4,809 research outputs found
Accretion of Small Satellites and Gas Inflows in a Disc Galaxy
Galaxy interactions can have an important effect in a galaxy's evolution.
Cosmological models predict a large number of small satellites around galaxies.
It is important to study the effect that these small satellites can have on the
host. The present work explores the effect of small N-body spherical satellites
with total mass ratios in the range approx 1:1000-1:100 in inducing gas flows
to the central regions of a disc galaxy with late-type morphology resembling
the Milky Way. Two model galaxies are considered: barred and non-barred models;
the latter one is motivated in order to isolate and understand better the
effects of the satellite. Several circular and non-circular orbits are
explored, considering both prograde and retrogade orientations. We show that
satellites with such small mass ratios can still produce observable distortions
in the gas and stellar components of the galaxy. In terms of gas flows, the
prograde circular orbits are more favourable for producing gas flows, where in
some cases up to $60% of the gas of the galaxy is driven to the central region.
We find, hence, that small satellites can induce significant gas flows to the
central regions of a disc galaxy, which is relevant in the context of fuelling
active galactic nuclei.Comment: Accepted for publication in MNRA
Negotiating Transaction Cost Economics: Oliver Williamson and his audiences
The article studies the interaction between Oliver Williamson and his audiences in the construction of Transaction Cost Economics (TCE). His attentiveness to the feedback from different groups has played a major role in the success of TCE. First we discuss briefly the relevance of rhetoric to the study of economics. Rhetoric stresses that economists talk not to a void, but to peers and lay people with their habits, interests, institutional conditionings and values. Using the toolbox of rhetoric we identify Williamson’s intended audiences. Next we discuss his lists of claimed antecedents and the changes made therein. We explore how those (changing) connections could possibly have incited different audiences. In what follows, we use citation data to delineate his actual readers. This helps compare intended and actual audiences as we close with a discussion of Williamson’s ability to modify his intended reader and widen the audience of TCE in the social sciences.
Semiclassical (Quantum Field Theory) and Quantum (String) de Sitter Regimes: New Results
We compute the quantum string entropy S_s(m, H) from the microscopic string
density of states rho_s (m,H) of mass m in de Sitter space-time. We find for
high m, a {\bf new} phase transition at the critical string temperature T_s=
(1/2 pi k_B)L c^2/alpha', higher than the flat space (Hagedorn) temperature
t_s. (L = c/H, the Hubble constant H acts at the transition as producing a
smaller string constant alpha' and thus, a higher tension). T_s is the precise
quantum dual of the semiclassical (QFT Hawking-Gibbons) de Sitter temperature
T_sem = hbar c /(2\pi k_B L). We find a new formula for the full de Sitter
entropy S_sem (H), as a function of the usual Bekenstein-Hawking entropy
S_sem^(0)(H). For L << l_{Planck}, ie. for low H << c/l_Planck,
S_{sem}^{(0)}(H) is the leading term, but for high H near c/l_Planck, a new
phase transition operates and the whole entropy S_sem (H) is drastically
different from the Bekenstein-Hawking entropy S_sem^(0)(H). We compute the
string quantum emission cross section by a black hole in de Sitter (or
asymptotically de Sitter) space-time (bhdS). For T_sem ~ bhdS << T_s, (early
evaporation stage), it shows the QFT Hawking emission with temperature T_sem ~
bhdS, (semiclassical regime). For T_sem ~ bhdS near T_{s}, it exhibits a phase
transition into a string de Sitter state of size L_s = l_s^2/L}, l_s=
\sqrt{\hbar alpha'/c), and string de Sitter temperature T_s. Instead of
featuring a single pole singularity in the temperature (Carlitz transition), it
features a square root branch point (de Vega-Sanchez transition). New bounds on
the black hole radius r_g emerge in the bhdS string regime: it can become r_g =
L_s/2, or it can reach a more quantum value, r_g = 0.365 l_s.Comment: New original materia
AC-driven double quantum dots as spin pumps and spin filters
We propose and analyze a new scheme of realizing both spin filtering and spin
pumping by using ac-driven double quantum dots in the Coulomb blockade regime.
By calculating the current through the system we demonstrate that the spin
polarization of the current can be controlled by tuning the parameters
(amplitude and frequency) of the ac field. We also discuss spin relaxation and
decoherence effects in the pumped current.Comment: 5 pages, 4 figure
Semiclassical (QFT) and Quantum (String) anti - de Sitter Regimes: New Results
We compute the quantum string entropy S_s(m, H) from the microscopic string
density of states of mass m in Anti de Sitter space-time. For high m, (high Hm
-->c/\alpha'), no phase transition occurs at the Anti de Sitter string
temperature T_{s} which is higher than the flat space (Hagedorn) temperature
t_{s}. (the Hubble constant H acts as producing a smaller string constant and
thus, a higher tension). T_s is the precise quantum dual of the semiclassical
(QFT) Anti de Sitter temperature scale . We compute the quantum string emission
by a black hole in Anti de Sitter space-time (bhAdS). In the early evaporation
stage, it shows the QFT Hawking emission with temperature T_{sem~bhAdS},
(semiclassical regime). For T_{sem~bhAdS}--> T_{s}, it exhibits a phase
transition into a Anti de Sitter string state. New string bounds on the black
hole emerge in the bhAdS string regime. We find a new formula for the full
(quantum regime included) Anti de Sitter entropy S_{sem}, as a function of the
usual Bekenstein-Hawking entropy S_{sem}^(0). For low H (semiclassical regime),
S_{sem}^(0) is the leading term but for high H (quantum regime), no phase
transition operates, in contrast to de Sitter space, and the entropy S_{sem} is
very different from the Bekenstein-Hawking term S_{sem}^(0).Comment: Comments 26 pages; no figure
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