arXiv subjects

D. R. G. Schleicher

Publications and source records attributed to D. R. G. Schleicher.

71 records · Page 4Linked to original sources

arXiv2014

Formation of carbon-enhanced metal-poor stars in the presence of far ultraviolet radiation

Recent discoveries of carbon-enhanced metal-poor stars like SMSS J031300.36-670839.3 provide increasing observational insights into the formation conditions of the first second-generation stars in the Universe, reflecting the chemical conditions after the first supernova explosion. Here, we present the first cosmological simulations with a detailed chemical network including primordial species as well as C, C$^+$, O, O$^+$, Si, Si$^+$, and Si$^{2+}$ following the formation of carbon-enhanced metal poor stars. The presence of background UV flux delays the collapse from $z=21$ to $z=15$ and cool the gas down to the CMB temperature for a metallicity of Z/Z$_\odot$=10$^{-3}$. This can potentially lead to the formation of lower mass stars. Overall, we find that the metals have a stronger effect on the collapse than the radiation, yielding a comparable thermal structure for large variations in the radiative background. We further find that radiative backgrounds are not able to delay the collapse for Z/Z$_\odot$=10$^{-2}$ or a carbon abundance as in SMSS J031300.36-670839.3.

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arXiv2014

The formation of massive primordial stars in the presence of moderate UV backgrounds

Radiative feedback from populations II stars played a vital role in early structure formation. Particularly, photons below the Lyman limit can escape the star forming regions and produce a background ultraviolet (UV) flux which consequently may influence the pristine halos far away from the radiation sources. These photons can quench the formation of molecular hydrogen by photo-detachment of $\rm H^{-}$. In this study, we explore the impact of such UV radiation on fragmentation in massive primordial halos of a few times $\rm 10^{7}$~M${_\odot}$. To accomplish this goal, we perform high resolution cosmological simulations for two distinct halos and vary the strength of the impinging background UV field in units of $\rm J_{21}$. We further make use of sink particles to follow the evolution for 10,000 years after reaching the maximum refinement level. No vigorous fragmentation is observed in UV illuminated halos while the accretion rate changes according to the thermal properties. Our findings show that a few 100-10, 000 solar mass protostars are formed when halos are irradiated by $\rm J_{21}=10-500$ at $\rm z>10$ and suggest a strong relation between the strength of UV flux and mass of a protostar. This mode of star formation is quite different from minihalos, as higher accretion rates of about $\rm 0.01-0.1$ M$_{\odot}$/yr are observed by the end of our simulations. The resulting massive stars are the potential cradles for the formation of intermediate mass black holes at earlier cosmic times and contribute to the formation of a global X-ray background.

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arXiv2014

Large-Scale Structure Formation: from the first non-linear objects to massive galaxy clusters

The large-scale structure of the Universe formed from initially small perturbations in the cosmic density field, leading to galaxy clusters with up to 10^15 Msun at the present day. Here, we review the formation of structures in the Universe, considering the first primordial galaxies and the most massive galaxy clusters as extreme cases of structure formation where fundamental processes such as gravity, turbulence, cooling and feedback are particularly relevant. The first non-linear objects in the Universe formed in dark matter halos with 10^5-10^8 Msun at redshifts 10-30, leading to the first stars and massive black holes. At later stages, larger scales became non-linear, leading to the formation of galaxy clusters, the most massive objects in the Universe. We describe here their formation via gravitational processes, including the self-similar scaling relations, as well as the observed deviations from such self-similarity and the related non-gravitational physics (cooling, stellar feedback, AGN). While on intermediate cluster scales the self-similar model is in good agreement with the observations, deviations from such self-similarity are apparent in the core regions, where numerical simulations do not reproduce the current observational results. The latter indicates that the interaction of different feedback processes may not be correctly accounted for in current simulations. Both in the most massive clusters of galaxies as well as during the formation of the first objects in the Universe, turbulent structures and shock waves appear to be common, suggesting them to be ubiquitous in the non-linear regime.

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arXiv2014

Dark-matter halo mergers as a fertile environment for low-mass Population III star formation

While Population III stars are typically thought to be massive, pathways towards lower-mass Pop III stars may exist when the cooling of the gas is particularly enhanced. A possible route is enhanced HD cooling during the merging of dark-matter halos. The mergers can lead to a high ionization degree catalysing the formation of HD molecules and may cool the gas down to the cosmic microwave background (CMB) temperature. In this paper, we investigate the merging of mini-halos with masses of a few 10$^5$ M$_\odot$ and explore the feasibility of this scenario. We have performed three-dimensional cosmological hydrodynamics calculations with the ENZO code, solving the thermal and chemical evolution of the gas by employing the astrochemistry package KROME. Our results show that the HD abundance is increased by two orders of magnitude compared to the no-merging case and the halo cools down to $\sim$60 K triggering fragmentation. Based on Jeans estimates the expected stellar masses are about 10 M$_\odot$. Our findings show that the merging scenario is a potential pathway for the formation of low-mass stars.

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arXiv2014

Impact of baryonic streaming velocities on the formation of supermassive black holes via direct collapse

Baryonic streaming motions produced prior to the epoch of recombination became supersonic during the cosmic dark ages. Various studies suggest that such streaming velocities change the halo statistics and also influence the formation of Population III stars. In this study, we aim to explore the impact of streaming velocities on the formation of supermassive black holes at $z>10$ via the direct collapse scenario. To accomplish this goal, we perform cosmological large eddy simulations for two halos of a few times $\rm 10^{7} M_{\odot}$ with initial streaming velocities of 3, 6 and 9 $\rm km/s$. These massive primordial halos illuminated by the strong Lyman Werner flux are the potential cradles for the formation of direct collapse seed black holes. To study the evolution for longer times, we employ sink particles and track the accretion for 10,000 years. Our findings show that higher streaming velocities increase the circular velocities from about 14 $\rm km/s$ to 16 $\rm km/s$. They also delay the collapse of halos for a few million years, but do not have any significant impact on the halo properties such as turbulent energy, radial velocity, density and accretion rates. Sink particles of about $\rm \sim 10^5 M_{\odot}$ are formed at the end of our simulations and no clear distribution of sink masses is observed in the presence of streaming motions. It is further found that the impact of streaming velocities is less severe in massive halos compared to the minihalos as reported in the previous studies.

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arXiv2014

Magnetic fields during the formation of supermassive black holes

Observations of quasars at $\rm z> 6$ report the existence of a billion solar mass black holes. Comprehending their formation in such a short time scale is a matter of ongoing research. One of the most promising scenarios to assemble supermassive black holes is a monolithic collapse of protogalactic gas clouds in atomic cooling halos with $\rm T_{vir} \geq 10^{4} K$. In this article, we study the amplification and impact of magnetic fields during the formation of seed black holes in massive primordial halos. We perform high resolution cosmological magnetohydrodynamics simulations for four distinct halos and follow their collapse for a few free-fall times until the simulations reach a peak density of $\rm 7 \times 10^{-10} g/cm^{3}$. Our findings show that irrespective of the initial seed field, the magnetic field strength reaches a saturated state in the presence of strong accretion shocks. Under such conditions, the growth time becomes very short and amplification occurs rapidly within a small fraction of the free-fall time. We find that the presence of such strong magnetic fields provides additional support against gravity and helps in suppressing fragmentation. Massive clumps of a few hundred solar masses are formed at the end of our simulations and high accretion rates of $\rm 1 M_{\odot}/yr$ are observed. We expect that in the presence of such accretion rates, the clumps will grow to form supermassive stars of $\rm \sim 10^{5} M_{\odot}$. Overall, the role of the magnetic fields seems supportive for the formation of massive black holes.

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arXiv2014

KROME - a package to embed chemistry in astrophysical simulations

Chemistry plays a key role in many astrophysical situations regulating the cooling and the thermal properties of the gas, which are relevant during gravitational collapse, the evolution of disks and the fragmentation process. In order to simplify the usage of chemical networks in large numerical simulations, we present the chemistry package KROME, consisting of a Python pre-processor which generates a subroutine for the solution of chemical networks which can be embedded in any numerical code. For the solution of the rate equations, we make use of the high-order solver DLSODES, which was shown to be both accurate and efficient for sparse networks, which are typical in astrophysical applications. KROME also provides a large set of physical processes connected to chemistry, including photochemistry, cooling, heating, dust treatment, and reverse kinetics. The package presented here already contains a network for primordial chemistry, a small metal network appropriate for the modelling of low metallicities environments, a detailed network for the modelling of molecular clouds, a network for planetary atmospheres, as well as a framework for the modelling of the dust grain population. In this paper, we present an extended test suite ranging from one-zone and 1D-models to first applications including cosmological simulations with ENZO and RAMSES and 3D collapse simulations with the FLASH code. The package presented here is publicly available at http://kromepackage.org/ and https://bitbucket.org/krome/krome_stable

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arXiv2013

Primordial star formation: relative impact of H2 three-body rates and initial conditions

Population III stars are the first stars in the Universe to form at z=20-30 out of a pure hydrogen and helium gas in minihalos of 10^5-10^6 M$_\odot$ . Cooling and fragmentation is thus regulated via molecular hydrogen. At densities above 10^8 cm$^{-3}$, the three-body H2 formation rates are particularly important for making the gas fully molecular. These rates were considered to be uncertain by at least a few orders of magnitude. We explore the impact of new accurate three-body H2 formation rates derived by Forrey (2013) for three different minihalos, and compare to the results obtained with three-body rates employed in previous studies. The calculations are performed with the cosmological hydrodynamics code ENZO (release 2.2) coupled with the chemistry package KROME (including a network for primordial chemistry), which was previously shown to be accurate in high resolution simulations. While the new rates can shift the point where the gas becomes fully molecular, leading to a different thermal evolution, there is no trivial trend in how this occurs. While one might naively expect the results to be inbetween the calculations based on Palla et al. (1983) and Abel et al. (2002), the behavior can be close to the former or the latter depending on the dark matter halo that is explored. We conclude that employing the correct three-body rates is about as equally important as the use of appropriate initial conditions, and that the resulting thermal evolution needs to be calculated for every halo individually.

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arXiv2013

The characteristic black hole mass resulting from direct collapse in the early universe

Black holes of a billion solar masses are observed in the infant universe a few hundred million years after the Big Bang. The direct collapse of protogalactic gas clouds in primordial halos with $\rm T_{vir} \geq 10^{4} K$ provides the most promising way to assemble massive black holes. In this study, we aim to determine the characteristic mass scale of seed black holes and the time evolution of the accretion rates resulting from the direct collapse model. We explore the formation of supermassive black holes via cosmological large eddy simulations (LES) by employing sink particles and following their evolution for twenty thousand years after the formation of the first sink. As the resulting protostars were shown to have cool atmospheres in the presence of strong accretion, we assume here that UV feedback is negligible during this calculation. We confirm this result in a comparison run without sinks. Our findings show that black hole seeds with characteristic mass of $\rm 10^{5} M_{\odot}$ are formed in the presence of strong Lyman Werner flux which leads to an isothermal collapse. The characteristic mass is a about two times higher in LES compared to the implicit large eddy simulations (ILES). The accretion rates increase with time and reach a maximum value of 10 $\rm M_{\odot}/yr$ after $\rm 10^{4}$ years. Our results show that the direct collapse model is clearly feasible as it provides the expected mass of the seed black holes.

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arXiv2013

The formation of massive Pop III stars in the presence of turbulence

Population III stars forming in the infant universe at z=30 heralded the end of the cosmic dark ages. They are presumed to be assembled in so-called minihaloes with virial temperatures of a few thousand K where collapse is triggered by molecular hydrogen cooling. A central question concerns their final masses, and whether fragmentation occurs during their formation. While studies employing Lagrangian codes suggest fragmentation via a self-gravitating disk, recent high resolution simulations indicated that disk formation is suppressed. Here we report the first high-resolution large-eddy simulations performed with the Eulerian grid-based code Enzo following the evolution beyond the formation of the first peak, to investigate the accretion of the central massive clump and potential fragmentation. For a total of 3 halos, we see that a disk forms around the first clump. The central clump reaches $\sim10$ solar masses after 40 years, while subsequent accretion is expected at a rate of $10^{-2}$ solar masses per year. In one of these halos, additional clumps form as a result of fragmentation which proceeds at larger scales. We note that subgrid-scale turbulence yields relevant contributions to the stability of the protostellar disks. We conclude that the first protostar may reach masses up to $\rm 40-100 M_{\odot}$, which are only limited by the effect of radiative feedback.

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arXiv2013

Impact of an accurate modeling of primordial chemistry in high resolution studies

The formation of the first stars in the Universe is regulated by a sensitive interplay of chemistry and cooling with the dynamics of a self-gravitating system. As the outcome of the collapse and the final stellar masses depend sensitively on the thermal evolution, it is necessary to accurately model the thermal evolution in high resolution simulations. As previous investigations raised doubts regarding the convergence of the temperature at high resolution, we investigate the role of the numerical method employed to model the chemistry and the thermodynamics. Here we compare the standard implementation in the adaptive-mesh refinement code \verb|ENZO|, employing a first order backward differentiation formula (BDF), with the 5th order accurate BDF solver \verb|DLSODES|. While the standard implementation in \verb|ENZO| shows a strong dependence on the employed resolution, the results obtained with \verb|DLSODES| are considerably more robust, both with respect to the chemistry and thermodynamics, but also for dynamical quantities such as density, total energy or the accretion rate. We conclude that an accurate modeling of the chemistry and thermodynamics is central for primordial star formation.

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arXiv2013

Black hole formation in the early universe

Supermassive black holes with up to a $\rm 10^{9}~M_{\odot}$ dwell in the centers of present-day galaxies, and their presence has been confirmed at z $\geq$ 6. Their formation at such early epochs is still an enigma. Different pathways have been suggested to assemble supermassive black holes in the first billion years after the Big Bang. Direct collapse has emerged as a highly plausible scenario to form black holes as it provides seed masses of $\rm 10^{5}-10^{6}~M_{\odot}$. Gravitational collapse in atomic cooling haloes with virial temperatures T$_{vir} \geq 10^{4}$~K may lead to the formation of massive seed black holes in the presence of an intense background UV flux. Turbulence plays a central role in regulating accretion and transporting angular momentum. We present here the highest resolution cosmological large-eddy simulations to date which track the evolution of high-density regions on scales of $0.25$~AU beyond the formation of the first peak, and study the impact of subgrid-scale turbulence. The peak density reached in these simulations is $\rm 1.2 \times 10^{-8}~g~cm^{-3}$. Our findings show that while fragmentation occasionally occurs, it does not prevent the growth of a central massive object resulting from turbulent accretion and occasional mergers. The central object reaches $\rm \sim 1000~M_{\odot}$ within $4$ free-fall times, and we expect further growth up to $\rm 10^{6}~M_{\odot}$ through accretion in about 1 million years. The direct collapse model thus provides a viable pathway of forming high-mass black holes at early cosmic times.

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arXiv2013

The small scale dynamo and the amplification of magnetic fields in massive primordial haloes

While present standard model of cosmology yields no clear prediction for the initial magnetic field strength, efficient dynamo action may compensate for initially weak seed fields via rapid amplification. In particular, the small-scale dynamo is expected to exponentially amplify any weak magnetic field in the presence of turbulence. We explore whether this scenario is viable using cosmological magneto-hydrodynamics simulations modeling the formation of the first galaxies, which are expected to form in so-called atomic cooling halos with virial temperatures $\rm T_{vir} \geq 10^{4}$ K. As previous calculations have shown that a high Jeans resolution is needed to resolve turbulent structures and dynamo effects, our calculations employ resolutions of up to 128 cells per Jeans length. The presence of the dynamo can be clearly confirmed for resolutions of at least 64 cells per Jeans length, while saturation occurs at approximate equipartition with turbulent energy. As a result of the large Reynolds numbers in primordial galaxies, we expect saturation to occur at early stages, implying magnetic field strengths of \sim0.1 $μ$G at densities of 10^4 cm^{-3}.

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arXiv2012

High resolution studies of massive primordial haloes

Atomic cooling haloes with virial temperatures $\rm T_{vir} \geq 10^{4}$ K are the most plausible sites for the formation of the first galaxies and the first intermediate mass black holes. It is therefore important to assess whether one can obtain robust results concerning their main properties from numerical simulations. A major uncertainty is the presence of turbulence, which is barely resolved in cosmological simulations. We explore the latter both by pursuing high-resolution simulations with up to 64 cells per Jeans length and by incorporating a subgrid-scale turbulence model to account for turbulent pressure and viscosity on unresolved scales. We find that the main physical quantities in the halo, in particular the density, temperature and energy density profile, are approximately converged. However, the morphologies in the central 500 AU change significantly with increasing resolution and appear considerably more turbulent. In a systematic comparison of three different haloes, we further found that the turbulence subgrid-scale model gives rise to more compact central structures, and decreases the amount of vorticity. Such compact morphologies may in particular favor the accretion onto the central object.

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arXiv2012

The impact of magnetic fields on the IMF in star-forming clouds near a supermassive black hole

Star formation in the centers of galaxies is thought to yield massive stars with a possibly top-heavy stellar mass distribution. It is likely that magnetic fields play a crucial role in the distribution of stellar masses inside star-forming molecular clouds. In this context, we explore the effects of magnetic fields, with a typical field strength of 38 microG, such as in RCW 38, and a field strength of 135 microG, similar to NGC 2024 and the infrared dark cloud G28.34+0.06, on the initial mass function (IMF) near (< 10 pc) a 10^7 solar mass black hole. Using these conditions, we perform a series of numerical simulations with the hydrodynamical code FLASH to elucidate the impact of magnetic fields on the IMF and the star-formation efficiency (SFE) emerging from an 800 solar mass cloud. We find that the collapse of a gravitationally unstable molecular cloud is slowed down with increasing magnetic field strength and that stars form along the field lines. The total number of stars formed during the simulations increases by a factor of 1.5-2 with magnetic fields. The main component of the IMF has a lognormal shape, with its peak shifted to sub-solar (< 0.3 M_sun) masses in the presence of magnetic fields, due to a decrease in the accretion rates from the gas reservoir. In addition, we see a top-heavy, nearly flat IMF above ~2 solar masses, from regions that were supported by magnetic pressure until high masses are reached. We also consider the effects of X-ray irradiation if the central black hole is active. X-ray feedback inhibits the formation of sub-solar masses and decreases the SFEs even further. Thus, the second contribution is no longer visible. We conclude that magnetic fields potentially change the SFE and the IMF both in active and inactive galaxies, and need to be taken into account in such calculations.

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arXiv2012

The implications of dust for high-redshift protogalaxies and the formation of binary disks

Numerical simulations suggest that the first galaxies are formed in protogalactic halos with virial temperatures >= 10^4 K. It is likely that such halos are polluted with trace amounts of metals produced by the first generation of stars. The presence of dust can significantly change the chemistry and dynamics of early galaxies. In this article, we aim to assess the role of dust on the thermal and dynamical evolution of the first galaxies in the presence of a background UV flux, and its implications for the observability of Lyman alpha emitters and sub-mm sources. We have performed high resolution cosmological simulations using the adaptive mesh refinement code FLASH to accomplish this goal. We have developed a chemical network appropriate for these conditions and coupled it with the FLASH code. The main ingredients of our chemical model include the formation of molecules, a multi-level treatment of atomic hydrogen, line trapping of Lyman alpha photons and, photoionization and photodissociation processes in a UV background. We found that the formation of molecules (H_{2} and HD) is significantly enhanced in the presence of dust grains. The presence of a background UV flux strongly influences the formation of molecules by photodissociating them. We explore the evolution after a major merger, leading to the formation of a binary disk. These disks have gas masses of ~10^{7} M_sun at a redshift of 5.4. Each disk lies in a separate subhalo as a result of the merger event. The disks are supported by turbulent pressure due to the highly supersonic turbulence present in the halo. The presence of dust does not significantly reduce the Lyman alpha emission. The emission of Lyman alpha is extended and originates from the envelope of the halo due to line trapping effects. We also find that dust masses of 10^8 M_sun are required to observe the dust continuum emission from z 5 galaxies with ALMA.

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arXiv2008

Effects of primordial chemistry on the cosmic microwave background

Previous works have demonstrated that the generation of secondary CMB anisotropies due to the molecular optical depth is likely too small to be observed. In this paper, we examine additional ways in which primordial chemistry and the dark ages might influence the CMB. We present a detailed and updated chemical network and give an overview of the interactions of molecules with the CMB. We consider the optical depth due to line absorption, photoionization, photodissociation and free-free processes, and estimate the resulting changes in the CMB temperature and its power spectrum. The most promising results are obtained for the negative hydrogen ion \HM and the \HeHII molecule. The free-free process of \HM yields a relative change in the CMB temperature of up to $2\times10^{-11}$, and leads to a frequency-dependent change in the power spectrum of the order $10^{-7}$ at 30 GHz. With a change of the order $10^{-10}$ in the power spectrum, our result for the bound-free process of \HM is significantly below a previous suggestion. \HeHII efficiently scatters CMB photons and smears out primordial fluctuations, leading to a change in the power spectrum of the order $10^{-8}$. Improvements in the accuracy of future CMB experiments may thus help to constrain and finally detect these interesting signals from the dark ages of the universe.

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