2,557 research outputs found
Determining the galactic mass distribution using tidal streams from globular clusters
We discuss how to use tidal streams from globular clusters to measure the
mass distribution of the Milky Way. Recent proper motion determinations for
globular clusters from plate measurements and Hipparcos astrometry provide
several good candidates for Galactic mass determinations in the intermediate
halo, far above the Galactic disk, including Pal 5, NGC 4147, NGC 5024 (M53)
and NGC 5466; the remaining Hipparcos clusters provide candidates for
measurements several kpc above and below the disk. These clusters will help
determine the profile and shape of the inner halo. To aid this effort, we
present two methods of mass determination: one, a generalization of
rotation-curve mass measurements, which gives the mass and potential from
complete position-velocity observations for stream stars; and another using a
simple chi^2 estimator, which can be used when only projected positions and
radial velocities are known for stream stars. We illustrate the use of the
latter method using simulated tidal streams from Pal 5 and find that fairly
accurate mass determinations are possible even for relatively poor data sets.
Follow-up observations of clusters with proper motion determinations may reveal
tidal streams; obtaining radial velocity measurements would enable accurate
measurements of the mass distribution in the inner Galaxy.Comment: 21 pages, 6 figures, published in A
The growth of galaxies in cosmological simulations of structure formation
We use hydrodynamic simulations to examine how the baryonic components of
galaxies are assembled, focusing on the relative importance of mergers and
smooth accretion in the formation of ~L_* systems. In our primary simulation,
which models a (50\hmpc)^3 comoving volume of a Lambda-dominated cold dark
matter universe, the space density of objects at our (64-particle) baryon mass
resolution threshold, M_c=5.4e10 M_sun, corresponds to that of observed
galaxies with L~L_*/4. Galaxies above this threshold gain most of their mass by
accretion rather than by mergers. At the redshift of peak mass growth, z~2,
accretion dominates over merging by about 4:1. The mean accretion rate per
galaxy declines from ~40 M_sun/yr at z=2 to ~10 M_sun/yr at z=0, while the
merging rate peaks later (z~1) and declines more slowly, so by z=0 the ratio is
about 2:1. We cannot distinguish truly smooth accretion from merging with
objects below our mass resolution threshold, but extrapolating our measured
mass spectrum of merging objects, dP/dM ~ M^a with a ~ -1, implies that
sub-resolution mergers would add relatively little mass. The global star
formation history in these simulations tracks the mass accretion rate rather
than the merger rate. At low redshift, destruction of galaxies by mergers is
approximately balanced by the growth of new systems, so the comoving space
density of resolved galaxies stays nearly constant despite significant mass
evolution at the galaxy-by-galaxy level. The predicted merger rate at z<~1
agrees with recent estimates from close pairs in the CFRS and CNOC2 redshift
surveys.Comment: Submitted to ApJ, 35 pp including 15 fig
Effect of the Milky Way on Magellanic Cloud structure
A combination of analytic models and n-body simulations implies that the
structural evolution of the Large Magellanic Cloud (LMC) is dominated by its
dynamical interaction with the Milky Way. Although expected at some level, the
scope of the involvement has significant observational consequences. First, LMC
disk orbits are torqued out of the disk plane, thickening the disk and
populating a spheroid. The torque results from direct forcing by the Milky Way
tide and, indirectly, from the drag between the LMC disk and its halo resulting
from the induced precession of the LMC disk. The latter is a newly reported
mechanism that can affect all satellite interations. However, the overall
torque can not isotropize the stellar orbits and their kinematics remains
disk-like. Such a kinematic signature is observed for nearly all LMC
populations. The extended disk distribution is predicted to increase the
microlensing toward the LMC. Second, the disk's binding energy slowly decreases
during this process, puffing up and priming the outer regions for subsequent
tidal stripping. Because the tidally stripped debris will be spatially
extended, the distribution of stripped stars is much more extended than the HI
Magellanic Stream. This is consistent with upper limits to stellar densities in
the gas stream and suggests a different strategy for detecting the stripped
stars. And, finally, the mass loss over several LMC orbits is predicted by
n-body simulation and the debris extends to tens of kiloparsecs from the tidal
boundary. Although the overall space density of the stripped stars is low,
possible existence of such intervening populations have been recently reported
and may be detectable using 2MASS.Comment: 15 pages, color Postscript figures, uses emulateapj.sty. Also
available from http://www-astro.phast.umass.edu/~weinberg/weinberg-pubs.htm
Globular Cluster Evolution in M87 and Fundamental Plane Ellipticals
The globular cluster population in M87 has decreased measurably through
dynamical evolution caused by relaxation, binary heating and time-dependent
tidal perturbation. For fundamental plane ellipticals in general, cluster
populations evolve more rapidly in smaller galaxies because of the higher mass
density. A simple evolutionary model reproduces the observed trend in specific
frequency with luminosity for an initially constant relationship.
Fits of theoretically evolved populations to M87 cluster data from McLaughlin
et al. (1994) show the following: 1) dynamical effects drive evolution in the
initial mass and space distributions and can account for the large core in the
spatial profile as well as producing radial-dependence in the mass spectrum; 2)
evolution reduces S_N by 50% within 16 kpc and 35% within 50 kpc, implying that
S_N was initially 26 in this region. We estimate that 15% of the `missing'
clusters lie below the detection threshold with mass less than 10^5 M_sun.Comment: Small but important typo. p.3, Table 2: M^{-\alpha+Kr} should read
M^{-(\alpha+Kr)} in both instances. Otherwise identical. Latex, 9 pages, 9
figures, mn.sty included. HTML version at
http://decoy.phast.umass.edu/preprints/m87/m87.html (Requires Netscape 1.1 or
better
The Effect of the Galactic Spheroid on Globular Cluster Evolution
We study the combined effects of relaxation, tidal heating and binary heating
on globular cluster evolution, exploring the physical consequences of external
effects and examining evolutionary trends in the Milky Way population. Our
analysis demonstrates that heating on circular and low-eccentricity orbits can
dominate cluster evolution. The results also predict rapid evolution on
eccentric orbits either due to strong relaxation caused by the high densities
needed for tidal limitation or due to efficient bulge shocking of low density
clusters.
The combination of effects leads to strong evolution of the population as a
whole. For example, within the solar circle, tidally-limited 10^5 M_sun
clusters lose at least 40% of their mass in 10 Gyr. At high eccentricity most
of these clusters evaporate completely. Bulge shocking disrupts clusters within
40 kpc which have less than 80% of their mass within their pericentric inner
Lagrange point. Our results are consistent with suggestions that the shape of
the cluster luminosity function results from evaporation and disruption of low
mass clusters; they further predict that the net velocity dispersion of the
cluster system in the inner Galaxy has decreased with time. Preliminary
constraints on formation models are also discussed. We conclude that the
observed cluster system has largely been shaped by dynamical selection.Comment: Latex, 17 pages, 16 figures, mn.st
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