24,647 research outputs found
Spatially resolved kinematics of the central regions of M83: hidden mass signatures and the role of supernovae
The barred grand-design spiral M83 (NGC 5236) is one of the most studied
galaxies given its proximity, orientation, and particular complexity.
Nonetheless, many aspects of the central regions remain controversial conveying
our limited understanding of the inner gas and stellar kinematics, and
ultimately of the nucleus evolution.
In this work, we present AO VLT-SINFONI data of its central ~235x140 pc with
an unprecedented spatial resolution of ~0.2 arcsec, corresponding to ~4 pc. We
have focused our study on the distribution and kinematics of the stars and the
ionised and molecular gas by studying in detail the Pa_alpha and Br_gamma
emission, the H_2 1-0S(1) line at 2.122 micron and the [FeII] line at 1.644
micron, together with the CO absorption bands at 2.293 micron and 2.323 micron.
Our results reveal a complex situation where the gas and stellar kinematics are
totally unrelated. Supernova explosions play an important role in shaping the
gas kinematics, dominated by shocks and inflows at scales of tens of parsecs
that make them unsuitable to derive general dynamical properties.
We propose that the location of the nucleus of M83 is unlikely to be related
to the off-centre 'optical nucleus'. The study of the stellar kinematics
reveals that the optical nucleus is a gravitationally bound massive star
cluster with M_dyn = (1.1 \pm 0.4)x10^7 M_sun, formed by a past starburst. The
kinematic and photometric analysis of the cluster yield that the stellar
content of the cluster is well described by an intermediate age population of
log T(yr) = 8.0\pm0.4, with a mass of M \simeq (7.8\pm2.4)x10^6 M_sun.Comment: 14 pages, 10 figures, accepted for publication in Ap
Normal form decomposition for Gaussian-to-Gaussian superoperators
In this paper we explore the set of linear maps sending the set of quantum
Gaussian states into itself. These maps are in general not positive, a feature
which can be exploited as a test to check whether a given quantum state belongs
to the convex hull of Gaussian states (if one of the considered maps sends it
into a non positive operator, the above state is certified not to belong to the
set). Generalizing a result known to be valid under the assumption of complete
positivity, we provide a characterization of these Gaussian-to-Gaussian (not
necessarily positive) superoperators in terms of their action on the
characteristic function of the inputs. For the special case of one-mode
mappings we also show that any Gaussian-to-Gaussian superoperator can be
expressed as a concatenation of a phase-space dilatation, followed by the
action of a completely positive Gaussian channel, possibly composed with a
transposition. While a similar decomposition is shown to fail in the multi-mode
scenario, we prove that it still holds at least under the further hypothesis of
homogeneous action on the covariance matrix
Derivation of some translation-invariant Lindblad equations for a quantum Brownian particle
We study the dynamics of a Brownian quantum particle hopping on an infinite
lattice with a spin degree of freedom. This particle is coupled to free boson
gases via a translation-invariant Hamiltonian which is linear in the creation
and annihilation operators of the bosons. We derive the time evolution of the
reduced density matrix of the particle in the van Hove limit in which we also
rescale the hopping rate. This corresponds to a situation in which both the
system-bath interactions and the hopping between neighboring sites are small
and they are effective on the same time scale. The reduced evolution is given
by a translation-invariant Lindblad master equation which is derived
explicitly.Comment: 28 pages, 4 figures, minor revisio
The Planck Surveyor mission: astrophysical prospects
Although the Planck Surveyor mission is optimized to map the cosmic microwave
background anisotropies, it will also provide extremely valuable information on
astrophysical phenomena. We review our present understanding of Galactic and
extragalactic foregrounds relevant to the mission and discuss on one side,
Planck's impact on the study of their properties and, on the other side, to
what extent foreground contamination may affect Planck's ability to accurately
determine cosmological parameters. Planck's multifrequency surveys will be
unique in their coverage of large areas of the sky (actually, of the full sky);
this will extend by two or more orders of magnitude the flux density interval
over which mm/sub-mm counts of extragalactic sources can be determined by
instruments already available (like SCUBA) or planned for the next decade (like
the LSA-MMA or the space mission FIRST), which go much deeper but over very
limited areas. Planck will thus provide essential complementary information on
the epoch-dependent luminosity functions. Bright radio sources will be studied
over a poorly explored frequency range where spectral signatures, essential to
understand the physical processes that are going on, show up. The
Sunyaev-Zeldovich effect, with its extremely rich information content, will be
observed in the direction of a large number of rich clusters of Galaxies.
Thanks again to its all sky coverage, Planck will provide unique information on
the structure and on the emission properties of the interstellar medium in the
Galaxy. At the same time, the foregrounds are unlikely to substantially limit
Planck's ability to measure the cosmological signals. Even measurements of
polarization of the primordial Cosmic Microwave background fluctuations appear
to be feasible.Comment: 20 pages, Latex (use aipproc2.sty, aipproc2.cls, epsfig.sty), 10
PostScript figures; invited review talk, Proc. of the Conference: "3 K
Cosmology", Roma, Italy, 5-10 October 1998, AIP Conference Proc, in press
Note: Figures 6 and 7 have been replaced by new and correct version
The Planetary Nebulae Population in the Nuclear Regions of M31: the SAURON view
Following a first study of the central regions of M32 that illustrated the
power of integral-field spectroscopy (IFS) in detecting and measuring the [O
III]{\lambda}5007 emission of PNe against a strong stellar background, we turn
to the very nuclear PN population of M31, within 80 pc of its centre. We show
that PNe can also be found in the presence of emission from diffuse gas and
further illustrate the excellent sensitivity of IFS in detecting extragalactic
PNe through a comparison with narrowband images obtained with the Hubble Space
Telescope. We find the nuclear PNe population of M31 is only marginally
consistent with the generally adopted form of the PNe luminosity function
(PNLF). In particular, this is due to a lack of PNe with absolute magnitude
M5007 brighter than -3, which would only result from a rather unfortunate draw
from such a model PNLF. We suggest that the observed lack of bright PNe in the
nuclear regions of M31 is due to a horizontal-branch population that is more
tilted toward less massive and hotter He-burning stars, so that its progeny
consists mostly of UV-bright stars that fail to climb back up the asymptotic
giant branch (AGB) and only of few, if any, bright PNe powered by central
post-AGB stars. These results are also consistent with recent reports on a
dearth of bright post-AGB stars towards the nucleus of M31, and lend further
support to the idea that the metallicity of a stellar population has an impact
on the way the horizontal branch is populated and to the loose anticorrelation
between the strength of the UV-upturn and the specific number of PNe that is
observed in early-type galaxies. Finally, our investigation also serves to
stress the importance of considering the same spatial scales when comparing the
PNe population of galaxies with the properties of their stellar populations.Comment: 11 pages, 10 figures, accepted for publication on Monthly Notices of
the Royal Astronomical Societ
The Energy-Momentum Tensor in Fulling-Rindler Vacuum
The energy density in Fulling-Rindler vacuum, which is known to be negative
"everywhere" is shown to be positive and singular on the horizons in such a
fashion as to guarantee the positivity of the total energy. The mechanism of
compensation is displayed in detail.Comment: 9 pages, ULB-TH-15/9
Surface Shape and Local Critical Behaviour in Two-Dimensional Directed Percolation
Two-dimensional directed site percolation is studied in systems directed
along the x-axis and limited by a free surface at y=\pm Cx^k. Scaling
considerations show that the surface is a relevant perturbation to the local
critical behaviour when k<1/z where z=\nu_\parallel/\nu is the dynamical
exponent. The tip-to-bulk order parameter correlation function is calculated in
the mean-field approximation. The tip percolation probability and the fractal
dimensions of critical clusters are obtained through Monte-Carlo simulations.
The tip order parameter has a nonuniversal, C-dependent, scaling dimension in
the marginal case, k=1/z, and displays a stretched exponential behaviour when
the perturbation is relevant. The k-dependence of the fractal dimensions in the
relevant case is in agreement with the results of a blob picture approach.Comment: 13 pages, Plain TeX file, epsf, 6 postscript-figures, minor
correction
The circumstellar disk, envelope, and bi-polar outflow of the Massive Young Stellar Object W33A
The Young Stellar Object (YSO) W33A is one of the best known examples of a
massive star still in the process of forming. Here we present Gemini North
ALTAIR/NIFS laser-guide star adaptive-optics assisted K-band integral-field
spectroscopy of W33A and its inner reflection nebula. In our data we make the
first detections of a rotationally-flattened outer envelope and fast bi-polar
jet of a massive YSO at near-infrared wavelengths. The predominant spectral
features observed are Br-gamma, H_2, and a combination of emission and
absorption from CO gas. We perform a 3-D spectro-astrometric analysis of the
line emission, the first study of its kind. We find that the object's Br-gamma
emission reveals evidence for a fast bi-polar jet on sub-milliarcsecond scales,
which is aligned with the larger-scale outflow. The hybrid CO features can be
explained as a combination of hot CO emission arising in a disk close to the
central star, while cold CO absorption originates in the cooler outer envelope.
Kinematic analysis of these features reveals that both structures are rotating,
and consistent with being aligned perpendicularly to both the ionised jet and
the large-scale outflow. Assuming Keplerian rotation, we find that the
circumstellar disk orbits a central mass of >10Msun, while the outer envelope
encloses a mass of ~15Msun. Our results suggest a scenario of a central star
accreting material from a circumstellar disk at the centre of a cool extended
rotating torus, while driving a fast bi-polar wind. These results therefore
provide strong supporting evidence for the hypothesis that the formation
mechanism for high-mass stars is qualitatively similar to that of low-mass
stars.Comment: 13 pages, 18 figs. Accepted for publication in MNRA
Electronic states and transport properties in the Kronig-Penney model with correlated compositional and structural disorder
We study the structure of the electronic states and the transport properties
of a Kronig-Penney model with weak compositional and structural disorder. Using
a perturbative approach we obtain an analytical expression for the localisation
length which is valid for disorder with arbitrary correlations. We show how to
generate disorder with self- and cross-correlations and we analyse both the
known delocalisation effects of the long-range self-correlations and new
effects produced by cross-correlations. We finally discuss how both kinds of
correlations alter the transport properties in Kronig-Penney models of finite
size.Comment: 23 pages, 5 figure
- …