9,684 research outputs found
The effect of baryonic streaming motions on the formation of the first supermassive black holes
Observations of quasars at redshifts z > 6 reveal that 10^9 Msol supermassive
black holes (SMBHs) had already formed when the Universe was < 0.9 Gyr old. One
hypothesis for the origins of these SMBHs is that they grew from the remnants
of the first generation of massive stars, which formed in low-mass (~ 10^5 to
10^6 Msol) dark matter minihaloes at z > 20. This is the regime where baryonic
streaming motions--the relative velocities of baryons with respect to dark
matter in the early Universe--most strongly inhibit star formation by
suppressing gas infall and cooling. We investigate the impact of this effect on
the growth of the first SMBHs using a suite of high-fidelity,
ellipsoidal-collapse Monte Carlo merger-tree simulations. We find that the
suppression of seed BH formation by the streaming motions significantly reduces
the number density of the most massive BHs at z > 15, but the residual effect
at lower redshifts is essentially negligible. The streaming motions can reduce
by a factor of few the number density of the most luminous quasars at z ~
10-11, where such objects could be detected by the James Webb Space Telescope.
We conclude, with minor theoretical caveats, that baryonic streaming motions
are unlikely to pose a significant additional obstacle to the formation of the
observed high-redshift quasar SMBHs. Nor do they appreciably affect the heating
and reionization histories of the Universe or the merger rates of nuclear BHs
in the mass and redshift ranges of interest for proposed gravitational-wave
detectors.Comment: 9 pages, 5 figures, accepted for publication in MNRA
The Impact of Type Ia Supernovae in Quiescent Galaxies: I. Formation of the Multiphase Interstellar medium
A cool phase of the interstellar medium has been observed in many giant
elliptical galaxies, but its origin remains unclear. We propose that uneven
heating from Type Ia supernovae (SNe Ia), together with radiative cooling, can
lead to the formation of the cool phase. The basic idea is that since SNe Ia
explode randomly, gas parcels which are not directly heated by SN shocks will
cool, forming multiphase gas. We run a series of idealized high-resolution
numerical simulations, and find that cool gas develops even when the overall
SNe heating rate exceeds the cooling rate by a factor as large as 1.4.
We also find that the time for multiphase gas development depends on the gas
temperature. When the medium has a temperature K, the cool
phase forms within one cooling time \tc; however, the cool phase formation is
delayed to a few times \tc\ for higher temperatures. The main reason for the
delay is turbulent mixing. Cool gas formed this way would naturally have a
metallicity lower than that of the hot medium. For constant , there is
more turbulent mixing for higher temperature gas. We note that this mechanism
of producing cool gas cannot be captured in cosmological simulations, which
usually fail to resolve individual SN remnants.Comment: 16 pages, 11 figures, published by ApJ. This work is part of the
SMAUG project, see more information at
https://www.simonsfoundation.org/flatiron/center-for-computational-astrophysics/galaxy-formation/smaug/papersplash
K-essence Explains a Lorentz Violation Experiment
Recently, a state of the art experiment shows evidence for Lorentz violation
in the gravitational sector. To explain this experiment, we investigate a
spontaneous Lorentz violation scenario with a generalized scalar field. We find
that when the scalar field is nonminimally coupled to gravity, the Lorentz
violation induces a deformation in the Newtonian potential along the direction
of Lorentz violation.Comment: 8 pages, the final version, discussion and references adde
Supernova Feedback and the Hot Gas Filling Fraction of the Interstellar Medium
Supernovae (SN), the most energetic stellar feedback mechanism, are crucial
for regulating the interstellar medium (ISM) and launching galactic winds. We
explore how supernova remnants (SNRs) create a multiphase medium by performing
3D hydrodynamical simulations at various SN rates, , and ISM average
densities, . The evolution of a SNR in a self-consistently generated
three-phase ISM is qualitatively different from that in a uniform or a
two-phase warm/cold medium. By travelling faster and further in the low-density
hot phase, the domain of a SNR increases by . Varying and
, we find that a steady state can only be achieved when the hot gas volume
fraction . Above that level, overlapping
SNRs render connecting topology of the hot gas, and the ISM is subjected to
thermal runaway. Photoelectric heating (PEH) has a surprisingly strong impact
on . For \bar{n}\gtrsim 3 \cm-3 , a reasonable PEH rate is
able to suppress the thermal runaway. Overall, we determine the critical SN
rate for the onset of thermal runaway to be S_{\rm{crit}} = 200
(\bar{n}/1\cm-3)^k (E_{\rm{SN}}/10^{51}\erg)^{-1} \kpc^{-3} \myr-1, where for and > 1\cm-3 , respectively. We present a
fitting formula of the ISM pressure , ), which can be used as an
effective equation of state in cosmological simulations. Despite the 5 orders
of magnitude span of , the average Mach number varies little:
for the hot, warm
and cold phases, respectively.Comment: 57 pages, 16 figures, 3 tables. ApJ accepte
Shock Waves and Cosmological Matrix Models
We find the shock wave solutions in a class of cosmological backgrounds with
a null singularity, each of these backgrounds admits a matrix description. A
shock wave solution breaks all supersymmetry meanwhile indicates that the
interaction between two static D0-branes cancel, thus provides basic evidence
for the matrix description. The probe action of a D0-brane in the background of
another suggests that the usual perturbative expansion of matrix model breaks
down.Comment: 10 pages, harvmav, v2: some comments on instability added, v3:
version to appear in JHE
Erosion-induced CO2 flux of small watersheds
Soil erosion not only results in severe ecological damage, but also interferes with soil organic carbon formation and decomposition, influencing the global green-house effect. However, there is controversy as to whether a typical small watershed presumed as the basic unit of sediment yield acts as a CO2 sink or source. This paper proposes a discriminant equation for the direction of CO2 flux in small watersheds, basing on the concept of Sediment Delivery Ratio (SDR). Using this equation, watersheds can be classified as Sink Watersheds, Source Watersheds, or Transition Watersheds, noting that small watersheds can act either as a CO2 sink or as a CO2 source. A mathematical model for calculating the two discriminant coefficients in the equation is set up to analyze the conditions under which each type of watershed would occur. After assigning the model parameter values at three levels (low, medium, and high), and considering 486 scenarios in total, the influences are examined for turnover rate of the carbon pool, erosion rate, deposition rate, cultivation depth and period. The effect of adopting conservation measures like residue return, contour farming, terracing, and conservation tillage is also analyzed. The results show that Sink Watersheds are more likely to result in conditions of high erosion rate, long cultivation period, high deposition rate, fast carbon pool turnover rate, and small depth of cultivation; otherwise, Source Watersheds would possibly occur. The results also indicate that residue return and conservation tillage are beneficial for CO2 sequestration. (C) 2012 Elsevier B.V. All rights reserved.Geography, PhysicalGeosciences, MultidisciplinarySCI(E)EI0ARTICLE101-11094-9
Weak gravity conjecture in the asymptotical dS and AdS background
The cosmological observations provide a strong evidence that there is a
positive cosmological constant in our universe and thus the spacetime is
asymptotical de Sitter space. The conjecture of gravity as the weakest force in
the asymptotical dS space leads to a lower bound on the U(1) gauge coupling
, or equivalently, the positive cosmological constant gets an upper bound
in order that the U(1) gauge theory can survive in four
dimensions. This result has a simple explanation in string theory, i.e. the
string scale should not be greater than the size of the
cosmic horizon. Our proposal in string theory can be generalized to U(N) gauge
theory and gives a guideline to the microscopic explanation of the de Sitter
entropy. The similar results are also obtained in the asymptotical anti-de
Sitter space.Comment: 4 pages; version for publication in JHEP (title changed
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