Constraining the contribution of active galactic nuclei to reionization

Recent results have suggested that active galactic nuclei (AGN) could provide enough photons to reionise the Universe. We assess the viability of this scenario using a semi-numerical framework for modeling reionisation, to which we add a quasar contribution by constructing a Quasar Halo Occupation Distribution (QHOD) based on Giallongo et al. observations. Assuming a constant QHOD, we find that an AGN-only model cannot simultaneously match observations of the optical depth $\tau_e$, neutral fraction, and ionising emissivity. Such a model predicts $\tau_e$ too low by $\sim 2\sigma$ relative to Planck constraints, and reionises the Universe at $z\lesssim 5$. Arbitrarily increasing the AGN emissivity to match these results yields a strong mismatch with the observed ionising emissivity at $z\sim 5$. If we instead assume a redshift-independent AGN luminosity function yielding an emissivity evolution like that assumed in Madau & Haardt model, then we can match $\tau_e$ albeit with late reionisation, however such evolution is inconsistent with observations at $z\sim 4-6$ and poorly motivated physically. These results arise because AGN are more biased towards massive halos than typical reionising galaxies, resulting in stronger clustering and later formation times. AGN-dominated models produce larger ionising bubbles that are reflected in $\sim\times 2$ more 21cm power on all scales. A model with equal parts galaxies and AGN contribution is still (barely) consistent with observations, but could be distinguished using next-generation 21cm experiments HERA and SKA-low. We conclude that, even with recent claims of more faint AGN than previously thought, AGN are highly unlikely to dominate the ionising photon budget for reionisation.Comment: 16 pages, 9 figures, matches the accepted version for publication in MNRAS, 201

Epoch of reionization 21 cm forecasting from MCMC-constrained semi-numerical models

The recent low value of Planck (2016) integrated optical depth to Thomson scattering suggests that the reionization occurred fairly suddenly, disfavoring extended reionization scenarios. This will have a significant impact on the 21cm power spectrum. Using a semi-numerical framework, we improve our model from Hassan et al. (2016) to include time-integrated ionisation and recombination effects, and find that this leads to more sudden reionisation. It also yields larger HII bubbles which leads to an order of magnitude more 21cm power on large scales, while suppressing the small scale ionization power. Local fluctuations in the neutral hydrogen density play the dominant role in boosting the 21cm power spectrum on large scales, while recombinations are subdominant. We use a Monte Carlo Markov Chain approach to constrain our model to observations of the star formation rate functions at z = 6,7,8 from Bouwens et al. (2015), the Planck (2016) optical depth measurements, and the Becker & Bolton (2013) ionising emissivity data at z~5. We then use this constrained model to perform 21cm forecasting for LOFAR, HERA, and SKA in order to determine how well such data can characterise the sources driving reionisation. We find that the 21cm power spectrum alone can somewhat constrain the halo mass dependence of ionising sources, the photon escape fraction and ionising amplitude, but combining the 21cm data with other current observations enables us to separately constrain all these parameters. Our framework illustrates how 21cm data can play a key role in understanding the sources and topology of reionisation as observations improve.Comment: 20 pages, 16 figues, matches the accepted version for publication in MNRA

Austenization stasis in Fe-12Cr-0.1C martensitic stainless steel

Residual ferrite, a common sub-product of the austenization process in martensitic stainless steels (MSS), has serious detrimental effects on the mechanical properties of these alloys [1]. Due to its technological relevance, austenization is one of the most well-known phase transformation in material science. For high-Cr steels, a transformation in multiple stages is often reported. However, the mechanisms dictating the onset of the different transformation rates are not entirely clear [2]. Here, using both experimental and simulation techniques, we show that the austenization reaction in MSS occurs in three stages: (1) fast growth of austenite driven by Cr diffusion in ferrite and partial dissolution of M23C6, (2) soft-impingement and reaction stasis, (3) slow austenite growth driven by Cr homogenization in austenite. The moving boundary model in DICTRA is used to study the transformation. Based on experimental observations, austenite is set to nucleate from ferrite grain boundaries and to grow towards the M23C6 particle, which is initially embedded in the ferrite matrix. DICTRA calculations are in good agreement with dilatometric experimental data, which show the presence of residual ferrite even after prolonged holding time in the austenite temperature range. An analysis of the Cr profiles in the simulation domain shows that the transformation stasis is caused by soft-impingement between M23C6 /α and α/γ interfaces in the residual ferrite matrix. Next, the effect of heating rate and initial M23C6 particle size are investigated to optimize the process parameters. For heating rates greater than 1°C/s, simulations predict ferrite growth, which immediately follow the point of maximum austenite volume fraction. Based on thermodynamic considerations, this phenomenon is qualitatively explained with Thermo-Calc. Moreover, as the initial carbide size is reduced, the volume fraction of austenite transformed before soft-impingement increased. Finally, a set of process parameters are optimized with the objective function of minimizing time while maximizing the final volume fraction of austenite

Aktualisasi Semangat Perjuangan Bangsa Guna Memantapkan Kepemimpinan TNI Dalam Rangka Menjaga Keutuhan NKRI (Executive Summary)

29cm;18ha

Computer Simulation of DNA Double-helix Dynamics

this paper, the nature of the dynamics of the DNA double helix was investigated using the technique of molecular dynamics simulation, which proved so illuminating when used on globular proteins (McCaramon et al. 1977; Levitt 1981b). This technique simulates the movement of atoms in the static X-ray structure and thus provides information about the amplitudes and frequencies of vibrations and the type, rate, and pathway of conformational changes. Results are presented for simulations of room-temperature atomic motion of 12-bp and 24-bp DNA double helices for periods of more than 90 psec. The hydrogen bonds between base pairs are all found to be stable on this time scale, and the motions of the torsion angles are found to be of small amplitude ( e 10). The length fluctuations of adjacent hydrogen bonds in the same base pair are weakly correlated, whereas the torsion angles of each nucleotide show stronger correlations that agree with those seen in the static X-ray structures. Both DNA fragments show cooperative overall bending and twisting motions of large amplitude that do not involve any major perturbation of the DNA torsion angles. This smooth bending differs from that expected of an isotropic elastic rod in that (1) it is asymmetric, always acting to close the major groove of DNA, and (2) it consists predominantly of a normal mode that has a wavelength close to the helical repeat length. The stack of base pairs is also seen to kink into the minor groove. The extent of this global motion is consistent with nuclear magnetic resonance measurements and explains the observed sensitivity of DNA conformation to local environment. These calculations have implications for the way the DNA double helix may interact with repressors, polyroerases, and other cellular proteins (Ande..