1606.04947
Models of low-mass helium white dwarfs including gravitational settling, thermal and chemical diffusion, and rotational mixing
Istrate, et al
A large number of extremely low-mass He white dwarfs (ELM WDs) have been discovered n recent years. The majority of them are found in close binary systems suggesting they are formed either through a common-envelope phase or via stable mass transfer in a low-mass X-ray binary (LMXB) or a cataclysmic variable (CV) system. Here, investigate the formation of these objects through the LMXB channel with emphasis on the proto-WD evolution in environments with different metallicities. Study the combined effects of rotational mixing and element diffusion (e.g. gravitational settling, thermal and chemical diffusion) on the evolution of proto-WDs and on the cooling properties of the resulting WDs. Present state-of-the-art binary stellar evolution models computed with MESA for metallicities between Z=0.0002 and Z=0.02, producing WDs with masses between 0.16-0.45 Msun. The results confirm that element diffusion plays a significant role in the evolution of proto-WDs that experience H shell flashes. The occurrence of these flashes produces a clear dichotomy in the cooling timescales of ELM WDs, which has important consequences e.g. for the age determination of binary millisecond pulsars. Rotational mixing is found to counteract the effect of gravitational settling in the surface layers of young, bloated ELM proto-WDs and therefore plays a key role in determining their surface chemical abundances. Predict that these proto-WDs have He-rich envelopes through a significant part of their lifetime, a crucial ingredient for understanding the newly observed ELM proto-WD pulsators. The H envelope at detachment, although small compared to the total mass of the WD, contains enough angular momentum such that the spin frequency of the resulting WD on the cooling track is well above the orbital frequency.
1606.05309
Absorption line spectroscopy of gravitationally-lensed galaxies: further evidence for an increased escape fraction of ionizing photons at high redshift
Leethochawalit et al
The fraction of ionizing photos that escape from high z SF galaxies remains a key obstacle in evaluating whether galaxies were the primary agents of cosmic reionization. In an earlier work, proposed using the covering fraction of low ionization gas, measured via deep absorption line spectroscopy, as a proxy. Present a significant update using this method, sampling seven gravitationally-lensed sources in the redshift range 4<z<6. Show the absorbing gas in the sources is spatially inhomogeneous with a median covering fraction of 66%. Correcting for reddening according to a dust-in-cloud model, estimate this implies an absolute escape fraction of ~19±6%. Recognizing this is higher than independent estimates based on recombination rate suites of the IGM from QSO absorption lines, quantify possible biases and uncertainties. Collectively find the average escape fraction could be reduced to no less than 11%, excluding the effect of spatial variations. For one of the lensed sources, have sufficient S/N to demonstrate the presence of such spatial variations as well as scatter in its dependence on the Lya equivalent width consistent with recent simulations. If this source is typical, the lower limit to the escape fraction could be reduced by a further factor ~2. Across the sample, find a modest anti-correlation between the inferred escape fraction and the local star formation rate consistent with a time delay between a burst and leaking LyC photons. The analysis demonstrates considerable variations in the proportion of ionizing photons that can escape consistent with being governed by the small scale behavior of star-forming regions which fluctuate in their activities over short timescales. This supports the suggestion that the escape fraction may increase toward the reionization era when star formation becomes more energetic and burst-like.
1606.05337
Calibration of weak-lensing shear in the Kilo-Degree Survey
Fenech Conti, Herbonnet, Hoekstra, Merten, Miller, Viola
Describe and test the pipeline used to measure the WL shear signal from KiDS. It includes a novel method of 'self-calibration' that partially corrects for the effect of noise bias. Also discuss the 'weight bias' that may arise in optimally-weighted measurements, and present a scheme to mitigate that bias. To study the residual biases arising from both galaxy selection and shear measurement, and to derive an empirical correction to reduce the shear biases to <~1%, create a suite of simulated images whose properties are close to those of the KIDS surveys observations. Find that the use of 'self-calibration' reduces the additive and multiplicative shear biases significantly, although further correction via a calibration scheme is required, which also corrects for a dependence of the bias on galaxy properties. Find that the calibration relation itself is biased by the use of noisy, measured galaxy properties, which may limit the final accuracy that can be achieved. Assess the accuracy of the calibration in the tomographic bins used for the KiDS cosmic shear analysis, testing in particular the effect of possible variations in the uncertain distributions of galaxy size, magnitude and ellipticity, and conclude that the calibration procedure is accurate at the level of multiplicative bias <~1% required for the KiDS cosmic shear analysis.
1606.05338
KiDS-450: Cosmological parameter constraints from tomographic weak gravitational lensing
Hildebrandt, Viola, Heymans, Joudaki, Kuijken, et al
Present cosmological parameter constraints from a tomographic WL analysis of ~450 deg^2 of imaging data from KiDS. For a flat LCDM cosmo with a prior on H0 that encompasses the most recent direct measurements, find S8=sigma_8 (Omega_m/0.3)^{1/2} = 0.745±0.039. This result is in good agreement with other low z probes of large scale structure, including recent cosmic shear results, along with pre-Planck cosmic microwave background constraints. A 2.3-sigma tension in S8 and 'substantial discordance' in the full parameter space is found with respect to the Planck 2015 results. Use shear measurements for nearly 15M galaxies, determined with a new improved 'self-calibrating' version of lensfit validated using an extensive suite of image simulations. Four-band ugri photometric redshifts are calibrated directly with deep spectroscopic surveys. The redshift calibration is confirmed using two independent techniques based on angular cross-correlations and the properties of the photometric redshift probability distributions. The covariance matrix is determined using an analytical approach, verified numerically with large mock galaxy catalogues. Account for uncertainties in the modeling of intrinsic galaxy alignments and their impact of baryon feedback on the shape of the non-linear matter power spectrum, in addition to the small residual uncertainties in the shear and redshift calibration. The cosmology analysis was performed blind. The high-level data products, including shear correlation functions, covariance matrices, redshift distributions, and MCMC chains are available at kids.strw.leidenuniv.nl.
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