ALBERT

Description: We present an analysis of probability distribution functions (pdfs) of column density in different zones of the star-forming region Perseus and its diffuse environment based on the map of dust opacity at 353 GHz available from the Planck archive. The pdf shape can be fitted by a combination of a lognormal function and an extended power-law tail at high densities, in zones centred at the molecular cloud Perseus. A linear combination of several lognormals fits very well the pdf in rings surrounding the cloud or in zones of its diffuse neighbourhood. The slope of the mean-density scaling law 〈 〉 L L α is steep (α = –1.93) in the former case and rather shallow (α = –0.77 ± 0.11) in the rings delineated around the cloud. We interpret these findings as signatures of two distinct physical regimes: (i) a gravoturbulent one which is characterized by nearly linear scaling of mass and practical lack of velocity scaling; and (ii) a predominantly turbulent one which is best described by steep velocity scaling and by invariant for compressible turbulence $\langle \rho \rangle _L u_L^3/L$ , describing a scale-independent flux of the kinetic energy per unit volume through turbulent cascade. The gravoturbulent spatial domain can be identified with the molecular cloud Perseus while a relatively sharp transition to predominantly turbulent regime occurs in its vicinity.

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: We present a model for describing the general structure of molecular clouds (MCs) at early evolutionary stages in terms of their mass–size relationship. Sizes are defined through threshold levels at which equipartitions between gravitational, turbulent and thermal energy | W | ~  f ( E kin  +  E th ) take place, adopting interdependent scaling relations of velocity dispersion and density and assuming a lognormal density distribution at each scale. Variations of the equipartition coefficient 1 ≤ f ≤ 4 allow for modelling of star-forming regions at scales within the size range of typical MCs (4 pc). Best fits are obtained for regions with low or no star formation (Pipe, Polaris) as well for such with star-forming activity but with nearly lognormal distribution of column density (Rosette). An additional numerical test of the model suggests its applicability to cloud evolutionary times prior to the formation of first stars.

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: We analyse the scaling properties of turbulent flows using a suite of three-dimensional numerical simulations. We model driven, compressible, isothermal, turbulence with Mach numbers ranging from the subsonic ( $\mathcal {M} \approx 0.5$ ) to the highly supersonic regime ( $\mathcal {M}\approx 16$ ). The forcing scheme consists of both solenoidal (transverse) and compressive (longitudinal) modes in equal parts. We confirm the relation $\sigma _{s}^2 = \ln {(1+b^2\mathcal {M}^2)}$ between the Mach number and the standard deviation of the logarithmic density with b  = 0.33. We find increasing deviations with higher Mach number from the predicted lognormal shape in the high-density wing of the density probability density function. The density spectra follow $\mathcal {D}(k,\,\mathcal {M}) \propto k^{\zeta (\mathcal {M})}$ with scaling exponents depending on the Mach number. We find $\zeta (\mathcal {M}) = \alpha \mathcal {M}^{\beta }$ with coefficients α = –2.1 and β = –0.33. The dependence of the scaling exponent on the Mach number implies a fractal dimension $D=2+1.05 \mathcal {M}^{-0.33}$ .

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: It has been shown that the behaviour of primordial gas collapsing in a dark matter minihalo can depend on the adopted choice of three-body H 2 formation rate. The uncertainties in this rate span two orders of magnitude in the current literature, and so it remains a source of uncertainty in our knowledge of Population III star formation. Here, we investigate how the amount of fragmentation in primordial gas depends on the adopted three-body rate. We present the results of calculations that follow the chemical and thermal evolution of primordial gas as it collapses in two dark matter minihaloes. Our results on the effect of three-body rate on the evolution until the first protostar forms agree well with previous studies. However, our modified version of gadget -2 smoothed particle hydrodynamics also includes sink particles, which allows us to follow the initial evolution of the accretion disc that builds up on the centre of each halo, and capture the fragmentation in gas as well as its dependence on the adopted three-body H 2 formation rate. We find that the fragmentation behaviour of the gas is only marginally affected by the choice of three-body rate co-efficient, and that halo-to-halo differences are of equal importance in affecting the final mass distribution of stars.

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.