Schleicher, Dominik R. G.

[["dc.bibliographiccitation.artnumber","A22"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","572"],["dc.contributor.author","van Borm, C."],["dc.contributor.author","Bovino, Stefano"],["dc.contributor.author","Latif, A. H. M. Mahbub"],["dc.contributor.author","Schleicher, Dominik R. G."],["dc.contributor.author","Spaans, M."],["dc.contributor.author","Grassi, T."],["dc.date.accessioned","2018-11-07T09:31:57Z"],["dc.date.available","2018-11-07T09:31:57Z"],["dc.date.issued","2014"],["dc.description.abstract","Context. The seeds of the first supermassive black holes may have resulted from the direct collapse of hot primordial gas in greater than or similar to 10(4) K haloes, forming a supermassive or quasi- star as an intermediate stage. Aims. We explore the formation of a protostar resulting from the collapse of primordial gas in the presence of a strong Lyman- Werner radiation background. Particularly, we investigate the impact of turbulence and rotation on the fragmentation behaviour of the gas cloud. We accomplish this goal by varying the initial turbulent and rotational velocities. Methods. We performed 3D adaptive mesh refinement simulations with a resolution of 64 cells per Jeans length using the ENZO code, simulating the formation of a protostar up to unprecedentedly high central densities of 10(21) cm(-3) and spatial scales of a few solar radii. To achieve this goal, we employed the KROME package to improve modelling of the chemical and thermal processes. Results. We find that the physical properties of the simulated gas clouds become similar on small scales, irrespective of the initial amount of turbulence and rotation. After the highest level of refinement was reached, the simulations have been evolved for an additional similar to 5 freefall times. A single bound clump with a radius of 2 x 10(-2) AU and a mass of similar to 7 x 10(-2) M fi is formed at the end of each simulation, marking the onset of protostar formation. No strong fragmentation is observed by the end of the simulations, regardless of the initial amount of turbulence or rotation, and high accretion rates of a few solar masses per year are found. Conclusions. Given such high accretion rates, a quasi- star of 105 M fi is expected to form within 105 years."],["dc.description.sponsorship","Deutsche Forschungsgemeinschaft (DFG) [SFB 963 / 1]"],["dc.identifier.doi","10.1051/0004-6361/201424658"],["dc.identifier.fs","609689"],["dc.identifier.isi","000346101700037"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/11406"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/31640"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","1432-0746"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Effects of turbulence and rotation on protostar formation as a precursor of massive black holes"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A46"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","545"],["dc.contributor.author","Hocuk, S."],["dc.contributor.author","Schleicher, Dominik R. G."],["dc.contributor.author","Spaans, M."],["dc.contributor.author","Cazaux, S."],["dc.date.accessioned","2018-11-07T09:06:06Z"],["dc.date.available","2018-11-07T09:06:06Z"],["dc.date.issued","2012"],["dc.description.abstract","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 mu G, such as in RCW 38, and a field strength of 135 mu G, 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-circle dot) 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 similar to 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. The presence of a flat component of the IMF would be a particularly relevant signature for the importance of magnetic fields, as it is usually not found in hydrodynamical simulations."],["dc.identifier.doi","10.1051/0004-6361/201219628"],["dc.identifier.fs","596754"],["dc.identifier.isi","000309254900046"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9614"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/25480"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","The impact of magnetic fields on the IMF in star-forming clouds near a supermassive black hole"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","3163"],["dc.bibliographiccitation.issue","3"],["dc.bibliographiccitation.journal","Monthly Notices of the Royal Astronomical Society"],["dc.bibliographiccitation.lastpage","3177"],["dc.bibliographiccitation.volume","446"],["dc.contributor.author","Latif, A. H. M. Mahbub"],["dc.contributor.author","Bovino, Stefano"],["dc.contributor.author","Grassi, T."],["dc.contributor.author","Schleicher, Dominik R. G."],["dc.contributor.author","Spaans, M."],["dc.date.accessioned","2018-11-07T10:02:02Z"],["dc.date.available","2018-11-07T10:02:02Z"],["dc.date.issued","2015"],["dc.description.abstract","Observations of high-redshift quasars at z > 6 indicate that they harbour supermassive black holes (SMBHs) of a billion solar masses. The direct collapse scenario has emerged as the most plausible way to assemble SMBHs. The nurseries for the direct collapse black holes are massive primordial haloes illuminated with an intense UV flux emitted by Population II (Pop II) stars. In this study, we compute the critical value of such a flux (J(21)(crit)) for realistic spectra of Pop II stars through three-dimensional cosmological simulations. We derive the dependence of J(21)(crit) on the radiation spectra, on variations from halo to halo, and on the impact of X-ray ionization. Our findings show that the value of J(21)(crit) is a few times 10(4) and only weakly depends on the adopted radiation spectra in the range between T-rad = 2 x 10(4) and 10(5) K. For three simulated haloes of a few times 10(7) M-circle dot, J(21)(crit) varies from 2 x 10(4) to 5 x 10(4). The impact of X-ray ionization is almost negligible and within the expected scatter of J(21)(crit) for background fluxes of J(X, 21) <= 0.1. The computed estimates of J(21)(crit) have profound implications for the quasar abundance at z = 10 as it lowers the number density of black holes forming through an isothermal direct collapse by a few orders of magnitude below the observed black hole density. However, the sites with moderate amounts of H-2 cooling may still form massive objects sufficient to be compatible with observations."],["dc.identifier.doi","10.1093/mnras/stu2244"],["dc.identifier.isi","000350272300076"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/38147"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Oxford Univ Press"],["dc.relation.issn","1365-2966"],["dc.relation.issn","0035-8711"],["dc.title","How realistic UV spectra and X-rays suppress the abundance of direct collapse black holes"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A101"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","540"],["dc.contributor.author","Latif, A. H. M. Mahbub"],["dc.contributor.author","Schleicher, Dominik R. G."],["dc.contributor.author","Spaans, M."],["dc.date.accessioned","2018-11-07T09:11:27Z"],["dc.date.available","2018-11-07T09:11:27Z"],["dc.date.issued","2012"],["dc.description.abstract","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 (both in the gas phase and on dust grains), a multi-level treatment of atomic hydrogen, line trapping of Ly-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 as compared to only gas phase reactions by up to two orders of magnitude. The presence of dust may thus establish a molecular ISM in high-redshift galaxies. 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 similar to 10(7) M-circle dot 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. For values of J(21) = 1000 (internal flux), we find that fragmentation may be enhanced due to thermal instabilities in the hot gas. The presence of dust does not significantly reduce the Ly-alpha emission. The emission of Ly-alpha is extended and originates from the envelope of the halo due to line trapping effects. We also find that dust masses of a few x10(8) M-circle dot are required to observe the dust continuum emission from z similar to 5 galaxies with ALMA."],["dc.identifier.doi","10.1051/0004-6361/201118295"],["dc.identifier.fs","596713"],["dc.identifier.isi","000303315400115"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9598"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/26725"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","The implications of dust for high-redshift protogalaxies and the formation of binary disks"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","1979"],["dc.bibliographiccitation.issue","3"],["dc.bibliographiccitation.journal","Monthly Notices of the Royal Astronomical Society"],["dc.bibliographiccitation.lastpage","1987"],["dc.bibliographiccitation.volume","443"],["dc.contributor.author","Latif, A. H. M. Mahbub"],["dc.contributor.author","Bovino, Stefano"],["dc.contributor.author","van Borm, C."],["dc.contributor.author","Grassi, T."],["dc.contributor.author","Schleicher, Dominik R. G."],["dc.contributor.author","Spaans, M."],["dc.date.accessioned","2018-11-07T09:35:06Z"],["dc.date.available","2018-11-07T09:35:06Z"],["dc.date.issued","2014"],["dc.description.abstract","The ability of metal-free gas to cool by molecular hydrogen in primordial haloes is strongly associated with the strength of ultraviolet (UV) flux produced by the stellar populations in the first galaxies. Depending on the stellar spectrum, these UV photons can either dissociate H-2 molecules directly or indirectly by photodetachment of H- as the latter provides the main pathway for H2 formation in the early universe. In this study, we aim to determine the critical strength of the UV flux above which the formation of molecular hydrogen remains suppressed for a sample of five distinct haloes at z > 10 by employing a higher order chemical solver and a Jeans resolution of 32 cells. We presume that such flux is emitted by Pop II stars implying atmospheric temperatures of 104 K. We performed three-dimensional cosmological simulations and varied the strength of the UV flux below the Lyman limit in units of J(21). Our findings show that the value of J(21)(crit) varies from halo to halo and is sensitive to the local thermal conditions of the gas. For the simulated haloes, it varies from 400 to 700 with the exception of one halo where J(21)(crit) >= 1500. This has important implications for the formation of direct collapse black holes and their estimated population at z > 6. It reduces the number density of direct collapse black holes by almost three orders of magnitude compared to the previous estimates."],["dc.identifier.doi","10.1093/mnras/stu1230"],["dc.identifier.isi","000342920400011"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/32320"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Oxford Univ Press"],["dc.relation.issn","1365-2966"],["dc.relation.issn","0035-8711"],["dc.title","A UV flux constraint on the formation of direct collapse black holes"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","78"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","The Astrophysical Journal"],["dc.bibliographiccitation.volume","792"],["dc.contributor.author","Latif, A. H. M. Mahbub"],["dc.contributor.author","Schleicher, Dominik R. G."],["dc.contributor.author","Bovino, Stefano"],["dc.contributor.author","Grassi, T."],["dc.contributor.author","Spaans, M."],["dc.date.accessioned","2018-11-07T09:36:05Z"],["dc.date.available","2018-11-07T09:36:05Z"],["dc.date.issued","2014"],["dc.description.abstract","Radiative feedback produced by stellar populations played a vital role in early structure formation. In particular, 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 photodetachment of H-. In this study, we explore the impact of such UV radiation on fragmentation in massive primordial halos of a few times 10(7) M-circle dot. 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 J(21) assuming a blackbody radiation spectrum with a characteristic temperature of T-rad = 10(4) K. We further make use of sink particles to follow the evolution for 10,000 yr 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 10(2)-10(4) solar mass protostars are formed when halos are irradiated by J(21) = 10-500 at z > 10 and suggest a strong relation between the strength of the UV flux and mass of a protostar. This mode of star formation is quite different from minihalos, as higher accretion rates of about 0.01-0.1 M-circle dot yr(-1) are observed by the end of our simulations. The resulting massive stars are 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."],["dc.identifier.doi","10.1088/0004-637X/792/1/78"],["dc.identifier.isi","000341172100078"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/32532"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Iop Publishing Ltd"],["dc.relation.issn","1538-4357"],["dc.relation.issn","0004-637X"],["dc.title","THE FORMATION OF MASSIVE PRIMORDIAL STARS IN THE PRESENCE OF MODERATE UV BACKGROUNDS"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]