72 research outputs found
Is the Bardeen-Petterson effect responsible for the warping and precession in NGC 4258?
Strong evidence for the presence of a warped Keplerian accretion disc in
NGC4258 (M 106) has been inferred from the kinematics of water masers detected
at sub-parsec scales. Assuming a power-law accretion disc and using constraints
on the disc parameters derived from observational data, we have analyzed the
relativistic Bardeen-Petterson effect driven by a Kerr black hole as the
potential physical mechanism responsible for the disc warping. We found that
the Bardeen-Petterson radius is comparable to or smaller than the inner radius
of the maser disc (independent of the allowed value for the black hole spin
parameter). Numerical simulations for a wide range of physical conditions have
shown that the evolution of a misaligned disc due to the Bardeen-Petterson
torques usually produces an inner flat disc and a warped transition region with
a smooth gradient in the tilt and twist angles. Since this structure is similar
to that seen in NGC 4258, we propose that the Bardeen-Petterson effect may be
responsible for the disc warping in this galaxy. We estimated the time-scale
necessary for the disc inside of the Bardeen-Petterson radius to align with the
black hole's equator, as a function of the black hole spin. Our results show
that the Bardeen-Petterson effect can align the disc within a few billion years
in the case of NGC 4258. Finally, we show that if the observed curvature of the
outer anomalous arms in the galactic disc of NGC 4258 is associated with the
precession of its radio jet/counterjet, then the Bardeen-Petterson effect can
provide the required precession period.Comment: 10 pages, 5 figures, 1 table, accepted for publication in The Monthly
Notices of the Royal Astronomical Societ
The mass loss process in dwarf galaxies from 3D hydrodynamical simulations: the role of dark matter and starbursts
Theoretical CDM cosmological models predict a much larger number of
low mass dark matter haloes than has been observed in the Local Group of
galaxies. One possible explanation is the increased difficulty of detecting
these haloes if most of the visible matter is lost at early evolutionary phases
through galactic winds. In this work we study the current models of triggering
galactic winds in dwarf spheroidal galaxies (dSph) from supernovae, and study,
based on 3D hydrodynamic numerical simulations, the correlation of the mass
loss rates and important physical parameters as the dark matter halo mass and
its radial profile, and the star formation rate. We find that the existence of
winds is ubiquitous, independent on the gravitational potential. Our
simulations revealed that the Rayleigh-Taylor Instability (RTI) may play a
major role on pushing matter out of these systems, even for very massive
haloes. The instability is responsible for 5 - 40% of the mass loss during the
early evolution of the galaxy, being less relevant at Myrs. There is
no significant difference in the mass loss rates obtained for the different
dark matter profiles studied (NFW and logarithmic). We have also found a
correlation between the mass loss rate and both the halo mass and the rate of
supernovae, as already reported in previous works. Besides, the epoch in which
most of the baryon galactic matter is removed from the galaxy varies depending
on the SN rate and gravitational potential. The later, combined to the
importance of the RTI in each model, may change our understanding about the
chemical evolution of dwarf galaxies, as well as in the heavy element
contamination of the intergalactic medium at high redshifts.Comment: MNRAS, accepte
Bardeen-Petterson effect and the disk structure of the Seyfert galaxy NGC 1068
VLBA high spatial resolution observations of the disk structure of the active
galactic nucleus NGC 1068 has recently revealed that the kinematics and
geometry of this AGN is well characterized by an outer disk of H2O maser
emission having a compact milliarcsecond (parsec) scale structure, which is
encircling a thin rotating inner disk surrounding a ~10^7 M_\sun compact
mass, likely a black hole. A curious feature in this source is the occurrence
of a misalignment between the inner and outer parts of the disk, with the
galaxy's radio jet being orthogonal to the inner disk. We interpret this
peculiar configuration as due to the Bardeen-Petterson effect, a general
relativistic effect that warps an initially inclined (to the black hole
equator) viscous disk, and drives the angular momentum vector of its inner part
into alignment with the rotating black hole spin. We estimate the time-scale
for both angular momenta to get aligned as a function the spin parameter of the
Kerr black hole. We also reproduce the shape of the parsec and kiloparsec scale
jets, assuming a model in which the jet is precessing with a period and
aperture angle that decrease exponentially with time, as expected from the
Bardeen-Petterson effect.Comment: 12 pages, 3 figures, accepted for publication in The Astrophysical
Journa
Centaurus A: The Nearest Blazar?
Abstract. Centaurus A (NGC5128), at a distance of 3.4 Mpc is the nearest Active Galaxy, classified as a low luminosity Fanaroff-Riley class I object. Although the central source is completely obscured at optical wavelengths, VLBI studies at radio frequencies show an unresolved core and an asymmetric jet at sub-parsec scales. Kinematical studies of the jet components show subluminal expansion velocities, which together with the jet-counterjet intensity ratio implies that the jet direction forms a large angle with the line of sight (50 o to 80 o ). The nuclear emission is highly variable at all wavelengths, from radio to Îł-rays. Single dish radio observations showed that the stronger, long duration outbursts (months to years) present a correlation at radio and X-rays, although it is not clear whether the emission mechanism is synchrotron radiation at both frequencies or if the inverse Compton process dominates at high energies. Moreover, no information is available about the correlation between the emission at these two frequencies at shorter timescales (days and hours), due to the lack of short term monitoring at radio frequencies. In this work we report 43 GHz monitoring of Cantaurus A at the Itapetinga Radio Observatory during the last year, with daily resolution during a three-month period. We found very large variations (factor of two) within a few days, which puts Centaurus A in the blazar category. These variations were superimposed to a continuous rise in flux density that lasted until the end of 2003, when it started a fast decline. No apparent correlation with the All Sky Monitor (ASM/RXTE) data was found at these short timescales
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