the sample, the observations, and the data reduction. We compare the
measured redshifts for the stars, the ionized gas as traced by nebular lines,
and the neutral gas as traced by absorption lines; we find the expected bulk
outflow of the neutral gas, and no systemic offset between the redshifts
measured from nebular lines and the redshifts measured from the stellar
continuum. We provide the MEGaSaURA spectra to the astronomical community
through a data release.Comment: Resubmitted to AAS Journals. Data release will accompany journal
publication. v2 addresses minor comments from refere
Spatially Resolved Outflows in a Seyfert Galaxy at z = 2.39
We present the first spatially resolved analysis of rest-frame optical and UV
imaging and spectroscopy for a lensed galaxy at z = 2.39 hosting a Seyfert
active galactic nucleus (AGN). Proximity to a natural guide star has enabled
high signal-to-noise VLT SINFONI + adaptive optics observations of rest-frame
optical diagnostic emission lines, which exhibit an underlying broad component
with FWHM ~ 700 km/s in both the Balmer and forbidden lines. Measured line
ratios place the outflow robustly in the region of the ionization diagnostic
diagrams associated with AGN. This unique opportunity - combining gravitational
lensing, AO guiding, redshift, and AGN activity - allows for a magnified view
of two main tracers of the physical conditions and structure of the
interstellar medium in a star-forming galaxy hosting a weak AGN at cosmic noon.
By analyzing the spatial extent and morphology of the Ly-alpha and
dust-corrected H-alpha emission, disentangling the effects of star formation
and AGN ionization on each tracer, and comparing the AGN induced mass outflow
rate to the host star formation rate, we find that the AGN does not
significantly impact the star formation within its host galaxy.Comment: 16 pages, 5 figures, accepted for publication in Ap
C III] Emission in Star-Forming Galaxies Near and Far
We measure C III Lambda Lambda 1907, 1909 Angstrom emission lines in eleven gravitationally-lensed star-forming galaxies at zeta at approximately 1.6-3, finding much lower equivalent widths than previously reported for fainter lensed galaxies (Stark et al. 2014). While it is not yet clear what causes some galaxies to be strong C III] emitters, C III] emission is not a universal property of distant star-forming galaxies. We also examine C III] emission in 46 star-forming galaxies in the local universe, using archival spectra from GHRS, FOS, and STIS on HST, and IUE. Twenty percent of these local galaxies show strong C III] emission, with equivalent widths less than 5 Angstrom. Three nearby galaxies show C III] emission equivalent widths as large as the most extreme emitters yet observed in the distant universe; all three are Wolf-Rayet galaxies. At all redshifts, strong C III] emission may pick out low-metallicity galaxies experiencing intense bursts of star formation. Such local C III] emitters may shed light on the conditions of star formation in certain extreme high-redshift galaxies
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