8,161 research outputs found
Structural properties of disk galaxies I. The intrinsic ellipticity of bulges
(Abridged) A variety of formation scenarios was proposed to explain the
diversity of properties observed in bulges. Studying their intrinsic shape can
help in constraining the dominant mechanism at the epochs of their assembly.
The structural parameters of a magnitude-limited sample of 148 unbarred S0--Sb
galaxies were derived in order to study the correlations between bulges and
disks as well as the probability distribution function (PDF) of the intrinsic
equatorial ellipticity of bulges. It is presented a new fitting algorithm
(GASP2D) to perform the two-dimensional photometric decomposition of galaxy
surface-brightness distribution. This was assumed to be the sum of the
contribution of a bulge and disk component characterized by elliptical and
concentric isophotes with constant (but possibly different) ellipticity and
position angles. Bulge and disk parameters of the sample galaxies were derived
from the J-band images which were available in the Two Micron All Sky Survey.
The PDF of the equatorial ellipticity of the bulges was derived from the
distribution of the observed ellipticities of bulges and misalignments between
bulges and disks. Strong correlations between the bulge and disk parameters
were found. About 80% of bulges in unbarred lenticular and
early-to-intermediate spiral galaxies are not oblate but triaxial ellipsoids.
Their mean axial ratio in the equatorial plane is = 0.85. There is not
significant dependence of their PDF on morphology, light concentration, and
luminosity. The interplay between bulge and disk parameters favors scenarios in
which bulges assembled from mergers and/or grew over long times through disk
secular evolution. But all these mechanisms have to be tested against the
derived distribution of bulge intrinsic ellipticities.Comment: 24 pages, 13 figures, accepted for publication in A&A, corrected
proof
Drastic Reduction of Shot Noise in Semiconductor Superlattices
We have found experimentally that the shot noise of the tunneling current
through an undoped semiconductor superlattice is reduced with respect to the
Poissonian noise value , and that the noise approaches 1/3 of that value
in superlattices whose quantum wells are strongly coupled. On the other hand,
when the coupling is weak or when a strong electric field is applied to the
superlattice the noise becomes Poissonian. Although our results are
qualitatively consistent with existing theories for one-dimensional mulitple
barriers, the theories cannot account for the dependence of the noise on
superlattice parameters that we have observed.Comment: 4 Pages, 3Figure
Shot Noise Enhancement in Resonant Tunneling Structures in a Magnetic Field
We have observed that the shot noise of tunnel current, I, in
GaSb-AlSb-InAs-AlSb-GaSb double-barrier structure under a magnetic field can
exceed 2qI. The measurements were done at T=4K in fields up to 5T parallel to
the current. The noise enhancement occurred at each of the several
negative-differential conductance regions induced by the tunneling of holes
through Landau levels in the InAs quantum well. The amount of the enhancement
increased with the strength of the negative conductance and reached values up
to 8qI. These results are explained qualitatively by fluctuations of the
density of states in the well, but point out the need for a detailed theory of
shot noise enhancement in resonant tunneling devices.Comment: 4 pages, RevTex, 3 figure
The relation between bar formation, galaxy luminosity, and environment
We derive the bar fraction in three different environments ranging from the
field to Virgo and Coma clusters, covering an unprecedentedly large range of
galaxy luminosities (or, equivalently, stellar masses). We confirm that the
fraction of barred galaxies strongly depends on galaxy luminosity. We also show
that the difference between the bar fraction distributions as a function of
galaxy luminosity (and mass) in the field and Coma cluster are statistically
significant, with Virgo being an intermediate case. We interpret this result as
a variation of the effect of environment on bar formation depending on galaxy
luminosity. We speculate that brighter disk galaxies are stable enough against
interactions to keep their cold structure, thus, the interactions are able to
trigger bar formation. For fainter galaxies the interactions become strong
enough to heat up the disks inhibiting bar formation and even destroying the
disks. Finally, we point out that the controversy regarding whether the bar
fraction depends on environment could be resolved by taking into account the
different luminosity ranges of the galaxy samples studied so far.Comment: 4 pages, 2 figures. To appear in the proceedings of EWASS 2012
Special Session 4, Structure of galaxy disks shaped by secular evolution and
environmental processes, ed. P. Di Matteo and C. Jog, Memorie della Societ\`a
Astronomica Italiana Supplement Serie
The intrinsic three-dimensional shape of galactic bars
We present the first statistical study on the intrinsic three-dimensional
(3D) shape of a sample of 83 galactic bars extracted from the CALIFA survey. We
use the galaXYZ code to derive the bar intrinsic shape with a statistical
approach. The method uses only the geometric information (ellipticities and
position angles) of bars and discs obtained from a multi-component photometric
decomposition of the galaxy surface-brightness distributions. We find that bars
are predominantly prolate-triaxial ellipsoids (68%), with a small fraction of
oblate-triaxial ellipsoids (32%). The typical flattening (intrinsic C/A
semiaxis ratio) of the bars in our sample is 0.34, which matches well the
typical intrinsic flattening of stellar discs at these galaxy masses. We
demonstrate that, for prolate-triaxial bars, the intrinsic shape of bars
depends on the galaxy Hubble type and stellar mass (bars in massive S0 galaxies
are thicker and more circular than those in less massive spirals). The bar
intrinsic shape correlates with bulge, disc, and bar parameters. In particular
with the bulge-to-total (B/T) luminosity ratio, disc g-r color, and central
surface brightness of the bar, confirming the tight link between bars and their
host galaxies. Combining the probability distributions of the intrinsic shape
of bulges and bars in our sample we show that 52% (16%) of bulges are thicker
(flatter) than the surrounding bar at 1 level. We suggest that these
percentages might be representative of the fraction of classical and disc-like
bulges in our sample, respectively.Comment: 18 pages, 11 figures, accepted for publication in MNRA
The Burst Spectra of EXO 0748-676 during a Long 2003 XMM-Newton Observation
Gravitationally redshifted absorption lines from highly ionized iron have
been previously identified in the burst spectra of the neutron star in EXO
0748-676. To repeat this detection we obtained a long, nearly 600 ks
observation of the source with XMM-Newton in 2003. The spectral features seen
in the burst spectra from the initial data are not reproduced in the burst
spectra from this new data. In this paper we present the spectra from the 2003
observations and discuss the sensitivity of the absorption structure to changes
in the photospheric conditions.Comment: 18 Pages, 3 Figures. Accepted for publication in Ap
Peanut-shaped bulges in face-on disk galaxies
We present high resolution absorption-line spectroscopy of 3 face-on
galaxies, NGC 98, NGC 600, and NGC 1703 with the aim of searching for
box/peanut (B/P)-shaped bulges. These observations test and confirm the
prediction of Debattista et al. (2005) that face-on B/P-shaped bulges can be
recognized by a double minimum in the profile of the fourth-order Gauss-Hermite
moment h_4. In NGC 1703, which is an unbarred control galaxy, we found no
evidence of a B/P bulge. In NGC 98, a clear double minimum in h_4 is present
along the major axis of the bar and before the end of the bar, as predicted. In
contrast, in NGC 600, which is also a barred galaxy but lacks a substantial
bulge, we do not find a significant B/P shape.Comment: 4 pages, 1 figure. To appear in "Tumbling, twisting, and winding
galaxies: Pattern speeds along the Hubble sequence", E. M. Corsini and V. P.
Debattista (eds.), Memorie della Societa` Astronomica Italian
Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells
We have shown experimentally that an electric field decreases the energy
separation between the two components of a dense spin-polarized exciton gas in
a coupled double quantum well, from a maximum splitting of meV to
zero, at a field of 35 kV/cm. This decrease, due to the field-induced
deformation of the exciton wavefunction, is explained by an existing
calculation of the change in the spin-dependent exciton-exciton interaction
with the electron-hole separation. However, a new theory that considers the
modification of screening with that separation is needed to account for the
observed dependence on excitation power of the individual energies of the two
exciton components.Comment: 5 pages, 4 eps figures, RevTeX, Physical Review Letters (in press
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