2,708 research outputs found
The formation of supermassive black holes in rapidly rotating disks
Massive primordial halos exposed to moderate UV backgrounds are the potential
birthplaces of supermassive black holes. In such a halo, an initially
isothermal collapse will occur, leading to high accretion rates of
~M~yr. During the collapse, the gas in the interior
will turn into a molecular state, and form an accretion disk due to the
conservation of angular momentum. We consider here the structure of such an
accretion disk and the role of viscous heating in the presence of high
accretion rates for a central star of , and ~M. Our
results show that the temperature in the disk increases considerably due to
viscous heating, leading to a transition from the molecular to the atomic
cooling phase. We found that the atomic cooling regime may extend out to
several ~AU for a ~M central star and provides substantial
support to stabilize the disk. It therefore favors the formation of a massive
central object. The comparison of clump migration and contraction time scales
shows that stellar feedback from these clumps may occur during the later stages
of the evolution. Overall, viscous heating provides an important pathway to
obtain an atomic gas phase within the center of the halo, and helps in the
formation of very massive objects. The latter may collapse to form a massive
black hole of about ~M.Comment: Accepted for publication in Astronomy & Astrophysics, comments are
still welcom
Impact of dust cooling on direct collapse black hole formation
Observations of quasars at suggest the presence of black holes with
a few times . Numerous models have been proposed to
explain their existence including the direct collapse which provides massive
seeds of . The isothermal direct collapse requires a strong
Lyman-Werner flux to quench formation in massive primordial halos. In
this study, we explore the impact of trace amounts of metals and dust
enrichment. We perform three dimensional cosmological simulations for two halos
of with illuminated
by an intense Lyman Werner flux of . Our results show that
initially the collapse proceeds isothermally with K but dust
cooling becomes effective at densities of and
brings the gas temperature down to a few 100-1000 K for . No gravitationally bound clumps are found in cases by the end of our simulations in contrast to the case with . Large inflow rates of are
observed for similar to a zero-metallicity case
while for the inflow rate starts to decline earlier
due to the dust cooling and fragmentation. For given large inflow rates a
central star of may form for .Comment: Accepted for publication in ApJ, comments are still welcom
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 . In this study, we explore the impact of such
UV radiation on fragmentation in massive primordial halos of a few times ~M. 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 . 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 at 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 M/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.Comment: Submitted to APJ, comments are welcome. High resolution copy is
available at http://www.astro.physik.uni-goettingen.de/~mlatif/IMBHs_apj.pd
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 M 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 60 K triggering
fragmentation. Based on Jeans estimates the expected stellar masses are about
10 M. Our findings show that the merging scenario is a potential
pathway for the formation of low-mass stars.Comment: Submitted to MNRA
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