7,731 research outputs found
The Kinematics of Galactic Stellar Disks
The disks of galaxies are primarily stellar systems, and fundamentally
dynamical entities. Thus, to fully understand galactic disks, we must study
their stellar kinematics as well as their morphologies. Observational
techniques have now advanced to a point where quite detailed stellar-kinematic
information can be extracted from spectral observations. This review presents
three illustrative examples of analyses that make use of such information to
study the formation and evolution of these systems: the derivation of the
pattern speed of the bar in NGC 936; the calculation of the complete velocity
ellipsoid of random motions in NGC 488; and the strange phenomenon of
counter-rotation seen in NGC 3593.Comment: 11 pages, LaTeX (including 7 figures), uses paspconf.sty and
epsf.sty, to be published in Proceedings of the EC Summer School on
'Astrophysical Discs', eds J. A. Sellwood and J. Goodman, ASP Conf. Serie
The pattern speed of the bar in NGC 936
We have used the Tremaine-Weinberg method to measure the angular speed of
rotation for the bar in the SB0 galaxy NGC 936. With this technique, the bar's
pattern speed, Omega_p, can be derived from the luminosity and
stellar-kinematic information in long-slit spectral observations taken parallel
to the major axis of the galaxy. The kinematic measurement required is the mean
line-of-sight velocity of all stellar light entering the slit. This quantity
can only be calculated reliably if any asymmetry in the shape of the broadening
function of the spectral lines is also measured, and so we present a method
which allows for such asymmetry. The technique also returns a true measure of
the RMS uncertainty in the estimate. Application of the analysis to a set of
long-slit spectra of NGC 936 returns four separate measures of Omega_p which
are mutually consistent. Combining these data produces a best estimate for the
bar pattern speed of Omega_p = 60 +/- 14 km/s/kpc (assuming a distance of 16.6
Mpc). This result refines the only previous attempt to make this measurement,
which yielded an estimate for Omega_p in NGC 936 of 104 +/- 37 km/s/kpc (Kent
1987). The new measurement places the co-rotation radius just beyond the end of
the bar, in agreement with theoretical calculations.Comment: uuencoded compressed postscript file. 6 pages. Accepted for
publication in MNRAS
Hidden Bars and Boxy Bulges
It has been suggested that the boxy and peanut-shaped bulges found in some
edge-on galaxies are galactic bars viewed from the side. We investigate this
hypothesis by presenting emission-line spectra for a sample of 10 edge-on
galaxies that display a variety of bulge morphologies. To avoid potential
biases in the classification of this morphology, we use an objective measure of
bulge shape. Generally, bulges classified as more boxy show the more
complicated kinematics characteristic of edge-on bars, confirming the intimate
relation between the two phenomena.Comment: 4 pages, 1 figure, to appear in A&ALett. Colour version of figure a
vailable from http://www.astro.rug.nl/~kuijken/nutkinfig2.p
Radar Derived Spatial Statistics of Summer Rain
Data reduction and analysis procedures are discussed along with the physical and statistical descriptors used. The statistical modeling techniques are outlined and examples of the derived statistical characterization of rain cells in terms of the several physical descriptors are presented. Recommendations concerning analyses which can be pursued using the data base collected during the experiment are included
Dissecting the active galactic nucleus in Circinus -- I. Peculiar mid-IR morphology explained by a dusty hollow cone
Recent high angular resolution observations resolved for the first time the
mid-infrared (MIR) structure of nearby active galactic nuclei (AGN).
Surprisingly, they revealed that a major fraction of their MIR emission comes
from the polar regions. This is at odds with the expectation based on AGN
unification, which postulates a dusty torus in the equatorial region. The
nearby, archetypical AGN in the Circinus galaxy offers one of the best
opportunities to study the MIR emission in greater detail. New, high quality
MIR images obtained with the upgraded VISIR instrument at the Very Large
Telescope show that the previously detected bar-like structure extends up to at
least 40 pc on both sides of the nucleus along the edges of the ionization
cone. Motivated by observations across a wide wavelength range and on different
spatial scales, we propose a phenomenological dust emission model for the AGN
in the Circinus galaxy consisting of a compact dusty disk and a large-scale
dusty cone shell, illuminated by a tilted accretion disk with an anisotropic
emission pattern. Undertaking detailed radiative transfer simulations, we
demonstrate that such a model is able to explain the peculiar MIR morphology
and account for the entire IR spectral energy distribution. Our results call
for caution when attributing dust emission of unresolved sources entirely to
the torus and warrant further investigation of the MIR emission in the polar
regions of AGN.Comment: Accepted to MNRAS. Version 2: typos correcte
Information gain versus state disturbance for a single qubit
The trade-off between the information gain and the state disturbance is
derived for quantum operations on a single qubit prepared in a uniformly
distributed pure state. The derivation is valid for a class of measures
quantifying the state disturbance and the information gain which satisfy
certain invariance conditions. This class includes in particular the Shannon
entropy versus the operation fidelity. The central role in the derivation is
played by efficient quantum operations, which leave the system in a pure output
state for any measurement outcome. It is pointed out that the optimality of
efficient quantum operations among those inducing a given operator-valued
measure is related to Davies' characterization of convex invariant functions on
hermitian operators.Comment: 17 pages, LaTeX, osid.sty. Substantially expanded and generalize
Resolving Phonon Fock States in a Multimode Cavity with a Double-Slit Qubit
We resolve phonon number states in the spectrum of a superconducting qubit
coupled to a multimode acoustic cavity. Crucial to this resolution is the sharp
frequency dependence in the qubit-phonon interaction engineered by coupling the
qubit to surface acoustic waves in two locations separated by acoustic
wavelengths. In analogy to double-slit diffraction, the resulting
self-interference generates high-contrast frequency structure in the
qubit-phonon interaction. We observe this frequency structure both in the
coupling rate to multiple cavity modes and in the qubit spontaneous emission
rate into unconfined modes. We use this sharp frequency structure to resolve
single phonons by tuning the qubit to a frequency of destructive interference
where all acoustic interactions are dispersive. By exciting several detuned yet
strongly-coupled phononic modes and measuring the resulting qubit spectrum, we
observe that, for two modes, the device enters the strong dispersive regime
where single phonons are spectrally resolved.Comment: 9 pages, 8 figures; revised arguments in paragraphs 3 and 8, added
Hamiltonian description, and corrected typo
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