ALBERT

Description: Supermassive primordial stars in hot, atomically cooling haloes at z ∼ 15–20 may have given birth to the first quasars in the Universe. Most simulations of these rapidly accreting stars suggest that they are red, cool hypergiants, but more recent models indicate that some may have been bluer and hotter, with surface temperatures of 20 000–40 000 K. These stars have spectral features that are quite distinct from those of cooler stars and may have different detection limits in the near-infrared today. Here, we present spectra and AB magnitudes for hot, blue supermassive primordial stars calculated with the tlusty and cloudy codes. We find that photometric detections of these stars by the James Webb Space Telescope will be limited to z ≲ 10–12, lower redshifts than those at which red stars can be found, because of quenching by their accretion envelopes. With moderate gravitational lensing, Euclid and the Wide-Field Infrared Space Telescope could detect blue supermassive stars out to similar redshifts in wide-field surveys.

Description: The SILCC (SImulating the Life-Cycle of molecular Clouds) project aims to self-consistently understand the small-scale structure of the interstellar medium (ISM) and its link to galaxy evolution. We simulate the evolution of the multiphase ISM in a (500 pc) 2   x  ±5 kpc region of a galactic disc, with a gas surface density of $\Sigma _{_{\rm GAS}} = 10 \;{\rm M}_{\odot }\,{\rm pc}^{-2}$ . The flash 4 simulations include an external potential, self-gravity, magnetic fields, heating and radiative cooling, time-dependent chemistry of H 2 and CO considering (self-) shielding, and supernova (SN) feedback but omit shear due to galactic rotation. We explore SN explosions at different rates in high-density regions ( peak ), in random locations with a Gaussian distribution in the vertical direction ( random ), in a combination of both ( mixed ), or clustered in space and time ( clus / clus2 ). Only models with self-gravity and a significant fraction of SNe that explode in low-density gas are in agreement with observations. Without self-gravity and in models with peak driving the formation of H 2 is strongly suppressed. For decreasing SN rates, the H 2 mass fraction increases significantly from 〈10 per cent for high SN rates, i.e. 0.5 dex above Kennicutt–Schmidt, to 70–85 per cent for low SN rates, i.e. 0.5 dex below KS. For an intermediate SN rate, clustered driving results in slightly more H 2 than random driving due to the more coherent compression of the gas in larger bubbles. Magnetic fields have little impact on the final disc structure but affect the dense gas ( n   10 cm –3 ) and delay H 2 formation. Most of the volume is filled with hot gas (~80 per cent within ±150 pc). For all but peak driving a vertically expanding warm component of atomic hydrogen indicates a fountain flow. We highlight that individual chemical species populate different ISM phases and cannot be accurately modelled with temperature-/density-based phase cut-offs.

Description: We employ the -variance analysis and study the turbulent gas dynamics of simulated molecular clouds (MCs). Our models account for a simplified treatment of time-dependent chemistry and the non-isothermal nature of the gas. We investigate simulations using three different initial mean number densities of n 0  = 30, 100 and 300 cm –3 that span the range of values typical for MCs in the solar neighbourhood. Furthermore, we model the CO line emission in a post-processing step using a radiative transfer code. We evaluate -variance spectra for centroid velocity (CV) maps as well as for integrated intensity and column density maps for various chemical components: the total, H 2 and 12 CO number density and the integrated intensity of both the 12 CO and 13 CO ( J  = 1 -〉 0) lines. The spectral slopes of the -variance computed on the CV maps for the total and H 2 number density are significantly steeper compared to the different CO tracers. We find slopes for the linewidth–size relation ranging from 0.4 to 0.7 for the total and H 2 density models, while the slopes for the various CO tracers range from 0.2 to 0.4 and underestimate the values for the total and H 2 density by a factor of 1.5–3.0. We demonstrate that optical depth effects can significantly alter the -variance spectra. Furthermore, we report a critical density threshold of ~100 cm –3 at which the -variance slopes of the various CO tracers change sign. We thus conclude that carbon monoxide traces the total cloud structure well only if the average cloud density lies above this limit.

Description: High-redshift quasars at z  〉 6 have masses up to ~10 9 M . One of the pathways to their formation includes direct collapse of gas, forming a supermassive star, precursor of the black hole seed. The conditions for direct collapse are more easily achievable in metal-free haloes, where atomic hydrogen cooling operates and molecular hydrogen (H 2 ) formation is inhibited by a strong external (ultraviolet) UV flux. Above a certain value of UV flux ( J crit ), the gas in a halo collapses isothermally at ~10 4 K and provides the conditions for supermassive star formation. However, H 2  can self-shield, reducing the effect of photodissociation. So far, most numerical studies used the local Jeans length to calculate the column densities for self-shielding. We implement an improved method for the determination of column densities in 3D simulations and analyse its effect on the value of J crit . This new method captures the gas geometry and velocity field and enables us to properly determine the direction-dependent self-shielding factor of H 2  against photodissociating radiation. We find a value of J crit that is a factor of 2 smaller than with the Jeans approach (~2000 J 21 versus ~4000 J 21 ). The main reason for this difference is the strong directional dependence of the H 2  column density. With this lower value of J crit , the number of haloes exposed to a flux 〉 J crit is larger by more than an order of magnitude compared to previous studies. This may translate into a similar enhancement in the predicted number density of black hole seeds.

Description: Population III (Pop III) stars can regulate star formation in the primordial Universe in several ways. They can ionize nearby haloes, and even if their ionizing photons are trapped by their own haloes, their Lyman–Werner (LW) photons can still escape and destroy H 2 in other haloes, preventing them from cooling and forming stars. LW escape fractions are thus a key parameter in cosmological simulations of early reionization and star formation but have not yet been parametrized for realistic haloes by halo or stellar mass. To do so, we perform radiation hydrodynamical simulations of LW UV escape from 9–120 M Pop III stars in 10 5 –10 7 M haloes with zeus-mp . We find that photons in the LW lines (i.e. those responsible for destroying H 2 in nearby systems) have escape fractions ranging from 0 to 85 per cent. No LW photons escape the most massive halo in our sample, even from the most massive star. Escape fractions for photons elsewhere in the 11.18–13.6 eV energy range, which can be redshifted into the LW lines at cosmological distances, are generally much higher, being above 60 per cent for all but the least massive stars in the most massive haloes. We find that shielding of H 2 by neutral hydrogen, which has been neglected in most studies to date, produces escape fractions that are up to a factor of 3 smaller than those predicted by H 2 self-shielding alone.

Description: With new observational facilities becoming available soon, discovering and characterizing supernovae from the first stars will open up alternative observational windows to the end of the cosmic dark ages. Based on a semi-analytical merger tree model of early star formation, we constrain Population III supernova rates. We find that our method reproduces the Population III supernova rates of large-scale cosmological simulations very well. Our computationally efficient model allows us to survey a large parameter space and to explore a wide range of different scenarios for Population III star formation. Our calculations show that observations of the first supernovae can be used to differentiate between cold and warm dark matter models and to constrain the corresponding particle mass of the latter. Our predictions can also be used to optimize survey strategies with the goal to maximize supernova detection rates.

Description: We investigate the temperature distribution of CO-dark molecular hydrogen (H 2 ) in a series of disc galaxies simulated using the arepo moving-mesh code. In conditions similar to those in the Milky Way, we find that H 2 has a flat temperature distribution ranging from 10 to 100 K. At T 〈 30 K, the gas is almost fully molecular and has a high CO content, whereas at T 〉 30 K, the H 2 fraction spans a broader range and the CO content is small, allowing us to classify gas in these two regimes as CO-bright and CO-dark, respectively. The mean sound speed in the CO-dark H 2 is c s, dark = 0.64 km s –1 , significantly lower than the value in the cold atomic gas ( c s, CNM = 1.15 km s –1 ), implying that the CO-dark molecular phase is more susceptible to turbulent compression and gravitational collapse than its atomic counterpart. We further show that the temperature of the CO-dark H 2 is highly sensitive to the strength of the interstellar radiation field, but that conditions in the CO-bright H 2 remain largely unchanged. Finally, we examine the usefulness of the [C ii ] and [O i ] fine-structure lines as tracers of the CO-dark gas. We show that in Milky Way-like conditions, diffuse [C ii ] emission from this gas should be detectable. However, it is a problematic tracer of this gas, as there is only a weak correlation between the brightness of the emission and the H 2 surface density. The situation is even worse for the [O i ] line, which shows no correlation with the H 2 surface density.

Description: We use detailed numerical simulations of a turbulent molecular cloud to study the usefulness of the [C i ] 609 and 370 μm fine structure emission lines as tracers of cloud structure. Emission from these lines is observed throughout molecular clouds, and yet they have attracted relatively little theoretical attention. We show that the widespread [C i ] emission results from the fact that the clouds are turbulent. Turbulence creates large density inhomogeneities, allowing radiation to penetrate deeply into the clouds. As a result, [C i ] emitting gas is found throughout the cloud. We examine how well [C i ] emission traces the cloud structure, and show that the 609 μm line traces column density accurately over a wide range of values. For visual extinctions greater than a few, [C i ] and 13 CO both perform well, but [C i ] performs better at A V  ≤ 3. We have also studied the distribution of [C i ] excitation temperatures. We show that these are typically smaller than the kinetic temperature, indicating that the carbon is subthermally excited. We discuss how best to estimate the excitation temperature and the carbon column density, and show that the latter tends to be systematically underestimated. Consequently, estimates of the atomic carbon content of real giant molecular clouds could be wrong by up to a factor of 2.

Description: We present a new near-field cosmological probe of the initial mass function (IMF) of the first stars. Specifically, we constrain the lower mass limit of the Population III (Pop III) IMF with the total number of stars in large, unbiased surveys of the Milky Way. We model the early star formation history in a Milky Way-like halo with a semi-analytic approach, based on Monte Carlo sampling of dark matter merger trees, combined with a treatment of the most important feedback mechanisms. Assuming a logarithmically flat Pop III IMF and varying its low-mass limit, we derive the number of expected survivors of these first stars, using them to estimate the probability to detect any such Pop III fossil in stellar archaeological surveys. Following our analysis, the most promising region to find possible Pop III survivors is the stellar halo of the Milky Way, which is the best target for future surveys. We find that if no genuine Pop III survivor is detected in a sample size of 4 10 6 (2 10 7 ) halo stars with well-controlled selection effects, then we can exclude the hypothesis that the primordial IMF extended down below 0.8 M at a confidence level of 68 per cent (99 per cent). With the sample size of the Hamburg/European Southern Observatory survey, we can tentatively exclude Pop III stars with masses below 0.65 M with a confidence level of 95 per cent, although this is subject to significant uncertainties. To fully harness the potential of our approach, future large surveys are needed that employ uniform, unbiased selection strategies for high-resolution spectroscopic follow-up.

Description: The direct collapse model for the formation of massive seed black holes in the early Universe attempts to explain the observed number density of supermassive black holes (SMBHs) at z  ~ 6 by assuming that they grow from seeds with masses M  〉 10 4 M that form by the direct collapse of metal-free gas in atomic cooling haloes in which H 2 cooling is suppressed by a strong extragalactic radiation field. The viability of this model depends on the strength of the radiation field required to suppress H 2 cooling, J crit : if this is too large, then too few seeds will form to explain the observed number density of SMBHs. In order to determine J crit reliably, we need to be able to accurately model the formation and destruction of H 2 in gas illuminated by an extremely strong radiation field. In this paper, we use a reaction-based reduction technique to analyse the chemistry of H 2 in these conditions, allowing us to identify the key chemical reactions that are responsible for determining the value of J crit . We construct a reduced network of 26 reactions that allows us to determine J crit accurately, and compare it with previous treatments in the literature. We show that previous studies have often omitted one or more important chemical reactions, and that these omissions introduce an uncertainty of up to a factor of 3 into previous determinations of J crit .