176 research outputs found
Anisotropies in the Motions and Positions of the Galactic Globular Clusters
The velocity ellipsoid for 38 globular clusters with [Fe/H] <= -1.0 is
derived and shown to be significantly anisotropic with major axis directed
towards low Galactic latitude. Principal axes of the spatial distribution of
different groups of clusters are derived and compared with the velocity
ellipsoid. The metal poor cluster spatial distribution is significantly
flattened along an axis which coincides within the uncertainties with the major
axis of the velocity ellipsoid. Given the observed steep age-metallicity
relation for metal poor clusters, one speculative interpretation of the data is
that an initially flattened filament underwent a relatively rapid initial
transverse collapse forming satellite galaxies and metal poor globular clusters
while the protogalaxy collapsed and assembled more slowly along the filament
acquiring and/or redistributing angular momentum in the process.Comment: 10 pages, 1 figure, accepted for publication in ApJ Letter
Some Global Characteristics of the Galactic Globular Cluster System
The relations between the luminosities , the metallicities ,
the Galactocentric radii , and the central concentration indices of
Galactic globular clusters are discussed. It is found that the most luminous
clusters rarely have collapsed cores. The reason for this might be that the
core collapse time scales for such populous clusters are greater than the age
of the Galaxy. Among those clusters, for which the structure has not been
modified by core collapse, there is a correlation between central concentration
and integrated luminosity, in the sense that the most luminous clusters have
the strongest central concentration. The outermost region of the Galaxy with
kpc was apparently not able to form metal-rich globular
clusters, whereas such clusters (of which Ter 7 is the prototype) were able to
form in some nearby dwarf spheroidal galaxies. It is not yet clear how the
popular hypothesis that globular clusters were initially formed with a single
power law mass spectrum can be reconciled with the observation that both (1)
Galactic globular clusters with kpc, and (2) the globulars associated
with the Sagittarius dwarf, appear to have bi-modal luminosity functions.Comment: 15 pages, 1 figur
Do the nearby BHB stars belong to the Thick Disk or the Halo?
We study the Milky Way region Z<3.0 kpc, where the thick disk and inner halo
overlap, by using the kinematics of local blue horizontal branch (BHB) stars
(within 1 kpc) and new samples of BHB stars and A-type stars from the Century
Survey. We derive Galactic U,V,W velocities for these BHB and A-type star
samples using proper motions from the NOMAD catalog. The mean velocities and
the velocity dispersions of the BHB samples (Z<3 kpc) are characteristic of the
halo, while those of the Century Survey A-type stars are characteristic of the
thick disk. There is no evidence from our samples that the BHB stars rotate
with the thick disk in the region Z<3 kpc. Nearly a third of the nearby local
RR Lyrae stars have disk kinematics and are more metal-rich than [Fe/H]~-1.
Only a few percent of the Century Survey BHB stars have these properties. Only
one nearby BHB star (HD 130201) is likely to be such a disk star but selection
based on high proper motions will have tended to exclude such stars from the
local sample. The scale height derived from a sample of local RR Lyrae stars
agrees with that of the Century Survey BHB stars. The local samples of BHB
stars and metal-weak red giants are too incomplete for a similar comparison.Comment: 14 pages, accepted to A
Cosmic Star Formation History from Local Observations and an Outline for Galaxy Formation and Evolution
The goal of this investigation is to reconstruct the cosmic star formation
rate density history from local observations and in doing so to gain insight
into how galaxies might have formed and evolved. A new chemical evolution model
is described which accounts for the formation of globular clusters as well as
the accompanying field stars. When this model is used in conjunction with the
observed age metallicity relations for the clusters and with input which allows
for the formation of the nearly universally observed bimodal distribution of
globular clusters, star formation rates are obtained. By confining attention to
a representative volume of the local universe, these rates allow a successful
reconstruction of the Madau plot while complementary results similtaneously
satisfy many local cosmological constraints. A physical framework for galaxy
formation is presented which incorporates the results from this chemical
evolution model and assumes an anisotropic collapse. In addition to providing
the `classical' halo, bulge and disk components, the model also predicts a new
stellar halo component with peak [Fe/H] ~ -0.8 and disk-like angular momentum
and allows for the formation of a thick disk as outlined by the group of metal
rich globular clusters. Milky Way counterparts of the latter two components are
identified.Comment: 32 pages, 6 figs accepted by Ap
The Kinematic Properties of BHB and RR Lyrae stars towards the Anticentre and the North Galactic Pole: The Transition between the Inner and the Outer Halo
We identify 51 blue horizontal branch (BHB) stars, 12 possible BHB stars and
58 RR Lyrae stars in Anticentre fields. Their selection does not depend on
their kinematics. Light curves and ephemerides are given for 7 previously
unknown RR Lyrae stars. All but 4 of the RR Lyrae stars are of Oosterhoff type
I. Our selection criteria for BHB stars give results that agree with those used
by Smith et al. (2010) and Ruhland et al. (2011). We use 5 methods to determine
distances for the BHB stars and 3 methods for the RR Lyrae stars to get
distances on a uniform scale. Absolute proper motions (largely derived from the
GSCII and SDSS (DR7) databases) are given for these stars; radial velocities
are given for 31 of the BHB stars and 37 of the RR Lyrae stars. Combining these
data for BHB and RR Lyrae stars with those previously found in fields at the
North Galactic Pole, we find that retrograde orbits dominate for galactocentric
distances greater than 12.5 kpc. The majority of metal-poor stars in the solar
neighbourhood are known to be concentrated in a Lperp vs. Lz angular momentum
plot. We show that the ratio of the number of outliers to the number in the
main concentration increases with galactocentric distance. The location of
these outliers with Lperp and Lz shows that the halo BHB and RR Lyrae stars
have more retrograde orbits and a more spherical distribution with increasing
galactocentric distance. Six RR Lyrae stars are identified in the H99 group of
outliers; the small spread in their [Fe/H] suggests that they could have come
from a single globular cluster. Another group of outliers contains two pairs of
RR Lyrae stars; the stars in each pair have similar properties.Comment: 40 pages, 19 figures, to be published in MNRA
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