2,328 research outputs found
Triaxial Black-Hole Nuclei
We demonstrate that the nuclei of galaxies containing supermassive black
holes can be triaxial in shape. Schwarzschild's method was first used to
construct self-consistent orbital superpositions representing nuclei with axis
ratios of 1:0.79:0.5 and containing a central point mass representing a black
hole. Two different density laws were considered, with power-law slopes of -1
and -2. We constructed two solutions for each power law: one containing only
regular orbits and the other containing both regular and chaotic orbits.
Monte-Carlo realizations of the models were then advanced in time using an
N-body code to verify their stability. All four models were found to retain
their triaxial shapes for many crossing times. The possibility that galactic
nuclei may be triaxial complicates the interpretation of stellar-kinematical
data from the centers of galaxies and may alter the inferred interaction rates
between stars and supermassive black holes.Comment: 4 pages, 4 postscript figures, uses emulateapj.st
Assessing the Formation Scenarios for the Double Nucleus of M31 Using Two-Dimensional Image Decomposition
The double nucleus geometry of M31 is currently best explained by the
eccentric disk hypothesis of Tremaine, but whether the eccentric disk resulted
from the tidal disruption of an inbounding star cluster by a nuclear black
hole, or by an m=1 perturbation of a native nuclear disk, remains debatable. I
perform detailed 2-D decomposition of the M31 double nucleus in the Hubble
Space Telescope V-band to study the bulge structure and to address competing
formation scenarios of the eccentric disk. I deblend the double nucleus (P1 and
P2) and the bulge simultaneously using five Sersic and one Nuker components. P1
and P2 appear to be embedded inside an intermediate component (r_e=3.2") that
is nearly spherical (q=0.97+/-m0.02), while the main galaxy bulge is more
elliptical (q=0.81+/-0.01). The spherical bulge mass of 2.8x10^7 M_sol is
comparable to the supermassive black hole mass (3x10^7 M_sol). In the 2-D
decomposition, the bulge is consistent with being centered near the UV peak of
P2, but the exact position is difficult to pinpoint because of dust in the
bulge. P1 and P2 are comparable in mass. Within a radius r=1\arcsec of P2, the
relative mass fraction of the nuclear components is M_BH:M_bulge:P1: P2 =
4.3:1.2:1:0.7, assuming the luminous components have a common mass-to-light
ratio of 5.7. The eccentric disk as a whole (P1+P2) is massive, M ~ 2.1x10^7
M_sol, comparable to the black hole and the local bulge mass. As such, the
eccentric disk could not have been formed entirely out of stars that were
stripped from an inbounding star cluster. Hence, the more favored scenario is
that of a disk formed in situ by an m=1 perturbation, caused possibly by the
passing of a giant molecular cloud, or the passing/accretion of a small
globular cluster.Comment: 19 pages, 8 figures. AJ accepted. For the version of this paper with
high resolution figures, go to:
http://zwicky.as.arizona.edu/~cyp/work/m31.ps.g
On the deprojection of the galactic bulge
An algorithm is developed and tested for the problem posed by photometric observations of the bulge of the Milky Way. The latter subtends a non-trivial solid angle on the sky, and we show that this permits inversion of the projected brightness distribution under the assumption that the bulge has three orthogonal mirror planes of specified orientation. A serious error in the assumed orientation of the mirror planes should be detectable
Metallicity of the polar disk in NGC4650A: constraints for cold accretion scenario
We used high resolution spectra in the optical and near-infrared wavelength
range to study the abundance ratios and metallicities of the HII regions
associated with the polar disk in NGC4650A, in order to put constraints on the
formation of the polar disk through cold gas accretion along a filament; this
might be the most realistic way by which galaxies get their gas. We have
compared the measured metallicities for the polar structure in NGC4650A with
those of different morphological types and we have found that they are similar
to those of late-type galaxies: such results is consistent with a polar disk
formed by accretion from cosmic web filaments of external cold gas.Comment: Proceeding of the conference "Hunting for the Dark: The Hidden Side
of Galaxy Formation", Malta, 19-23 Oct. 200
Shape of the Galactic Orbits in the CNOC1 Clusters
We present an analysis of the orbital properties in 9 intermediate-redshifts
cluster of the CNOC1 survey and we compare them to a control sample of 12
nearby clusters. Similar to the nearby elliptical galaxies, the bulge-dominated
galaxies in clusters at redshifts ~0.1-0.4 present orbits that are more
eccentric than those for disk-dominated galaxies. However, the orbital
segregation is less significant than that found for elliptical and spiral
galaxies in nearby cluster. When galaxies are separated by colors - red
galaxies with colors in the rest frame (U-V)_o > 1.4, and blue galaxies with
(U-V)_o =< 1.4 - the strongest orbital segregation is found. Therefore, the
segregation we found seems to modify more efficiently the star formation
activity than the internal shape of the galaxies. When we compare the orbits of
early-type galaxies at intermediate-redshift with those for z=0, they seem to
develop significant changes getting much more eccentric. A different behavior
is observed in the late-type galaxies, which present no-significant evolution
in their orbit shapes.Comment: Accepted for publication in ApJ, April 2000. Latex with aaspp4.sty,
20 pages, 4 tables, 6 eps figure
The Evolution of Cuspy Triaxial Galaxies Harboring Central Black Holes
We use numerical simulations to study the evolution of triaxial elliptical
galaxies with central black holes. In contrast to earlier numerical studies
which used galaxy models with central density ``cores,'' our galaxies have
steep central cusps, like those observed in real ellipticals. As a black hole
grows in these cuspy triaxial galaxies, the inner regions become rounder owing
to chaos induced in the orbit families which populate the model. At larger
radii, however, the models maintain their triaxiality, and orbital analyses
show that centrophilic orbits there resist stochasticity over many dynamical
times. While black hole induced evolution is strong in the inner regions of
these galaxies, and reaches out beyond the nominal ``sphere of influence'' of a
black hole, our simulations do not show evidence for a rapid {\it global}
transformation of the host. The triaxiality of observed elliptical galaxies is
therefore not inconsistent with the presence of supermassive black holes at
their centers.Comment: 15 pages, 7 figures (1 color). Accepted for publication in Ap
Comparing and calibrating black hole mass estimators for distant active galactic nuclei
Black hole mass is a fundamental property of active galactic nuclei (AGNs).
In the distant universe, black hole mass is commonly estimated using the MgII,
Hbeta, or Halpha emission line widths and the optical/UV continuum or line
luminosities, as proxies for the characteristic velocity and size of the
broad-line region. Although they all have a common calibration in the local
universe, a number of different recipes are currently used in the literature.
It is important to verify the relative accuracy and consistency of the recipes,
as systematic changes could mimic evolutionary trends when comparing various
samples. At z=0.36, all three lines can be observed at optical wavelengths,
providing a unique opportunity to compare different empirical recipes. We use
spectra from the Keck Telescope and the Sloan Digital Sky Survey to compare
black hole mass estimators for a sample of nineteen AGNs at this redshift. We
compare popular recipes available from the literature, finding that mass
estimates can differ up to 0.38+-0.05 dex in the mean (or 0.13+-0.05 dex, if
the same virial coefficient is adopted). Finally, we provide a set of 30
internally self consistent recipes for determining black hole mass from a
variety of observables. The intrinsic scatter between cross-calibrated recipes
is in the range 0.1-0.3 dex. This should be considered as a lower limit to the
uncertainty of the black hole mass estimators.Comment: ApJ in press, 11 pages, 10 figure
Stellar kinematics for the central spheroid in the Polar Disk Galaxy NGC4650A
We have obtained high angular resolution, high signal-to-noise spectra of the
Calcium triplet absorption lines on the photometric axes of the stellar
spheroid in the polar disk galaxy NGC4650A. Along the major axis, the observed
rotation and velocity dispersion measurements show the presence of a
kinematically decoupled nucleus, and a flat velocity dispersion profile. The
minor axis kinematics is determined for the first time: along this direction
some rotation is measured, and the velocity dispersion is nearly constant and
slightly increases at larger distances from the center. The new high resolution
kinematic data suggest that the stellar component in NGC4650A resembles a
nearly-exponential oblate spheroid supported by rotation. The main implications
of these results on the previous mass models for NGC4650A are discussed.
Moreover, the new kinematic data set constraints on current models for the
formation scenarios of Polar Ring Galaxies (PRGs), supporting a slow accretion
rather then a secondary strong dissipative event.Comment: 25 pages, 8 figures, accepted for publication in the Astrophysical
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