3,581 research outputs found
The Formation of Galactic Bulges
We summarise some recent results about nearby galactic bulges that are
relevant to their formation. We highlight a number of significant advances in
our understanding of the surface brightness profiles, stellar populations, and
especially the very centers of spiral galaxies. We also view our own Milky Way
as if it were an external galaxy. Our main conclusions are that bulges of
early-type spirals (S0 -- Sb) contain central nuclear components, just like
late-type spirals and most other types of galaxies. The luminosities of these
central components correlate best with total bulge luminosity, and not as well
with morphological type. Bulges of early-type spiral galaxies follow the
fundamental plane and the colour/line strength vs. luminosity relations of
elliptical galaxies. Although we have a reasonable idea about bulges of
early-type spirals we know much less about late-type bulges. However, the close
resemblance of our Milky Way Bulge to bulges in external disk galaxies makes us
suspect that bulges of late-type spirals might be very similar as well.Comment: 10 pages, invited review presented at 'Baryons in Dark Matter',
Novigrad Oct 5-9, 2004; eds. R. Dettmar, U. Klein and P. Salucci. On-line
publication at http://pos.sissa.i
Mean field convergence of a model of multiple TCP connections through a buffer implementing RED
RED (Random Early Detection) has been suggested when multiple TCP sessions
are multiplexed through a bottleneck buffer. The idea is to detect congestion
before the buffer overflows by dropping or marking packets with a probability
that increases with the queue length. The objectives are reduced packet loss,
higher throughput, reduced delay and reduced delay variation achieved through
an equitable distribution of packet loss and reduced synchronization. Baccelli,
McDonald and Reynier [Performance Evaluation 11 (2002) 77--97] have proposed a
fluid model for multiple TCP connections in the congestion avoidance regime
multiplexed through a bottleneck buffer implementing RED. The window sizes of
each TCP session evolve like independent dynamical systems coupled by the queue
length at the buffer. The key idea in [Performance Evaluation 11 (2002) 77--97]
is to consider the histogram of window sizes as a random measure coupled with
the queue. Here we prove the conjecture made in [Performance Evaluation 11
(2002) 77--97] that, as the number of connections tends to infinity, this
system converges to a deterministic mean-field limit comprising the window size
density coupled with a deterministic queue.Comment: Published at http://dx.doi.org/10.1214/105051605000000700 in the
Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute
of Mathematical Statistics (http://www.imstat.org
Thermodynamics of Lithium Intercalation into Graphites and Disordered Carbons
The temperature dependence of the open-circuit potential of lithium half-cells was measured for electrodes of carbon materials having different amounts of structural disorder. The entropy of lithium intercalation, DeltaS, and enthalpy of intercalation, DeltaH, were determined over a broad range of lithium concentrations. For the disordered carbons, DeltaS is small. For graphite, an initially large DeltaS decreases with lithium concentration, becomes negative, and then shows two plateaus associated with the formation of intercalation compounds. For all carbons DeltaH is negative, and decreases in magnitude with increased lithium concentration. For lithium concentrations less than x = 0.5 in LixC6, for the disordered carbons the magnitude of DeltaH is significantly more negative than for graphite (i.e., intercalation is more exothermic). The measurements of DeltaH provide an energy spectrum of chemical environments for lithium. This spectrum can be used to understand some of the concentration dependence of configurational entropy, but the negative values of DeltaS require another contribution to entropy, perhaps vibrational in origin
Synthesis of Data Word Transducers
In reactive synthesis, the goal is to automatically generate an
implementation from a specification of the reactive and non-terminating
input/output behaviours of a system. Specifications are usually modelled as
logical formulae or automata over infinite sequences of signals
(-words), while implementations are represented as transducers. In the
classical setting, the set of signals is assumed to be finite. In this paper,
we consider data -words instead, i.e., words over an infinite alphabet.
In this context, we study specifications and implementations respectively given
as automata and transducers extended with a finite set of registers. We
consider different instances, depending on whether the specification is
nondeterministic, universal or deterministic, and depending on whether the
number of registers of the implementation is given or not.
In the unbounded setting, we show undecidability for both universal and
nondeterministic specifications, while decidability is recovered in the
deterministic case. In the bounded setting, undecidability still holds for
nondeterministic specifications, but can be recovered by disallowing tests over
input data. The generic technique we use to show the latter result allows us to
reprove some known result, namely decidability of bounded synthesis for
universal specifications
Deficiency of `Thin' Stellar Bars in Seyfert Host Galaxies
Using all available major samples of Seyfert galaxies and their corresponding
control samples of closely matched non-active galaxies, we find that the bar
ellipticities (or axial ratios) in Seyfert galaxies are systematically
different from those in non-active galaxies. Overall, there is a deficiency of
bars with large ellipticities (i.e., `fat' or `weak' bars) in Seyferts,
compared to non-active galaxies. Accompanied with a large dispersion due to
small number statistics, this effect is strictly speaking at the `2sigma'
level. To obtain this result, the active galaxy samples of near-infrared
surface photometry were matched to those of normal galaxies in type, host
galaxy ellipticity, absolute magnitude, and, to some extent, in redshift. We
discuss possible theoretical explanations of this phenomenon within the
framework of galactic evolution, and, in particular, of radial gas
redistribution in barred galaxies. Our conclusions provide further evidence
that Seyfert hosts differ systematically from their non-active counterparts on
scales of a few kpc.Comment: Astrophysical Journal Letters, in press. Latex, 2 postscript figure
Velocity Fields of Spiral Galaxies in z~0.5 Clusters
Spiral galaxies can be affected by interactions in clusters, that also may
distort the internal velocity field. If unrecognized from single-slit
spectroscopy, this could lead to a wrong determination of the maximum rotation
velocity as pointed out by Ziegler et al. (2003). This parameter directly
enters into the Tully-Fisher relation, an important tool to investigate the
evolution of spiral galaxies. To overcome this problem, we measure the
2D-velocity fields by observing three different slit positions per galaxy using
FORS2 at the VLT providing us with full coverage of each galaxy and an adequate
spatial resolution. The kinematic properties are compared to structural
features determined on the HST/ACS images to assess possible interaction
processes. As a next step, the whole analysis will be performed for three more
clusters, so that we will be able to establish a high-accuracy TFR for spirals
at z~0.5.Comment: 2 pages, 2 figures, going to be published in the proceedings of the
IAU Symp. 241, "Stellar Populations as Building Blocks of Galaxies
- …