34 research outputs found
Long-Lived Double-Barred Galaxies: Critical Mass and Length Scales
A substantial fraction of disk galaxies is double-barred. We analyze the
dynamical stability of such nested bar systems by means of Liapunov
exponents,by fixing a generic model and varying the inner (secondary) bar mass.
We show that there exists a critical mass below which the secondary bar cannot
sustain its own orbital structure, and above which it progressively destroys
the outer (primary) bar-supporting orbits. In this critical state, a large
fraction of the trajectories (regular and chaotic) are aligned with either bar,
suggesting the plausibility of long-lived dynamical states when
secondary-to-primary bar mass ratio is of the order of a few percent.
Qualitatively similar results are obtained by varying the size of the secondary
bar, within certain limits, while keeping its mass constant. In both cases, an
important role appears to be played by chaotic trajectories which are trapped
around (especially) the primary bar for long periods of time.Comment: 7 pages, 1 figure, to be published in Astrophysical Journal Letters
(Vol. 595, 9/20/03 issue). Replaced by revised figure and corrected typo
Density Waves Inside Inner Lindblad Resonance: Nuclear Spirals in Disk Galaxies
We analyze formation of grand-design two-arm spiral structure in the nuclear
regions of disk galaxies. Such morphology has been recently detected in a
number of objects using high-resolution near-infrared observations. Motivated
by the observed (1) continuity between the nuclear and kpc-scale spiral
structures, and by (2) low arm-interarm contrast, we apply the density wave
theory to explain the basic properties of the spiral nuclear morphology. In
particular, we address the mechanism for the formation, maintenance and the
detailed shape of nuclear spirals. We find, that the latter depends mostly on
the shape of the underlying gravitational potential and the sound speed in the
gas. Detection of nuclear spiral arms provides diagnostics of mass distribution
within the central kpc of disk galaxies. Our results are supported by 2D
numerical simulations of gas response to the background gravitational potential
of a barred stellar disk. We investigate the parameter space allowed for the
formation of nuclear spirals using a new method for constructing a
gravitational potential in a barred galaxy, where positions of resonances are
prescribed.Comment: 18 pages, 9 figures, higher resolution available at
http://www.pa.uky.edu/~ppe/papers/nucsp.ps.g
Nested Bars in Disk Galaxies: No Offset Dust Lanes in Secondary Nuclear Bars
Under certain conditions, sub-kpc nuclear bars form inside large-scale
stellar bars of disk galaxies. These secondary bars spend a fraction of their
lifetime in a dynamically-decoupled state, tumbling in the gravitational field
of the outer bars. We analyze the flow pattern in such nested bar systems and
find that secondary bars differ fundamentally from their large-scale
counterparts. In particular the gas flow across the bar-bar interface in these
systems can be more chaotic or more regular in nature, and, contrary to
predictions, has no difficulty in penetrating the secondary bars. The outer
parts of both short and long nuclear bars (with respect to their corotation)
appear to be depopulated of gas, while deep inside them the flow exhibits low
Mach numbers and follows ovally-shaped orbits with little dissipation. We find
that gas-dominated and star-dominated nuclear bars avoid the bar-bar interface,
making both types of bars short relative to their corotation. Furthermore, our
earlier work has shown that dynamically-coupled secondary bars exhibit a
similarly relaxed low-dissipation flow as well. Therefore, no large-scale
shocks form in the nuclear bars, and consequently, no offset dust lanes are
expected there. We find that offset dust lanes cannot be used in the search for
secondary (nuclear) bars.Comment: 13 pages, 8 figures, to be published in ApJ 564, January 10, 200
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
Interplay Between Chaotic and Regular Motion in a Time-Dependent Barred Galaxy Model
We study the distinction and quantification of chaotic and regular motion in
a time-dependent Hamiltonian barred galaxy model. Recently, a strong
correlation was found between the strength of the bar and the presence of
chaotic motion in this system, as models with relatively strong bars were shown
to exhibit stronger chaotic behavior compared to those having a weaker bar
component. Here, we attempt to further explore this connection by studying the
interplay between chaotic and regular behavior of star orbits when the
parameters of the model evolve in time. This happens for example when one
introduces linear time dependence in the mass parameters of the model to mimic,
in some general sense, the effect of self-consistent interactions of the actual
N-body problem. We thus observe, in this simple time-dependent model also, that
the increase of the bar's mass leads to an increase of the system's chaoticity.
We propose a new way of using the Generalized Alignment Index (GALI) method as
a reliable criterion to estimate the relative fraction of chaotic vs. regular
orbits in such time-dependent potentials, which proves to be much more
efficient than the computation of Lyapunov exponents. In particular, GALI is
able to capture subtle changes in the nature of an orbit (or ensemble of
orbits) even for relatively small time intervals, which makes it ideal for
detecting dynamical transitions in time-dependent systems.Comment: 21 pages, 9 figures (minor typos fixed) to appear in J. Phys. A:
Math. Theo
Bar Diagnostics in Edge-On Spiral Galaxies. I. The Periodic Orbits Approach
We develop diagnostics to detect the presence and orientation of a bar in an
edge-on disk, using its kinematical signature in the position-velocity diagram
(PVD) of a spiral galaxy observed edge-on. Using a well-studied barred spiral
galaxy mass model, we briefly review the orbital properties of two-dimensional
non-axisymmetric disks and identify the main families of periodic orbits. We
use those families as building blocks to model real galaxies and calculate the
PVDs obtained for various realistic combinations of periodic orbit families and
for a number of viewing angles with respect to the bar. We show that the global
structure of the PVD is a reliable bar diagnostic in edge-on disks.
Specifically, the presence of a gap between the signatures of the families of
periodic orbits in the PVD follows directly from the non-homogeneous
distribution of the orbits in a barred galaxy. Similarly, material in the two
so-called forbidden quadrants of the PVD results from the elongated shape of
the orbits. We show how the shape of the signatures of the dominant x1 and x2
families of periodic orbits in the PVD can be used efficiently to determine the
viewing angle with respect to the bar and, to a lesser extent, to constrain the
mass distribution of an observed galaxy. We also address the limitations of the
models when interpreting observational data.Comment: 22 pages, 9 figures (AASTeX, aaspp4.sty). Accepted for publication in
The Astrophysical Journa
Dynamical Decoupling of Nested Bars: Self-Gravitating Gaseous Nuclear Bars
A substantial fraction of barred galaxies host additional nuclear bars which
tumble with pattern speeds exceeding those of the large-scale (primary) stellar
bars. We have investigated the mechanism of formation and dynamical decoupling
in such nested bars which include gaseous (secondary) nuclear bars within the
full size galactic disks, hosting a double inner Lindblad resonance. Becoming
increasingly massive and self-gravitating, the nuclear bars lose internal
(circulation) angular momentum to the primary bars and increase their strength.
Developing chaos within these bars triggers a rapid gas collapse -- bar
contraction. During this time period, the secondary bar pattern speed
Omega_s~a^{-1}, where "a" stands for the bar size. As a result, Omega_s
increases dramatically until a new equilibrium is reached (if at all), while
the gas specific angular momentum decreases -- demonstrating the dynamical
decoupling of nested bars. Viscosity, and therefore the gas presence, appears
to be a necessary condition for the prograde decoupling of nested bars. This
process maintains an inflow rate of ~1 M_o/yr over ~10^8 yrs across the central
200 pc and has important implications for fueling the nuclear starbursts and
AGN.Comment: 7 pages, 4 postscript figures. (The associated MPEG movie can be
requested from the authors directly.) Submitted to Astrophysical Journal
Letter
Dynamics of Barred Galaxies
Some 30% of disc galaxies have a pronounced central bar feature in the disc
plane and many more have weaker features of a similar kind. Kinematic data
indicate that the bar constitutes a major non-axisymmetric component of the
mass distribution and that the bar pattern tumbles rapidly about the axis
normal to the disc plane. The observed motions are consistent with material
within the bar streaming along highly elongated orbits aligned with the
rotating major axis. A barred galaxy may also contain a spheroidal bulge at its
centre, spirals in the outer disc and, less commonly, other features such as a
ring or lens. Mild asymmetries in both the light and kinematics are quite
common. We review the main problems presented by these complicated dynamical
systems and summarize the effort so far made towards their solution,
emphasizing results which appear secure. (Truncated)Comment: This old review appeared in 1993. Plain tex with macro file. 82 pages
18 figures. A pdf version with figures at full resolution (3.24MB) is
available at http://www.physics.rutgers.edu/~sellwood/bar_review.pd
A review of elliptical and disc galaxy structure, and modern scaling laws
A century ago, in 1911 and 1913, Plummer and then Reynolds introduced their
models to describe the radial distribution of stars in `nebulae'. This article
reviews the progress since then, providing both an historical perspective and a
contemporary review of the stellar structure of bulges, discs and elliptical
galaxies. The quantification of galaxy nuclei, such as central mass deficits
and excess nuclear light, plus the structure of dark matter halos and cD galaxy
envelopes, are discussed. Issues pertaining to spiral galaxies including dust,
bulge-to-disc ratios, bulgeless galaxies, bars and the identification of
pseudobulges are also reviewed. An array of modern scaling relations involving
sizes, luminosities, surface brightnesses and stellar concentrations are
presented, many of which are shown to be curved. These 'redshift zero'
relations not only quantify the behavior and nature of galaxies in the Universe
today, but are the modern benchmark for evolutionary studies of galaxies,
whether based on observations, N-body-simulations or semi-analytical modelling.
For example, it is shown that some of the recently discovered compact
elliptical galaxies at 1.5 < z < 2.5 may be the bulges of modern disc galaxies.Comment: Condensed version (due to Contract) of an invited review article to
appear in "Planets, Stars and Stellar
Systems"(www.springer.com/astronomy/book/978-90-481-8818-5). 500+ references
incl. many somewhat forgotten, pioneer papers. Original submission to
Springer: 07-June-201
Does the intermediate-mass black hole in LEDA 87300 (RGG 118) follow the near-quadratic Mbh-Mspheroid relation?
The mass scaling relation between supermassive black holes and their host spheroids has previously been described by a quadratic or steeper relation at low masses (105 < Mbh/Mo âČ 107). How this extends into the realm of intermediate-mass black holes (102 < Mbh/Mo < 105) is not yet clear, although for the barred Sm galaxy LEDA 87300, Baldassare et al. recently reported a nominal virial mass of Mbh = 5 104 Mo residing in a "spheroid" of stellar mass equal to 6.3 108 Mo. We point out, for the first time, that LEDA 87300 therefore appears to reside on the near-quadratic Mbh-Msph,â relation. However, Baldassare et al. modeled the bulge and bar as the single spheroidal component of this galaxy. Here we perform a 3-component bulge+bar+disk decomposition and find a bulge luminosity which is 7.7 times fainter than the published "bulge" luminosity. After correcting for dust, we find that Mbulge = 0.9 108 Mo and Mbulge/Mdisk = 0.04 - which is now in accord with ratios typically found in Scd-Sm galaxies. We go on to discuss slight revisions to the stellar velocity dispersion (40 11 km s-1) and black hole mass () and show that LEDA 87300 remains consistent with the Mbh-Ï relation, and also the near-quadratic Mbh-Msph,â relation when using the reduced bulge mass. LEDA 87300 therefore offers the first support for the rapid but regulated (near-quadratic) growth of black holes, relative to their host bulge/spheroid, extending into the domain of intermediate-mass black holes