Radiative cooling is central to a wide range of astrophysical problems. Despite its importance, cooling rates are generally computed using very restrictive assumptions, such as collisional ionization equilibrium and solar relative abundances. We simultaneously relax both assumptions and investigate the effects of photoionization of heavy elements by the metagalactic ultraviolet (UV)/X‐ray background and of variations in relative abundances on the cooling rates of optically thin gas in ionization equilibrium. We find that photoionization by the metagalactic background radiation reduces the net cooling rates by up to an order of magnitude for gas densities and temperatures typical of the shock‐heated intergalactic medium and proto‐galaxies (104 K ≲T≲ 106 K, ρ/〈ρ〉≲ 100). In addition, photoionization changes the relative contributions of different elements to the cooling rates. We conclude that photoionization by both the ionizing background and heavy elements needs to be taken into account in order for the cooling rates to be correct to an order of magnitude. Moreover, if the rates need to be known to better than a factor of a few, then departures of the relative abundances from solar need to be taken into account. We propose a method to compute cooling rates on an element‐by‐element basis by interpolating pre‐computed tables that take photoionization into account. We provide such tables for a popular model of the evolving UV/X‐ray background radiation, computed using the photoionization package cloudy.

The analysis of complex multiphysics astrophysical simulations presents a unique and rapidly growing set of challenges: reproducibility, parallelization, and vast increases in data size and complexity chief among them. In order to meet these challenges, and in order to open up new avenues for collaboration between users of multiple simulation platforms, we present yt a , an open source, communitydeveloped astrophysical analysis and visualization toolkit. Analysis and visualization with yt are oriented around physically relevant quantities rather than quantities native to astrophysical simulation codes. While originally designed for handling Enzo's structure adaptive mesh refinement (AMR) data, yt has been extended to work with several different simulation methods and simulation codes including Orion, RAMSES, and FLASH. We report on its methods for reading, handling, and visualizing data, including projections, multivariate volume rendering, multi-dimensional histograms, halo finding, light cone generation and topologically-connected isocontour identification. Furthermore, we discuss the underlying algorithms yt uses for processing and visualizing data, and its mechanisms for parallelization of analysis tasks.

This paper describes the open-source code Enzo, which uses block-structured adaptive mesh refinement to provide high spatial and temporal resolution for modeling astrophysical fluid flows. The code is Cartesian, can be run in 1, 2, and 3 dimensions, and supports a wide variety of physics including hydrodynamics, ideal and non-ideal magnetohydrodynamics, N-body dynamics (and, more broadly, self-gravity of fluids and particles), primordial gas chemistry, optically-thin radiative cooling of primordial and metal-enriched plasmas (as well as some optically-thick cooling models), radiation transport, cosmological expansion, and models for star formation and feedback in a cosmological context. In addition to explaining the algorithms implemented, we present solutions for a wide range of test problems, demonstrate the code's parallel performance, and discuss the Enzo collaboration's code development methodology.

Starlight from galaxies plays a pivotal role throughout the process of cosmic reionisation. We present the statistics of dwarf galaxy properties at z > 7 in haloes with masses up to 10 9 M , using a cosmological radiation hydrodynamics simulation that follows their buildup starting with their Population III progenitors. We find that metal-enriched star formation is not restricted to atomic cooling (T vir 10 4 K) haloes, but can occur in haloes down to masses ∼ 10 6 M , especially in neutral regions. Even though these smallest galaxies only host up to 10 4 M of stars, they provide nearly 30 per cent of the ionising photon budget. We find that the galaxy luminosity function flattens above M UV ∼ −12 with a number density that is unchanged at z 10. The fraction of ionising radiation escaping into the intergalactic medium is inversely dependent on halo mass, decreasing from 50 to 5 per cent in the mass range log M/M = 7.0 − 8.5. Using our galaxy statistics in a semi-analytic reionisation model, we find a Thomson scattering optical depth consistent with the latest Planck results, while still being consistent with the UV emissivity constraints provided by Lyα forest observations at z = 4 − 6.

Although galaxies, groups, and clusters contain ∼ 10% of the baryons, many more reside in the photoionized and shocked-heated intergalactic medium (IGM) and in the circumgalactic medium (CGM). We update the baryon census in the (H I) Lyα forest and warm-hot IGM (WHIM) at 10 5−6 K traced by O VI λ1032, 1038 absorption. Using robust cosmological simulations of heating, cooling, and metal transport, we improve the O VI baryon surveys with spatially averaged corrections for metallicity (Z/Z ) and O VI ionization fraction (f OVI ). Statistically, their product correlates with column density, (Z/Z ) f OVI ≈ (0.015)(N OVI /10 14 cm −2 ) 0.70 , with a N OVI -weighted mean of 0.01, which doubles previous estimates of WHIM baryon content. We also update the Lyα forest contribution to baryon density out to z = 0.4, correcting for the (1 + z) 3 increase in absorber density, the (1 + z) 4.4 rise in photoionizing background, and cosmological proper length d /dz. We find substantial baryon fractions in the photoionized Lyα forest (28 ± 11%) and WHIM traced by O VI and broad-Lyα absorbers (25 ± 8%). The collapsed phase (galaxies, groups, clusters, CGM) contains 18 ± 4%, leaving an apparent baryon shortfall of 29 ± 13%. Our simulations suggest that ∼ 15% reside in hotter WHIM (T ≥ 10 6 K). Additional baryons could be detected in weaker Lyα and O VI absorbers. Further progress requires higher-precision baryon surveys of weak absorbers, down to minimum column densities N HI ≥ 10 12.0 cm −2 , N OVI ≥ 10 12.5 cm −2 , N OVII ≥ 10 14.5 cm −2 , using high-S/N data from high-resolution UV and X-ray spectrographs.

We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright AGN at redshifts z AGN < 0.85 to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H I and 25 different metal-ion species grouped into 2611 distinct redshift systems at z abs < 0.75 covering total redshift pathlengths ∆z HI = 21.7 and ∆z OVI = 14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively-defined as possible. The cumulative column-density distribution of H I systems can be parametrized dN (> N )/dz = C 14 (N/10 14 cm −2 ) −(β−1) , with C 14 = 25 ± 1 and β = 1.65 ± 0.02. This distribution is seen to evolve both in amplitude, C 14 ∝ (1 + z) 2.3±0.1 , and slope β(z) = 1.75 − 0.31 z for z ≤ 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N HI . The distribution of O VI absorbers appear to evolve in the same sense as the Lyα forest. We calculate contributions to Ω b from different components of the low-z IGM and determine the Lyα decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H I absorbers on scales of ∆v = 50 − 300 km s −1 with no significant clustering at ∆v 1000 km s −1 . Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N HI 10 13.5 cm −2 , metal-bearing IGM systems. The full catalog of absorption lines and fully-reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at

We present the Grackle chemistry and cooling library for astrophysical simulations and models. Grackle provides a treatment of non-equilibrium primordial chemistry and cooling for H, D, and He species, including H 2 formation on dust grains; tabulated primordial and metal cooling; multiple UV background models; and support for radiation transfer and arbitrary heat sources. The library has an easily implementable interface for simulation codes written in C, C++, and Fortran as well as a Python interface with added convenience functions for semi-analytical models. As an open-source project, Grackle provides a community resource for accessing and disseminating astrochemical data and numerical methods. We present the full details of the core functionality, the simulation and Python interfaces, testing infrastructure, performance, and range of applicability. Grackle is a fully open-source project and new contributions are welcome.

We perform a series of cosmological simulations using Enzo, an Eulerian adaptive-mesh refinement, N-body + hydrodynamical code, applied to study the warm/hot intergalactic medium. The WHIM may be an important component of the baryons missing observationally at low redshift. We investigate the dependence of the global star formation rate and mass fraction in various baryonic phases on spatial resolution and methods of incorporating stellar feedback. Although both resolution and feedback significantly affect the total mass in the WHIM, all of our simulations find that the WHIM fraction peaks at z ∼ 0.5, declining to 35-40% at z = 0. We construct samples of synthetic O VI absorption lines from our highest-resolution simulations, using several models of oxygen ionization balance. Models that include both collisional ionization and photoionization provide excellent fits to the observed number density of absorbers per unit redshift over the full range of column densities (10 13 cm −2 N OVI 10 15 cm −2 ). Models that include only collisional ionization provide better fits for high column density absorbers (N OVI 10 14 cm −2 ). The distribution of O VI in density and temperature exhibits two populations: one at T ∼ 10 5.5 K (collisionally ionized, 55% of total O VI) and one at T ∼ 10 4.5 K (photoionized, 37%) with the remainder located in dense gas near galaxies. While not a perfect tracer of hot gas, O VI provides an important tool for a WHIM baryon census.