600 research outputs found
Disc galaxies with multiple triaxial structures. II. JHK surface photometry and numerical simulations
We present detailed JHK surface photometry with ellipse fits of 13 galaxies
selected from previous optical observations as likely candidates for having a
secondary bar or a triaxial bulge within the primary bar. We have found 7
double-barred galaxies, 3 double-barred galaxies with an additional
intermediate structure with twisted isophotes, and 3 galaxies with a bar and
central twisted isophotes. A global analysis of the structural parameter
characteristics in the I- and K-bands is presented. Various numerical models of
galaxies with bars within bars are also analysed using the ellipse fitting
technique and compared to the observations. A thorough review of the possible
hypotheses able to explain this phenomenon is given with emphasis on the most
likely ones.Comment: 12 pages, AATEX. Accepted for publication in A&A. Large color
postscript figures omitted (Figs. 1), figures 2-9 included; gzip'ed
postscript files of the paper and Figs. 1 available via anonymous ftp at
ftp://obsftp.unige.ch/pub/fri/aasjhk/ , files fri_aasjhk.ps.gz and ngc*.ps.g
Photometric and dynamic evolution of an isolated disc galaxy simulation
We present a detailed analysis of the evolution of a simulated isolated disc
galaxy. The simulation includes stars, gas, star formation and simple chemical
yields. Stellar particles are split in two populations: the old one is present
at the beginning of the simulation and is calibrated according to various ages
and metallicities; the new population borns in the course of the simulation and
inherits the metallicity of the gas particles. The results have been calibrated
in four wavebands with the spectro-photometric evolutionary model GISSEL2000
(Bruzual & Charlot 1993). Dust extinction has also been taken into account. A
rest-frame morphological and bidimensional photometric analysis has been
performed on simulated images, with the same tools as for observations. The
effects of the stellar bar formation and the linked star formation episode on
the global properties of the galaxy (mass and luminosity distribution, colours,
isophotal radii) have been analysed. In particular, we have disentangled the
effects of stellar evolution from dynamic evolution to explain the cause of the
isophotal radii variations. We show that the dynamic properties (e.g. mass) of
the area enclosed by any isophotal radius depends on the waveband and on the
level of star formation activity. It is also shown that the bar isophotes
remain thinner than mass isodensities a long time (> 0.7 Gyr) after the maximum
of star formation rate. We show that bar ellipticity is very wavelength
dependent as suggested by real observations. Effects of dust extinction on
photometric and morphological measurements are systematically quantified.Comment: 14 pages, 16 figures (13 in eps, 3 in jpg format). Accepted for
publication in A&
Direct Confirmation of Two Pattern Speeds in the Double Barred Galaxy NGC 2950
We present surface photometry and stellar kinematics of NGC 2950, which is a
nearby and undisturbed SB0 galaxy hosting two nested stellar bars. We use the
Tremaine-Weinberg method to measure the pattern speed of the primary bar. This
also permits us to establish directly and for the first time that the two
nested bars are rotating with different pattern speeds, and in particular that
the rotation frequency of the secondary bar is higher than that of the primary
one.Comment: 12 pages, 4 figures. To appear in ApJ Letter
Double Bars, Inner Disks, and Nuclear Rings in Early-Type Disk Galaxies
We present results from an imaging survey of an unbiased sample of
thirty-eight early-type (S0--Sa), low-inclination, optically barred galaxies in
the field. Our goal was to find and characterize central stellar and gaseous
structures: secondary bars, inner disks, and nuclear rings. Bars inside bars
are surprisingly common: at least one quarter of the sample galaxies (possibly
as many as 40%) are double-barred, with no preference for Hubble type or the
strength of the primary bar. A typical secondary bar is ~12% of the size of its
primary bar and 240--750 pc in radius. We see no significant effect of
secondary bars on nuclear activity. We also find kiloparsec-scale inner disks
in at least 20% of our sample, almost exclusively in S0 galaxies. These disks
are on average 20% the size of their host bar, and show a wider range of
relative sizes than do secondary bars. Nuclear rings are present in about a
third of our sample. Most are dusty, sites of current or recent star formation,
or both; such rings are preferentially found in Sa galaxies. Three S0 galaxies
(15% of the S0's) appear to have purely stellar nuclear rings, with no evidence
for dust or recent star formation. The fact that these central stellar
structures are so common indicates that the inner regions of early-type barred
galaxies typically contain dynamically cool and disklike structures. This is
especially true for S0 galaxies, where secondary bars, inner disks, and/or
stellar nuclear rings are present at least two thirds of the time. (abridged)Comment: LaTeX, 15 pages, 7 EPS figures; to appear in The Astronomical Journal
(July 2002
Reexamination of the Radial Abundance Gradient Break in NGC 3359
In this contribution, we reexamine the radial oxygen abundance gradient in
the strongly barred spiral galaxy NGC 3359, for which, using an imaging
spectrophotometric technique, Martin & Roy detected a break near the effective
radius of the galaxy. We have new emission line flux measurements of HII
regions in NGC 3359 from spectra obtained with the Subaru telescope to further
investigate this claim. We find that there are small systematic variations in
the line ratios determined from narrow-band imaging as compared to our
spectroscopic measurements. We derive and apply a correction to the line ratios
found by Martin & Roy and statistically examine the validity of the gradient
break proposed for NGC 3359 using recently developed metallicity diagnostics.
We find that, with a high degree of confidence, a model with a break fits the
data significantly better than one without it. This suggests that the presence
of a strong bar in spiral galaxies can generate measurable changes in the
radial distribution of metals.Comment: Accepted to A
Induced Nested Galactic Bars Inside Assembling Dark Matter Halos
We investigate the formation and evolution of nested bar systems in disk
galaxies in a cosmological setting by following the development of an isolated
dark matter (DM) and baryon density perturbation. The disks form within the
assembling triaxial DM halos and the feedback from the stellar evolution is
accounted for in terms of supernovae and OB stellar winds. Focusing on a
representative model, we show the formation of an oval disk and of a first
generation of nested bars with characteristic sub-kpc and a few kpc sizes. The
system evolves through successive dynamical couplings and decouplings, forcing
the gas inwards and settles in a state of resonant coupling. The inflow rate
can support a broad range of activity within the central kpc, from quasar- to
Seyfert-types, supplemented by a vigorous star formation as a by-product. The
initial bar formation is triggered in response to the tidal torques from the
triaxial DM halo, which acts as a finite perturbation. This first generation of
bars does not survive for more than 4--5 Gyr: by that time the secondary bar
has totally dissolved, while the primary one has very substantially weakened,
reduced to a fat oval. This evolution is largely due to chaos introduced by the
interaction of the multiple non-axisymmetric components.Comment: 4 pages, 4 figures, 1 mpeg animation. To be published by the
Astrophysical Journal Letters. The animation can be found at
http://www.pa.uky.edu/~shlosman/research/galdyn/movies.html Replaced with an
updated version (small text corrections
Towards the Secondary Bar: Gas Morphology and Dynamics in NGC 4303
The bulk of the molecular line emission in the double barred galaxy NGC4303
as observed in its CO(1-0) line with the OVRO mm-interferometer comes from two
straight gas lanes which run north-south along the leading sides of the
large-scale primary bar. Inside a radius of ~ 400 pc the molecular gas forms a
spiral pattern which, for the northern arm, can be traced to the nucleus.
Comparison of the OVRO and archival HST data with dynamical models of gas flow
in the inner kiloparsec of single- and double-barred galaxies shows that the
observed global properties of the molecular gas are in agreement with models
for the gas flow in a strong, large-scale bar, and the two-arm spiral structure
seen in CO in the inner kiloparsec can already be explained by a density wave
initiated by the potential of that bar. Only a weak correlation between the
molecular gas distribution and the extinction seen in the HST V-H map is found
in the inner 400 pc of NGC4303: The innermost part of one arm of the nuclear CO
spiral correlates with a weak dust filament in the color map, while the overall
dust distribution follows a ring or single-arm spiral pattern well correlated
with the UV continuum. This complicated nuclear geometry of the stellar and
gaseous components allows for two scenarios: (A) A self-gravitating m=1 mode is
present forming the spiral structure seen in the UV continuum. In this case the
gas kinematics would be unaffected by the small (~ 4'') inner bar. (B) The UV
continuum traces a complete ring which is heavily extincted north of the
nucleus. Such a ring forms in hydrodynamic models of double bars, but the
models cannot account for the UV emission observed on the leading side of the
inner bar. (abridged)Comment: 47 pages, 14 figures, accepted for publication in Ap
Analytical Approximation for the Inductance of Circularly Cylindrical Two-Wire Transmission Lines with Proximity Effect
The paper describes a simple analytical approximation for the inductance of two-wire transmission lines of circularly cylindrical wires with proximity effect. It yields precise results up to very high frequencies, and also at all interaxial distances between the wires above some limit. Its accuracy is established by comparison to numerical computations and to measurements. It is shown that the proximity effect cannot be neglected unless the interaxial distance between the wires amounts to at least four wire diameters. Further, images of the current distribution in various situations are discussed
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