1,780 research outputs found
Radial Velocities of Stars in the Galactic Center
We present results from K band slit scan observations of a ~20''x20'' region
of the Galactic center (GC) in two separate epochs more than five years apart.
The high resolution (R>=14,000) observations allow the most accurate radial
velocity and acceleration measurements of the stars in the central parsec of
the Galaxy. Detected stars can be divided into three groups based on the CO
absorption band heads at ~2.2935 microns and the He I lines at ~2.0581 microns
and ~2.112, 2.113 microns: cool, narrow-line hot and broad-line hot. The radial
velocities of the cool, late-type stars have approximately a symmetrical
distribution with its center at ~-7.8(+/-10.3) km/s and a standard deviation
~113.7(+/-10.3) km/s. Although our statistics are dominated by the brightest
stars, we estimate a central black hole mass of 3.9(+/-1.1) million solar
masses, consistent with current estimates from complete orbits of individual
stars. Our surface density profile and the velocity dispersion of the late type
stars support the existence of a low density region at the Galactic center
suggested by earlier observations. Many hot, early-type stars show radial
velocity changes higher than maximum values allowed by pure circular orbital
motions around a central massive object, suggesting that the motions of these
stars greatly deviate from circular orbital motions around the Galactic center.
The correlation between the radial velocities of the early type He I stars and
their declination offsets from Sagittarius A* suggests that a systematic
rotation is present for the early-type population. No figure rotation around
the Galactic center for the late type stars is supported by the new
observations.Comment: 61 pages, 18 figures, 7 tables; accepted for publication in
Astrophysical Journa
Guidance, Flight Mechanics and Trajectory Optimization. Volume 5 - State Determination And/or Estimation
Guidance, flight mechanics, and trajectory optimizatio
The SAMI Galaxy Survey: gravitational potential and surface density drive stellar populations -- I. early-type galaxies
The well-established correlations between the mass of a galaxy and the
properties of its stars are considered evidence for mass driving the evolution
of the stellar population. However, for early-type galaxies (ETGs), we find
that color and stellar metallicity [Z/H] correlate more strongly with
gravitational potential than with mass , whereas stellar population
age correlates best with surface density . Specifically, for our sample
of 625 ETGs with integral-field spectroscopy from the SAMI Galaxy Survey,
compared to correlations with mass, the color--, [Z/H]--, and
age-- relations show both smaller scatter and less residual trend with
galaxy size. For the star formation duration proxy [/Fe], we find
comparable results for trends with and , with both being
significantly stronger than the [/Fe]- relation. In determining the
strength of a trend, we analyze both the overall scatter, and the observational
uncertainty on the parameters, in order to compare the intrinsic scatter in
each correlation. These results lead us to the following inferences and
interpretations: (1) the color-- diagram is a more precise tool for
determining the developmental stage of the stellar population than the
conventional color--mass diagram; and (2) gravitational potential is the
primary regulator of global stellar metallicity, via its relation to the gas
escape velocity. Furthermore, we propose the following two mechanisms for the
age and [/Fe] relations with : (a) the age-- and
[/Fe]-- correlations arise as results of compactness driven
quenching mechanisms; and/or (b) as fossil records of the
relation in their disk-dominated progenitors.Comment: 9 pages, 4 figures, 1 table Accepted to Ap
Recovering Velocity Distributions via Penalized Likelihood
Line-of-sight velocity distributions are crucial for unravelling the dynamics
of hot stellar systems. We present a new formalism based on penalized
likelihood for deriving such distributions from kinematical data, and evaluate
the performance of two algorithms that extract N(V) from absorption-line
spectra and from sets of individual velocities. Both algorithms are superior to
existing ones in that the solutions are nearly unbiased even when the data are
so poor that a great deal of smoothing is required. In addition, the
discrete-velocity algorithm is able to remove a known distribution of
measurement errors from the estimate of N(V). The formalism is used to recover
the velocity distribution of stars in five fields near the center of the
globular cluster Omega Centauri.Comment: 18 LATEX pages, 10 Postscript figures, uses AASTEX, epsf.sty.
Submitted to The Astronomical Journal, May 199
Comparison of an approximately isothermal gravitational potentials of elliptical galaxies based on X-ray and optical data
We analyze six X-ray bright elliptical galaxies, observed with Chandra and
XMM-Newton, and approximate their gravitational potentials by isothermal
spheres phi(r)=v_c^2 ln(r) over a range of radii from ~0.5 to ~25 kpc. We then
compare the circular speed v_c derived from X-ray data with the estimators
available from optical data. In particular we discuss two simple and robust
procedures for evaluating the circular speed of the galaxy using the observed
optical surface brightness and the line-of-sight velocity dispersion profiles.
The best fitting relation between the circular speeds derived from optical
observations of stars and X-ray observations of hot gas is v_{c,opt}~ \eta *
v_{c,X}, where \eta=1.10-1.15 (depending on the method), suggesting, albeit
with large statistical and systematic uncertainties, that non-thermal pressure
on average contributes ~20-30% of the gas thermal pressure.Comment: 24 pages, 15 figures; Accepted for publication in MNRA
The Structure and Dynamical Evolution of Dark Matter Halos
(Shortened) We use N-body simulations to investigate the structure and
dynamical evolution of dark matter halos in galaxy clusters. Our sample
consists of nine massive halos from an EdS universe with scale free power
spectrum and n = -1. Halos are resolved by ~20000 particles each, with a
dynamical resolution of 20-25 kpc. Large scale tidal fields are included up to
L=150 Mpc using background particles. The halo formation process can be
characterized by the alternation of two dynamical configurations: a merging
phase and a relaxation phase, defined by their signature on the evolution of
the total mass and rms velocity. Halos spend on average one 1/3 of their
evolution in the merging phase and 2/3 in the relaxation phase. Using this
definition, we study the density profiles and their change during the halo
history. The average density profiles are fitted by the NFW analytical model
with an rms residual of 17% between the virial radius Rv and 0.01 Rv. The
Hernquist (1990) profiles fits the same halos with an rms residual of 26%. The
trend with mass of the scale radius of these fits is marginally consistent with
that found by Cole & Lacey (1996): in comparison our halos are more centrally
concentrated, and the relation between scale radius and halo mass is slightly
steeper. We find a moderately large scatter in this relation, due both to
dynamical evolution within halos and to fluctuations in the halo population. We
analyze the dynamical equilibrium of our halos using the Jeans' equation, and
find that on average they are approximately in equilibrium within their virial
radius. Finally, we find that the projected mass profiles of our simulated
halos are in very good agreement with the profiles of three rich galaxy
clusters derived from strong and weak gravitational lensing observations.Comment: 20 pages, Latex, with all figures included. Modified to match the
published versio
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