146 research outputs found
Using frequency maps to constrain the distribution function of the Milky Way stellar halo
Resolved surveys of the Milky Way's stellar halo can obtain all 6 phase space
coordinates of tens of thousands of individual stars, making it possible to
compute their 3-dimensional orbits. Spectral analysis of large numbers of halo
orbits can be used to construct frequency maps which are a compact, yet
informative representation of their phase space distribution function (DF).
Such maps can be used to infer the major types of orbit families that
constitute the DF of stellar halo and their relative abundances. The structure
of the frequency maps, especially the resonant orbits, reflects the formation
history and shape of the dark matter potential and its orientation relative to
the disk. The application of frequency analysis to cosmological hydrodynamic
simulations of disk galaxies shows that the orbital families occupied by halo
stars and dark matter particles are very similar, implying that stellar halo
orbits can be used to constrain the DF of the dark matter halo, possibly
impacting future direct dark matter detection experiments. An application of
these methods to a sample of \sim 16,000 Milky Way halo and thick disk stars
from the SDSS-SEGUE survey yields a frequency map with strong evidence for
resonant trapping of halo stars by the Milky Way disk, in a manner predicted by
controlled simulations in which the disk grows adiabatically. The application
of frequency analysis methods to current and future phase space data for Milky
Way halo stars will provide new insights into the formation history of the
dierent components of the Galaxy and the DF of the halo.Comment: 4 pages, 3 figures, to appear in the proceedings of the conference
"Assembling the Puzzle of the Milky Way", to be published electronically by
the European Physical Journal. Eds. Celine Reyle, Annie Robin and Mathias
Schulthei
Cluster tidal fields: Effects on disk galaxies
A variety of observations of galaxies in clusters indicate that the gas in these galaxies is strongly affected by the cluster environment. We present results of a study of the dynamical effects of the mean cluster tidal field on a disk galaxy as it falls into a cluster for the first time on a bound orbit with constant angular momentum (Valluri 1992). The problem is studied in the restricted 3-body framework. The cluster is modelled by a modified Hubble potential and the disk galaxy is modelled as a flattened spheroid
Orbital instability and relaxation in stellar systems
The orbits of stars in galaxies are generically chaotic: the chaotic behavior
arises in part from the intrinsically grainy nature of a potential that is
composed of point masses. Even if the potential is assumed to be smooth,
however, orbits in non-axisymmetric galaxies can be chaotic due to the presence
of central density cusps or black holes. The chaotic nature of orbits implies
that perturbations will grow exponentially and this in turn is expected to
result in a diffusion in phase space. We show that the degree of orbital
evolution is not well predicted by the growth rate of infinitesimal
perturbations, i.e. by the Liapunov exponent. A more useful criterion is
whether perturbations continue to grow exponentially until their scale is of
order the size of the system. We illustrate these ideas in a potential
consisting of N fixed point masses. Liapunov exponents are large for all values
of N, but orbits become increasingly regular in their behavior as N increases;
the reason is that the exponential divergence saturates at smaller and smaller
distances as N is increased. The objects which impede diffusion are the
invariant tori. When there are few stable tori, an ensemble of chaotic orbits
evolves rapidly toward a nearly stationary state. This mixing process occurs on
timescales of a few crossing times in triaxial potentials containing massive
central singularities, consistent with the rapid evolution observed in N-body
simulations of galaxies with central black holes.Comment: Invited review to appear in "The Chaotic Universe", eds. R. Ruffini,
V.G. Gurzadyan (World Scientific) 1999, 9 pages, 4 figure
Collisional removal of HI from the inner disks of Virgo cluster galaxies
There is sufficient observational evidence to show that many Virgo Cluster spirals are HI deficient in their inner disks (in addition to being HI deficient globally, as previously established). It is shown here that collisions between galaxies in a cluster can lead to the removal of HI gas from these galaxies while leaving the H2 gas, undisturbed. This follows directly from the application of the Spitzer-Baade collisional gas removal mechanism to galaxies consisting of stars and a two-component interstellar medium (ISM) consisting of HI and H2, with HI having the largest filling factor. This can account for both the observed HI deficiency in the inner regions and the normal H2 content of these galaxies. The frequency of galaxy collisions in the Virgo Cluster is shown to be large enough to make collisional gas removal a viable mechanism
The Low End of the Supermassive Black Hole Mass Function: Constraining the Mass of a Nuclear Black Hole in NGC 205 via Stellar Kinematics
Hubble Space Telescope (HST) images and spectra of the nucleated dwarf
elliptical galaxy NGC 205 are combined with 3-integral axisymmetric dynamical
models to constrain the mass (M_BH) of a putative nuclear black hole. This is
only the second attempt, after M33, to use resolved stellar kinematics to
search for a nuclear black hole with mass below 10^6 solar masses. We are
unable to identify a best-fit value of M_BH in NGC 205; however, the data
impose a upper limit of 2.2x10^4 M_sun (1sigma confidence) and and upper limit
of 3.8x10^4 M_sun (3sigma confidence). This upper limit is consistent with the
extrapolation of the M_BH-sigma relation to the M_BH < 10^6 M_sunregime. If we
assume that NGC 205 and M33 both contain nuclear black holes, the upper limits
on M_BH in the two galaxies imply a slope of ~5.5 or greater for the M_BH-sigma
relation. We use our 3-integral models to evaluate the relaxation time (T_r)
and stellar collision time (T_coll) in NGC 205; T_r~10^8 yr or less in the
nucleus and T_coll~10^11 yr. The low value of T_r is consistent with core
collapse having already occurred, but we are unable to draw conclusions from
nuclear morphology about the presence or absence of a massive black hole.Comment: Latex emulateapj, 15 pages, 16 figures, Version accepted for
Publication in ApJ, 20 July 2005, v628. Minor changes to discussion
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