290 research outputs found
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
Microlensing Optical Depth of the COBE Bulge
We examine the left-right asymmetry in the cleaned COBE/DIRBE near-infrared
data of the inner Galaxy and show (i) that the Galactic bar is probably not
seen very nearly end-on, and (ii) that even if it is, it is not highly
elongated. The assumption of constant mass-to-light ratio is used to derive
simulated terminal-velocity plots for the ISM from our model luminosity
distributions. By comparing these plots with observed terminal velocities we
determine the mass-to-light ratio of the near-IR bulge and disk.
Assuming that all this mass contributes to gravitational microlensing we
compute optical depths for microlensing in Galactic-centre fields. For
three models with bar major axis between from the Sun-Galactic
Center line, the resulting optical depths in Baade's window lie in the range
0.83\times10^{-6} \lta \tau \lta 0.89\times10^{-6} for main-sequence stars
and 1.2\times10^{-6} \lta \tau \lta 1.3\times10^{-6} for red-clump giants. We
discuss a number of uncertainties including possible variations of the
near-infrared mass-to-light ratio. We conclude that, although the values
predicted from analyzing the COBE and gas velocity data are inconsistent at the
level with recent observational determinations of , we
believe they should be taken seriously.Comment: 9 pages, TeX. 7 figures (gif). Submitted to MNRAS. Also available
with full resolution figures as ps-file at
http://www.astro.unibas.ch/dynamics/papers.htm
Nuclear Bar, Star Formation and Gas Fueling in the Active Galaxy NGC 4303
A combination of Hubble Space Telescope (HST) WFPC2 and NICMOS images are
used to investigate the gas/dust and stellar structure inside the central 300
pc of the nearby active galaxy NGC 4303.
The NICMOS H-band (F160W) image reveals a bright core and a nuclear elongated
bar-like structure of 250 pc in diameter. The bar is centered on the bright
core, and its major axis is oriented in proyection along the spin axis of the
nuclear gaseous rotating disk recently detected (Colina & Arribas 1999).
The V-H (F606W - F160W) image reveals a complex gas/dust distribution with a
two-arm spiral structure of about 225 pc in radius. The southwestern arm is
traced by young star-forming knots while the northeastern arm is detected by
the presence of dust lanes. These spirals do not have a smooth structure but
rather they are made of smaller flocculent spirals or filament-like structures.
The magnitudes and colors of the star-forming knots are typical of clusters of
young stars with masses of 0.5 to 1 x yr^{-1} for about 80 Myr.Comment: ApJ, in press (February 1, 2000
Comparison of bar strengths in active and non-active galaxies
Bar strengths are compared between active and non-active galaxies for a
sample of 43 barred galaxies. The relative bar torques are determined using a
new technique (Buta and Block 2001), where maximum tangential forces are
calculated in the bar region, normalized to the axisymmetric radial force
field. We use JHK images of the 2 Micron All Sky Survey. We show a first clear
empirical indication that the ellipticies of bars are correlated with the
non-axisymmetric forces in the bar regions. We found that nuclear activity
appears preferentially in those early type galaxies in which the maximum bar
torques are weak and appear at quite large distances from the galactic center.
Most suprisingly the galaxies with the strongest bars are non-active. Our
results imply that the bulges may be important for the onset of nuclear
activity, but that the correlation between the nuclear activity and the early
type galaxies is not straightforward.Comment: MNRAS macro in tex format, 9 pages, 10 figure
Feeding AGN: new results from the NUGA survey
The NUGA project is a high-resolution (0.5''-1'') CO survey of low luminosity
AGN including the full sequence of activity types (Seyferts, LINERs and
transition objects). NUGA aims to systematically study the different mechanisms
for gas fueling of AGNs in the Local Universe. In this paper we discuss the
latest results of this recently completed survey, which now includes newly
acquired subarcsec resolution observations for all targets of the sample. The
large variety of circumnuclear disk morphologies found in NUGA galaxies (m=1,
m=2 and stochastic instabilities) is a challenging result that urges the
refinement of current dynamical models. In this paper we report on new results
obtained in 4 study cases for NUGA: NGC4826, NGC7217, NGC4579 and NGC6951Comment: 4 pages, 2 figures. Contributed talk to appear in "The Interplay
among Black Holes, Stars and ISM in Galactic Nuclei," Proc. IAU 222 (Gramado,
Brazil), eds. Th. Storchi Bergmann, L.C. Ho, H.R. Schmit
NUGA: the IRAM survey of AGN spiral hosts
The NUclei of GAlaxies (NUGA) project is a combined effort to carry out a
high-resolution (<1'') interferometer CO survey of a sample of 12 nearby AGN
spiral hosts, using the IRAM array. We map the distribution and dynamics of
molecular gas in the inner 1 kpc of the nuclei with resolutions of 10-50 pc,
and study the mechanisms for gas fueling of the different low-luminosity AGN.
First results show evidence for the occurrence of strong m=1 gas instabilities
in Seyferts. NUGA maps allow us to address the origin/nature of m=1 modes and
their link with m=2 modes and acoustic instabilities, present in other targets.Comment: 1 gzipped tar file containing 1 Latex file + 3 eps figures.
Proceedings of ''Active Galactic Nuclei: from Central Engine to Host
Galaxy'', meeting held in Meudon, France, July 23-27, 2002, Eds.: S. Collin,
F. Combes and I. Shlosman. To be published in ASP Conference Serie
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
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