Contingency Model Predictive Control for Automated Vehicles

We present Contingency Model Predictive Control (CMPC), a novel and implementable control framework which tracks a desired path while simultaneously maintaining a contingency plan -- an alternate trajectory to avert an identified potential emergency. In this way, CMPC anticipates events that might take place, instead of reacting when emergencies occur. We accomplish this by adding an additional prediction horizon in parallel to the classical receding MPC horizon. The contingency horizon is constrained to maintain a feasible avoidance solution; as such, CMPC is selectively robust to this emergency while tracking the desired path as closely as possible. After defining the framework mathematically, we demonstrate its effectiveness experimentally by comparing its performance to a state-of-the-art deterministic MPC. The controllers drive an automated research platform through a left-hand turn which may be covered by ice. Contingency MPC prepares for the potential loss of friction by purposefully and intuitively deviating from the prescribed path to approach the turn more conservatively; this deviation significantly mitigates the consequence of encountering ice.Comment: American Control Conference, July 2019; 6 page

Modeling the HeII Transverse Proximity Effect: Constraints on Quasar Lifetime and Obscuration

The HeII transverse proximity effect - enhanced HeII Ly{\alpha} transmission in a background sightline caused by the ionizing radiation of a foreground quasar - offers a unique opportunity to probe the emission properties of quasars, in particular the emission geometry (obscuration, beaming) and the quasar lifetime. Building on the foreground quasar survey published in Schmidt+2017, we present a detailed model of the HeII transverse proximity effect, specifically designed to include light travel time effects, finite quasar ages, and quasar obscuration. We post-process outputs from a cosmological hydrodynamical simulation with a fluctuating HeII UV background model, plus the added effect of the radiation from a single bright foreground quasar. We vary the age $t_\mathrm{age}$ and obscured sky fractions $\Omega_\mathrm{obsc}$ of the foreground quasar, and explore the resulting effect on the HeII transverse proximity effect signal. Fluctuations in IGM density and the UV background, as well as the unknown orientation of the foreground quasar, result in a large variance of the HeII Ly{\alpha} transmission along the background sightline. We develop a fully Bayesian statistical formalism to compare far UV HeII Ly{\alpha} transmission spectra of the background quasars to our models, and extract joint constraints on $t_\mathrm{age}$ and $\Omega_\mathrm{obsc}$ for the six Schmidt+2017 foreground quasars with the highest implied HeII photoionization rates. Our analysis suggests a bimodal distribution of quasar emission properties, whereby one foreground quasar, associated with a strong HeII transmission spike, is relatively old $(22\,\mathrm{Myr})$ and unobscured $\Omega_\mathrm{obsc}<35\%$, whereas three others are either younger than $(10\,\mathrm{Myr})$ or highly obscured $(\Omega_\mathrm{obsc}>70\%)$.Comment: 19 pages, 6 figures, submitted to Ap

He II Proximity Effect and the Lifetime of Quasars

The lifetime of quasars is fundamental for understanding the growth of supermassive black holes, and is an important ingredient in models of the reionization of the intergalactic medium. However, despite various attempts to determine quasar lifetimes, current estimates from a variety of methods are uncertain by orders of magnitude. This work combines cosmological hydrodynamical simulations and 1D radiative transfer to investigate the structure and evolution of the He II Ly$\alpha$ proximity zones around quasars at $z \simeq 3-4$. We show that the time evolution in the proximity zone can be described by a simple analytical model for the approach of the He II fraction $x_{\rm HeII}\left( t \right)$ to ionization equilibrium, and use this picture to illustrate how the transmission profile depends on the quasar lifetime, quasar UV luminosity, and the ionization state of helium in the ambient IGM (i.e. the average He II fraction, or equivalently the metagalactic He II ionizing background). A significant degeneracy exists between the lifetime and the average He II fraction, however the latter can be determined from measurements of the He II Ly$\alpha$ optical depth far from quasars, allowing the lifetime to be measured. We advocate stacking existing He II quasar spectra at $z\sim 3$, and show that the shape of this average proximity zone profile is sensitive to lifetimes as long as $\sim 30$ Myr. At higher redshift $z\sim 4$ where the He II fraction is poorly constrained, degeneracies will make it challenging to determine these parameters independently. Our analytical model for He II proximity zones should also provide a useful description of the properties of H I proximity zones around quasars at $z \simeq 6-7$.Comment: 26 pages, 18 figures, accepted to Ap

The candidate cluster and protocluster catalog (CCPC) of spectroscopically identified structures spanning 2.74<z<3.712.74 < z < 3.71

We have developed a search methodology to identify galaxy protoclusters at $z>2.74$, and implemented it on a sample of $\sim$14,000 galaxies with previously measured redshifts. The results of this search are recorded in the Candidate Cluster and Protocluster Catalog (CCPC). The catalog contains 12 clusters that are highly significant overdensities ($\delta_{gal}>7$), 6 of which are previously known. We also identify another 31 candidate protoclusters (including 4 previously identified structures) of lower overdensity. CCPC systems vary over a wide range of physical sizes and shapes, from small, compact groups to large, extended, and filamentary collections of galaxies. This variety persists over the range from $z=3.71$ to $z=2.74$. These structures exist as galaxy overdensities ($\delta_{gal}$) with a mean value of 2, similar to the values found for other protoclusters in the literature. The median number of galaxies for CCPC systems is 11. Virial mass estimates are large for these redshifts, with thirteen cases apparently having $M > 10^{15}\, M_{\odot}$. If these systems are virialized, such masses would pose a challenge to $\Lambda$CDM.Comment: Accepted for publication in ApJ. 31 Pages, 4 Tables, 91 Figure

On the rapid demise of Lyman-alpha emitters at z>7 due to the increasing incidence of optically thick absorption systems

A variety of independent observational studies have now reported a significant decline in the fraction of Lyman-break galaxies which exhibit Ly-a emission over the redshift interval z=6-7. In combination with the strong damping wing extending redward of Ly-a in the spectrum of the bright z=7.085 quasar ULAS 1120+0641, this has strengthened suggestions that the hydrogen in the intergalactic medium (IGM) is still substantially neutral at z~7. Current theoretical models imply HI fractions as large as 40-90 per cent may be required to explain these data assuming there is no intrinsic evolution in the Ly-a emitter population. We propose that such large neutral fractions are not necessary. Based on a hydrodynamical simulation which reproduces the absorption spectra of high-redshift (z~6-7) quasars, we demonstrate that the opacity of the intervening IGM redward of rest-frame Ly-a can rise rapidly in average regions of the Universe simply because of the increasing incidence of absorption systems which are optically thick to Lyman continuum photons as the tail-end of reionisation is approached. Our simulations suggest these data do not require a large change in the IGM neutral fraction by several tens of per cent from z=6-7, but may instead be indicative of the rapid decrease in the typical mean free path for ionising photons expected during the final stages of reionisation.Comment: 11 pages, 6 figures, accepted to MNRA

Clustering of Lyman-alpha Emitters Around Quasars at z∼4z\sim4

The strong observed clustering of $z>3.5$ quasars indicates they are hosted by massive ($M_{\rm{halo}}\gtrsim10^{12}\,h^{-1}\,\rm{M_{\odot}}$) dark matter halos. Assuming quasars and galaxies trace the same large-scale structures, this should also manifest as strong clustering of galaxies around quasars. Previous work on high-redshift quasar environments, mostly focused at $z>5$, have failed to find convincing evidence for these overdensities. Here we conduct a survey for Lyman alpha emitters (LAEs) in the environs of 17 quasars at $z\sim4$ probing scales of $R\lesssim7\,h^{-1}\,{\rm{Mpc}}$. We measure an average LAE overdensity around quasars of 1.4 for our full sample, which we quantify by fitting the quasar-LAE cross-correlation function. We find consistency with a power-law shape with correlation length of $r^{QG}_{0}=2.78^{+1.16}_{-1.05}\,h^{-1}\,{\rm{cMpc}}$ for a fixed slope of $\gamma=1.8$. We also measure the LAE auto-correlation length and find $r^{GG}_{0}=9.12^{+1.32}_{-1.31}\,h^{-1}$\,cMpc ($\gamma=1.8$), which is $3.3$ times higher than the value measured in blank fields. Taken together our results clearly indicate that LAEs are significantly clustered around $z\sim4$ quasars. We compare the observed clustering with the expectation from a deterministic bias model, whereby LAEs and quasars probe the same underlying dark matter overdensities, and find that our measurements fall short of the predicted overdensities by a factor of 2.1. We discuss possible explanations for this discrepancy including large-scale quenching or the presence of excess dust in galaxies near quasars. Finally, the large cosmic variance from field-to-field observed in our sample (10/17 fields are actually underdense) cautions one from over-interpreting studies of $z\sim6$ quasar environments based on a single or handful of quasar fields.Comment: 19 pages, 12 figures, submitted to the Ap

Light cone effect on the reionization 21-cm power spectrum

Observations of redshifted 21-cm radiation from neutral hydrogen during the epoch of reionization (EoR) are considered to constitute the most promising tool to probe that epoch. One of the major goals of the first generation of low frequency radio telescopes is to measure the 3D 21-cm power spectrum. However, the 21-cm signal could evolve substantially along the line of sight (LOS) direction of an observed 3D volume, since the received signal from different planes transverse to the LOS originated from different look-back times and could therefore be statistically different. Using numerical simulations we investigate this so-called light cone effect on the spherically averaged 3D 21-cm power spectrum. For this version of the power spectrum, we find that the effect mostly `averages out' and observe a smaller change in the power spectrum compared to the amount of evolution in the mean 21-cm signal and its rms variations along the LOS direction. Nevertheless, changes up to 50% at large scales are possible. In general the power is enhanced/suppressed at large/small scales when the effect is included. The cross-over mode below/above which the power is enhanced/suppressed moves toward larger scales as reionization proceeds. When considering the 3D power spectrum we find it to be anisotropic at the late stages of reionization and on large scales. The effect is dominated by the evolution of the ionized fraction of hydrogen during reionization and including peculiar velocities hardly changes these conclusions. We present simple analytical models which explain qualitatively all the features we see in the simulations.Comment: 15 pages, 15 figures, 4 tables, moderate revision, added results on anisotropies in the power spectra arising from the light cone effect and a discussion on the foreground subtraction effect. MNRAS (in press