The First Billion Years project - IV: Proto-galaxies reionising the Universe

The contribution of stars in galaxies to cosmic reionisation depends on the star formation history in the Universe, the abundance of galaxies during reionisation, the escape fraction of ionising photons and the clumping factor of the inter-galactic medium (IGM). We compute the star formation rate and clumping factor during reionisation in a cosmological volume using a high-resolution hydrodynamical simulation. We post-process the output with detailed radiative transfer simulations to compute the escape fraction of ionising photons. Together, this gives us the opportunity to assess the contribution of galaxies to reionisation self-consistently. The strong mass and redshift dependence of the escape fraction indicates that reionisation occurred between z=15 and z=10 and was mainly driven by proto-galaxies forming in dark-matter haloes with masses between 1e7 and 1e8 solar mass. More massive galaxies that are rare at these redshifts and have significantly lower escape fractions contribute less photons to the reionisation process than the more-abundant low-mass galaxies. Star formation in the low-mass haloes is suppressed by radiative feedback from reionisation, therefore these proto-galaxies only contribute when the part of the Universe they live in is still neutral. After z~10, massive galaxies become more abundant and provide most of the ionising photons. In addition, we find that Population (Pop) III stars are too short-lived and not frequent enough to have a major contribution to reionisation. Although the stellar component of the proto-galaxies that produce the bulk of ionising photons during reionisation is too faint to be detected by the James Webb Space Telescope (JWST), these sources are brightest in the H-alpha and Ly-alpha recombination lines, which will likely be detected by JWST in deep surveys.Comment: 5 pages, 4 figures, accepted for publication in MNRAS letter

Galactic wind X-ray heating of the intergalactic medium during the Epoch of Reionization

The diffuse soft X-ray emissivity from galactic winds is computed during the Epoch of Reionization (EoR). We consider two analytic models, a pressure-driven wind and a superbubble model, and a 3D cosmological simulation including gas dynamics from the First Billion Years (FiBY) project. The analytic models are normalized to match the diffuse X-ray emissivity of star-forming galaxies in the nearby Universe. The cosmological simulation uses physically motivated star formation and wind prescriptions, and includes radiative transfer corrections. The models and the simulation all are found to produce sufficient heating of the Intergalactic Medium to be detectable by current and planned radio facilities through 21 cm measurements during the EoR. While the analytic models predict a 21 cm emission signal relative to the Cosmic Microwave Background sets in by $z_{\rm trans} \simeq 8 - 10$, the predicted signal in the FiBY simulation remains in absorption until reionization completes. The 21 cm absorption differential brightness temperature reaches a minimum of $\Delta T \simeq -130$ to $-200$ mK, depending on model. Allowing for additional heat from high mass X-ray binaries pushes the transition to emission to $z_{\rm trans} \simeq 10 - 12$, with shallower absorption signatures having a minimum of $\Delta T \simeq -110$ to $-140$ mK. The 21 cm signal may be a means of distinguishing between the wind models, with the superbubble model favouring earlier reheating. While an early transition to emission may indicate X-ray binaries dominate the reheating, a transition to emission as early as $z_{\rm trans} > 12$ would suggest the presence of additional heat sources.Comment: 16 pages. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review. The version of record, MNRAS, 2017, volume 471, issue 3, is available online at: https://academic.oup.com/mnras/article/471/3/3632/4002694/Galactic-wind-X-ray-heating-of-the-intergalacti

PRISMA: A Novel Approach for Deriving Probabilistic Surrogate Safety Measures for Risk Evaluation

Surrogate Safety Measures (SSMs) are used to express road safety in terms of the safety risk in traffic conflicts. Typically, SSMs rely on assumptions regarding the future evolution of traffic participant trajectories to generate a measure of risk. As a result, they are only applicable in scenarios where those assumptions hold. To address this issue, we present a novel data-driven Probabilistic RISk Measure derivAtion (PRISMA) method. The PRISMA method is used to derive SSMs that can be used to calculate in real time the probability of a specific event (e.g., a crash). Because we adopt a data-driven approach to predict the possible future evolutions of traffic participant trajectories, less assumptions on these trajectories are needed. Since the PRISMA is not bound to specific assumptions, multiple SSMs for different types of scenarios can be derived. To calculate the probability of the specific event, the PRISMA method uses Monte Carlo simulations to estimate the occurrence probability of the specified event. We further introduce a statistical method that requires fewer simulations to estimate this probability. Combined with a regression model, this enables our derived SSMs to make real-time risk estimations. To illustrate the PRISMA method, an SSM is derived for risk evaluation during longitudinal traffic interactions. It is very difficult, if not impossible, to objectively compare the relative merits of two SSMs. Instead, we provide a method for benchmarking our derived SSM with respect to expected risk trends. The application of the benchmarking illustrates that the SSM matches the expected risk trends. Whereas the derived SSM shows the potential of the PRISMA method, future work involves applying the approach for other types of traffic conflicts, such as lateral traffic conflicts or interactions with vulnerable road users.Comment: 26 pages, 4 figures, 1 tabl

Conditions for Reionizing the Universe with A Low Galaxy Ionizing Photon Escape Fraction

We explore scenarios for reionizing the intergalactic medium with low galaxy ionizing photon escape fractions. We combine simulation-based halo-mass dependent escape fractions with an extrapolation of the observed galaxy rest-ultraviolet luminosity functions to solve for the reionization history from z=20 to z=4. We explore the posterior distributions for key unknown quantities, including the limiting halo mass for star-formation, the ionizing photon production efficiency, and a potential contribution from active galactic nuclei (AGN). We marginalize over the allowable parameter space using a Markov Chain Monte Carlo method, finding a solution which satisfies the most model-independent constraints on reionization. Our fiducial model can match observational constraints with an average escape fraction of <5% throughout the bulk of the epoch of reionization if: i) galaxies form stars down to the atomic cooling limit before reionization and a photosuppression mass of log(M_h/Msol)~9 during/after reionization (-13<M_UV,lim<-11); ii) galaxies become more efficient producers of ionizing photons at higher redshifts and fainter magnitudes, and iii) there is a significant, but sub-dominant, contribution by AGN at z -15) dominate the ionizing emissivity, leading to an earlier start to reionization and a smoother evolution of the ionized volume filling fraction than models which assume a single escape fraction at all redshifts and luminosities. The ionizing emissivity from this model is consistent with observations at z=4-5 (and below, when extrapolated), in contrast to some models which assume a single escape fraction. Our predicted ionized volume filling fraction at z=7 of Q_HII=78% (+\- 8%) is in ~1-2 sigma tension with observations of Lya emitters at z~7 and the damping wing analyses of the two known z>7 quasars, which prefer Q_HII,z=7~40-50%.Comment: 45 pages, 21 figures, accepted for publication in the Astrophysical Journa

The Spectral Evolution of the First Galaxies. III. Simulated James Webb Space Telescope Spectra of Reionization-epoch Galaxies with Lyman-continuum Leakage

Using four different suites of cosmological simulations, we generate synthetic spectra for galaxies with different Lyman continuum escape fractions (fesc) at redshifts z=7-9, in the rest-frame wavelength range relevant for the James Webb Space Telescope (JWST) NIRSpec instrument. By investigating the effects of realistic star formation histories and metallicity distributions on the EW(Hb)-beta diagram (previously proposed as a tool for identifying galaxies with very high fesc), we find that neither of these effects are likely to jeopardize the identification of galaxies with extreme Lyman continuum leakage. Based on our models, we expect essentially all z=7-9 galaxies that exhibit rest-frame EW(Hb)0.5. Incorrect assumptions concerning the ionizing fluxes of stellar populations or the dust properties of z>6 galaxies can in principle bias the selection, but substantial model deficiencies of this type will at the same time reveal themselves as an offset between the observed and simulated distribution of z>6 galaxies in the EW(Hb)-beta diagram. Such offsets would thereby allow JWST/NIRSpec measurements of these observables to serve as input for further model refinement.Comment: 12 pages, 8 figures, v.2: ApJ, accepted. Model grids are available from http://www.astro.uu.se/~ez/lycan/lycan.htm

Assessment of Automated Driving Systems using real-life scenarios

More and more Advanced Driver Assistance Systems (ADAS) are entering the market for improving both safety and comfort by assisting the driver with their driving task. An important aspect in developing future ADAS and Automated Driving Systems (ADS) is testing and validation. Validating the failure rate of an ADS requires so many operational hours that testing in real time is almost impossible. One way to reduce this test load is virtual testing or hardware in-The-loop testing. The major challenge is to create realistic test cases that closely resemble the situation on the road. We present a way to use data of naturalistic driving to generate test cases for Monte-Carlo simulations of ADS. Because real-life data is used, the assessment allows to draw conclusions on how the ADS would perform in real traffic. The method, developed in EU AdaptIVe, is demonstrated by testing an Adaptive Cruise Control (ACC) system in scenarios where the predecessor of the ego vehicle is braking. We show that the probability of the occurrence of unsafe situations with the ACC system can be accurately and efficiently determined