191 research outputs found
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
A 60 pc counter-rotating core in NGC 4621
We present adaptive optics assisted OASIS integral field spectrography of the
S0 galaxy NGC 4621. Two-dimensional stellar kinematical maps (mean velocity and
dispersion) reveal the presence of a 60 pc diameter counter-rotating core
(CRC), the smallest observed to date. The OASIS data also suggests that the
kinematic center of the CRC is slightly offset from the center of the outer
isophotes. This seems to be confirmed by archival HST/STIS data. We also
present the HST/WFPC2 V-I colour map, which exhibits a central elongated red
structure, also slightly off-centered in the same direction as the kinematic
centre. We then construct an axisymmetric model of NGC 4621: the two-integral
distribution function is derived using the Multi-Gaussian Expansion and the
Hunter & Qian (1993) formalisms. Although the stellar velocities are reasonably
fitted, including the region of the counter-rotating core, significant
discrepancies between the model and the observations demonstrate the need for a
more general model (e.g. a three-integral model).Comment: 9 pages, 8 figure
The M 31 double nucleus probed with OASIS and HST. A natural m=1 mode?
We present observations with the adaptive optics assisted integral field
spectrograph OASIS of the M 31 double nucleus at a spatial resolution better
than 0.5 arcsec FWHM. These data are used to derive the two-dimensional stellar
kinematics within the central 2 arcsec. Archival WFPC2/HST images are revisited
to perform a photometric decomposition of the nuclear region. We also present
STIS/HST kinematics obtained from the archive. The luminosity distribution of
the central region is well separated into the respective contributions of the
bulge, the nucleus including P1 and P2, and the so-called UV peak. We then show
that the axis joining P1 and P2, the two local surface brightness maxima, does
not coincide with the kinematic major-axis, which is also the major-axis of the
nuclear isophotes (excluding P1). We also confirm that the velocity dispersion
peak is offset by ~ 0.2 arcsec from the UV peak, assumed to mark the location
of the supermassive black hole. The newly reduced STIS/HST velocity and
dispersion profiles are then compared to OASIS and other published kinematics.
We find significant offsets with previously published data. Simple parametric
models are then built to successfully reconcile all the available kinematics.
We finally interpret the observations using new N-body simulations. The nearly
keplerian nuclear disk of M31 is subject to a natural m=1 mode, with a very
slow pattern speed (3 km/s/pc for M_BH = 7 10^7~\Msun), that can be maintained
during more than a thousand dynamical times. The resulting morphology and
kinematics of the mode can reproduce the M~31 nuclear-disk photometry and mean
stellar velocity, including the observed asymmetries. It requires a central
mass concentration and a cold disk system representing between 20 and 40% of
its mass. Abridged..Comment: 21 pages. accepted for publication in A&
Formation of central massive objects via tidal compression
For a density that is not too sharply peaked towards the center, the local
tidal field becomes compressive in all three directions. Available gas can then
collapse and form a cluster of stars in the center, including or even being
dominated by a central black hole. We show that for a wide range of
(deprojected) Sersic profiles in a spherical potential, the tidal forces are
compressive within a region which encloses most of the corresponding light of
observed nuclear clusters in both late-type and early-type galaxies. In such
models, tidal forces become disruptive nearly everywhere for relatively large
Sersic indices n >= 3.5. We also show that the mass of a central massive object
(CMO) required to remove all radial compressive tidal forces scales linearly
with the mass of the host galaxy. If CMOs formed in (progenitor) galaxies with
n ~ 1, we predict a mass fraction of ~ 0.1-0.5%, consistent with observations
of nuclear clusters and super-massive black holes. While we find that tidal
compression possibly drives the formation of CMOs in galaxies, beyond the
central regions and on larger scales in clusters disruptive tidal forces might
contribute to prevent gas from cooling.Comment: 19 pages, 4 figures. Accepted for publication in ApJ. High resolution
version available at
http://www-obs.univ-lyon1.fr/labo/perso/eric.emsellem/preprint
Axisymmetric dynamical models for SAURON and OASIS observations of NGC3377
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
SAURON: An Innovative Look at Early-Type Galaxies
A summary of the SAURON project and its current status is presented. SAURON
is a panoramic integral-field spectrograph designed to study the stellar
kinematics, gaseous kinematics, and stellar populations of spheroids. Here, the
sample of galaxies and its properties are described. The instrument is detailed
and its capabilities illustrated through observational examples. These includes
results on the structure of central stellar disks, the kinematics and
ionization state of gaseous disks, and the stellar populations of galaxies with
decoupled cores.Comment: 10 pages, 6 figures. To appear in "The Dynamics, Structure & History
of Galaxies", eds. G. S. Da Costa & E. M. Sadler (San Francisco: ASP).
Version with full resolution images available at
http://www.strw.leidenuniv.nl/~dynamics/Instruments/Sauron/pub_list.htm
SAURON Observations of Disks in Spheroids
The panoramic integral-field spectrograph SAURON is currently being used to
map the stellar kinematics, gaseous kinematics, and stellar populations of a
large number of early-type galaxies and bulges. Here, we describe SAURON
observations of cold stellar disks embedded in spheroids (NGC3384, NGC4459,
NGC4526), we illustrate the kinematics and ionization state of large-scale
gaseous disks (NGC4278, NGC7742), and we show preliminary comparisons of SAURON
data with barred galaxy N-body simulations (NGC3623).Comment: 8 pages including 5 figures. To appear in Galaxies: The Third
Dimension, eds. M. Rosado, L. Binnette, & L. Arias (ASP: San Francisco
SAURON Observations of Disks in Early-Type Galaxies
We briefly describe the SAURON project, aimed at determining the intrinsic
shape and internal dynamics of spheroids. We focus here on the ability of
SAURON to identify gaseous and stellar disks and to measure their morphology
and kinematics. We illustrate some of our results with complete maps of
NGC3377, NGC3623, and NGC4365.Comment: 4 pages, 4 figures (newpasp.sty). To appear in ASP Conf. Series
"Galaxy Disks and Disk Galaxies", eds. J.G. Funes S.J. & E.M. Corsini.
Version with full resolution images available at
http://www.strw.leidenuniv.nl/~bureau/pub_list.htm
A SAURON view of galaxies
We have measured the two-dimensional kinematics and line-strength
distributions of 72 representative nearby early-type galaxies, out to
approximately one effective radius, with our panoramic integral-field
spectrograph SAURON. The resulting maps reveal a rich variety in kinematical
structures and linestrength distributions, indicating that early-type galaxies
are more complex systems than often assumed. We are building detailed dynamical
models for these galaxies, to derive their intrinsic shape and dynamical
structure, and to determine the mass of the supermassive central black hole.
Here we focus on two examples, the compact elliptical M32 and the E3 galaxy
NGC4365. These objects represent two extreme cases: M32 has very regular
kinematics which can be represented accurately by an axisymmetric model in
which all stars rotate around the short axis, while NGC4365 is a triaxial
galaxy with a prominent kinematically decoupled core, with an inner core that
rotates about an axis that is nearly perpendicular to the rotation axis of the
main body of the galaxy. Our dynamical models for these objects demonstrate
that two-dimensional observations are essential for deriving the intrinsic
orbital structure and dark matter content of galaxies.Comment: 7 pages (3 figures, full resolution Fig. 1 available at
http://www.strw.leidenuniv.nl/~verolme/M32.ps). Contributed talk to the
Athens Workshop on Galaxies and Chaos, Theory and Observations; Proceedings
to appear in "Galaxies and Chaos", eds. G. Contopoulos and N. Vogli
OASIS High-Resolution Integral Field Spectroscopy of the SAURON Ellipticals and Lenticulars
We present a summary of high-spatial resolution follow-up observations of the
elliptical (E) and lenticular (S0) galaxies in the SAURON survey using the
OASIS integral field spectrograph. The OASIS observations explore the central
8x10" regions of these galaxies using a spatial sampling four times higher than
SAURON, often revealing previously undiscovered features. Around 75% (31/48) of
the SAURON E/S0s with central velocity dispersion >= 120 km/s were observed
with OASIS, covering well the original SAURON representative sample. We present
here an overview of this follow-up survey, and some preliminary results on
individual objects, including a previously unreported counter-rotating core in
NGC 4382; the decoupled stellar and gas velocity fields of NGC 2768; and the
strong age gradient towards the centre of NGC 3489.Comment: 4 pages, 5 figures. Accepted for publication in Astron. Nachr. as
refereed proceedings of Euro3D Science Workshop, IoA Cambridge, May 200
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