1,605 research outputs found
Polar ring galaxies as tests of gravity
Polar ring galaxies are ideal objects with which to study the
three-dimensional shapes of galactic gravitational potentials since two
rotation curves can be measured in two perpendicular planes. Observational
studies have uncovered systematically larger rotation velocities in the
extended polar rings than in the associated host galaxies. In the dark matter
context, this can only be explained through dark halos that are systematically
flattened along the polar rings. Here, we point out that these objects can also
be used as very effective tests of gravity theories, such as those based on
Milgromian dynamics (MOND). We run a set of polar ring models using both
Milgromian and Newtonian dynamics to predict the expected shapes of the
rotation curves in both planes, varying the total mass of the system, the mass
of the ring with respect to the host, as well as the size of the hole at the
center of the ring. We find that Milgromian dynamics not only naturally leads
to rotation velocities being typically higher in the extended polar rings than
in the hosts, as would be the case in Newtonian dynamics without dark matter,
but that it also gets the shape and amplitude of velocities correct. Milgromian
dynamics thus adequately explains this particular property of polar ring
galaxies.Comment: 9 pages, 8 Figures, 1 Table, Accepted for publication by MNRA
Galaxy size trends as a consequence of cosmology
We show that recently documented trends in galaxy sizes with mass and
redshift can be understood in terms of the influence of underlying cosmic
evolution; a holistic view which is complimentary to interpretations involving
the accumulation of discreet evolutionary processes acting on individual
objects. Using standard cosmology theory, supported with results from the
Millennium simulations, we derive expected size trends for collapsed cosmic
structures, emphasising the important distinction between these trends and the
assembly paths of individual regions. We then argue that the observed variation
in the stellar mass content of these structures can be understood to first
order in terms of natural limitations of cooling and feedback. But whilst these
relative masses vary by orders of magnitude, galaxy and host radii have been
found to correlate linearly. We explain how these two aspects will lead to
galaxy sizes that closely follow observed trends and their evolution, comparing
directly with the COSMOS and SDSS surveys. Thus we conclude that the observed
minimum radius for galaxies, the evolving trend in size as a function of mass
for intermediate systems, and the observed increase in the sizes of massive
galaxies, may all be considered an emergent consequence of the cosmic
expansion.Comment: 14 pages, 13 figures. Accepted by MNRA
Correlations among global photometric properties of disk galaxies
Using a two-dimensional galaxy image decomposition technique, we extract
global bulge and disk parameters for a complete sample of early type disk
galaxies in the near infrared K band. We find significant correlation of the
bulge parameter n with the central bulge surface brightness and with
effective radius r_e. Using bivar iate analysis techniques, we find that , and are distributed in a plane with small scatter. We
do not find a strong correlation of n with bulge-to-disk luminosity ratio,
contrary to earlier reports. r_e and the disk scale length r_d are well
correlated for these early type disk galaxies, but with large scatter. We
examine the implications of our results to various bulge formation scenarios in
disk galaxies.Comment: 14 pages, LaTeX including 14 figures. To appear in the Astrophysical
Journa
Atomic Hydrogen Properties of AGN Host Galaxies: HI in 16 NUclei of GAlaxies (NUGA) Sources
We present a comprehensive spectroscopic imaging survey of the distribution
and kinematics of atomic hydrogen (HI) in 16 nearby spiral galaxies hosting low
luminosity AGN, observed with high spectral and spatial resolution (resolution:
~20 arcsec, 5 km/s) using the NRAO Very Large Array (VLA). The sample contains
a range of nuclear types, ranging from Seyfert to star-forming nuclei and was
originally selected for the NUclei of GAlaxies project (NUGA) - a spectrally
and spatially resolved interferometric survey of gas dynamics in nearby
galaxies designed to identify the fueling mechanisms of AGN and the relation to
host galaxy evolution. Here we investigate the relationship between the HI
properties of these galaxies, their environment, their stellar distribution and
their AGN type. The large-scale HI morphology of each galaxy is classified as
ringed, spiral, or centrally concentrated; comparison of the resulting
morphological classification with AGN type reveals that ring structures are
significantly more common in LINER than in Seyfert host galaxies, suggesting a
time evolution of the AGN activity together with the redistribution of the
neutral gas. Dynamically disturbed HI disks are also more prevalent in LINER
host galaxies than in Seyfert host galaxies. While several galaxies are
surrounded by companions (some with associated HI emission), there is no
correlation between the presence of companions and the AGN type
(Seyfert/LINER).Comment: 54 pages, 7 figures, accepted for publication in AJ. The
full-resolution version is available at
http://www.mpia.de/homes/haan/research.htm
Loss of mass and stability of galaxies in MOND
The self-binding energy and stability of a galaxy in MOND-based gravity are
curiously decreasing functions of its center of mass acceleration towards
neighbouring mass concentrations. A tentative indication of this breaking of
the Strong Equivalence Principle in field galaxies is the RAVE-observed escape
speed in the Milky Way. Another consequence is that satellites of field
galaxies will move on nearly Keplerian orbits at large radii (100 - 500 kpc),
with a declining speed below the asymptotically constant naive MOND prediction.
But consequences of an environment-sensitive gravity are even more severe in
clusters, where member galaxies accelerate fast: no more Dark-Halo-like
potential is present to support galaxies, meaning that extended axisymmetric
disks of gas and stars are likely unstable. These predicted reappearance of
asymptotic Keplerian velocity curves and disappearance of "stereotypic
galaxies" in clusters are falsifiable with targeted surveys.Comment: 4 pages, 2 figures, ApJ Letter
Simplicity of eigenvalues in Anderson-type models
We show almost sure simplicity of eigenvalues for several models of
Anderson-type random Schr\"odinger operators, extending methods introduced by
Simon for the discrete Anderson model. These methods work throughout the
spectrum and are not restricted to the localization regime. We establish
general criteria for the simplicity of eigenvalues which can be interpreted as
separately excluding the absence of local and global symmetries, respectively.
The criteria are applied to Anderson models with matrix-valued potential as
well as with single-site potentials supported on a finite box.Comment: 20 page
Dynamical Friction and the Distribution of Dark Matter in Barred Galaxies
We use fully self-consistent N-body simulations of barred galaxies to show
that dynamical friction from a dense dark matter halo dramatically slows the
rotation rate of bars. Our result supports previous theoretical predictions for
a bar rotating within a massive halo. On the other hand, low density halos,
such as those required for maximum disks, allow the bar to continue to rotate
at a high rate. There is somewhat meager observational evidence indicating that
bars in real galaxies do rotate rapidly and we use our result to argue that
dark matter halos must have a low central density in all high surface
brightness disk galaxies, including the Milky Way. Bars in galaxies that have
larger fractions of dark matter should rotate slowly, and we suggest that a
promising place to look for such candidate objects is among galaxies of
intermediate surface brightness.Comment: 6 pages, Latex, 3 figures, Accepted by Ap.J.L., revised copy,
includes an added paragrap
Bar Diagnostics in Edge-On Spiral Galaxies. II. Hydrodynamical Simulations
We develop diagnostics based on gas kinematics to identify the presence of a
bar in an edge-on spiral galaxy and determine its orientation. We use
position-velocity diagrams (PVDs) obtained by projecting edge-on
two-dimensional hydrodynamical simulations of the gas flow in a barred galaxy
potential. We show that when a nuclear spiral is formed, the presence of a gap
in the PVDs, between the signature of the nuclear spiral and that of the outer
parts of the disk, reliably indicates the presence of a bar. This gap is due to
the presence of shocks and inflows in the simulations, leading to a depletion
of the gas in the outer bar region. If no nuclear spiral signature is present
in a PVD, only indirect arguments can be used to argue for the presence of a
bar. The shape of the signature of the nuclear spiral, and to a lesser extent
that of the outer bar region, allows to determine the orientation of the bar
with respect to the line-of-sight. The presence of dust can also help to
discriminate between viewing angles on either side of the bar. Simulations
covering a large fraction of parameter space constrain the bar properties and
mass distribution of observed galaxies. The strongest constraint comes from the
presence or absence of the signature of a nuclear spiral in the PVD.Comment: 25 pages (AASTeX, aaspp4.sty), 11 jpg figures. Accepted for
publication in The Astrophysical Journal. Online manuscript with PostScript
figures available at: http://www.strw.leidenuniv.nl/~bureau/pub_list.htm
Quantum enhancement of N-photon phase sensitivity by interferometric addition of down-converted photon pairs to weak coherent light
It is shown that the addition of down-converted photon pairs to coherent
laser light enhances the N-photon phase sensitivity due to the quantum
interference between components of the same total photon number. Since most of
the photons originate from the coherent laser light, this method of obtaining
non-classical N-photon states is much more efficient than methods based
entirely on parametrically down-converted photons. Specifically, it is possible
to achieve an optimal phase sensitivity of about delta phi^2=1/N^(3/2), equal
to the geometric mean of the standard quantum limit and the Heisenberg limit,
when the average number of down-converted photons contributing to the N-photon
state approaches (N/2)^(1/2).Comment: 21 pages, including 6 figures. Extended version gives more details on
down-conversion efficiencies and clarifies the relation between phase
sensitivity and squeezing. The title has been changed in order to avoid
misunderstandings regarding these concept
Polar Ring Galaxies and the Tully Fisher relation: implications for the dark halo shape
We have investigated the Tully-Fisher relation for Polar Ring Galaxies
(PRGs), based on near infrared, optical and HI data available for a sample of
these peculiar objects. The total K-band luminosity, which mainly comes from
the central host galaxy, and the measured HI linewidth at 20% of the peak line
flux density, which traces the potential in the polar plane, place most polar
rings of the sample far from the Tully-Fisher relation defined for spiral
galaxies, with many PRGs showing larger HI linewidths than expected for the
observed K band luminosity. This result is confirmed by a larger sample of
objects, based on B-band data. This observational evidence may be related to
the dark halo shape and orientation in these systems, which we study by
numerical modeling of PRG formation and dynamics: the larger rotation
velocities observed in PRGs can be explained by a flattened polar halo, aligned
with the polar ring.Comment: 22 pages, 8 postscript figures, accepted for publication in
Astrophysical Journa
- …