3,281 research outputs found
The Bifurcated Age-Metallicity Relation of Milky Way Globular Clusters and its Implications For the Accretion History of the Galaxy
We use recently derived ages for 61 Milky Way (MW) globular clusters (GCs) to
show that their age-metallicity relation (AMR) can be divided into two
distinct, parallel sequences at [Fe/H] \ga -1.8. Approximately one-third of
the clusters form an offset sequence that spans the full range in age (--13 Gyr), but is more metal rich at a given age by dex in
[Fe/H]. All but one of the clusters in the offset sequence show orbital
properties that are consistent with membership in the MW disk. They are not
simply the most metal-rich GCs, which have long been known to have disk-like
kinematics, but they are the most metal-rich clusters at all ages. The slope of
the mass-metallicity relation (MMR) for galaxies implies that the offset in
metallicity of the two branches of the AMR corresponds to a mass decrement of 2
dex, suggesting host galaxy masses of M_{*} \sim 10^{7-8} \msol for GCs that
belong to the more metal-poor AMR. We suggest that the metal-rich branch of the
AMR consists of clusters that formed in-situ in the disk, while the metal-poor
GCs were formed in relatively low-mass (dwarf) galaxies and later accreted by
the MW. The observed AMR of MW disk stars, and of the LMC, SMC and WLM dwarf
galaxies are shown to be consistent with this interpretation, and the relative
distribution of implied progenitor masses for the halo GC clusters is in
excellent agreement with the MW subhalo mass function predicted by simulations.
A notable implication of the bifurcated AMR, is that the identical mean ages
and spread in ages, for the metal rich and metal poor GCs are difficult to
reconcile with an in-situ formation for the latter population.Comment: 16 pages, 9 figures, accepted for publication in MNRA
Bulge plus disc and S\'ersic decomposition catalogues for 16,908 galaxies in the SDSS Stripe 82 co-adds: A detailed study of the structural measurements
Quantitative characterization of galaxy morphology is vital in enabling
comparison of observations to predictions from galaxy formation theory.
However, without significant overlap between the observational footprints of
deep and shallow galaxy surveys, the extent to which structural measurements
for large galaxy samples are robust to image quality (e.g., depth, spatial
resolution) cannot be established. Deep images from the Sloan Digital Sky
Survey (SDSS) Stripe 82 co-adds provide a unique solution to this problem -
offering magnitudes improvement in depth with respect to SDSS Legacy
images. Having similar spatial resolution to Legacy, the co-adds make it
possible to examine the sensitivity of parametric morphologies to depth alone.
Using the Gim2D surface-brightness decomposition software, we provide public
morphology catalogs for 16,908 galaxies in the Stripe 82 co-adds. Our
methods and selection are completely consistent with the Simard et al. (2011)
and Mendel et al. (2014) photometric decompositions. We rigorously compare
measurements in the deep and shallow images. We find no systematics in total
magnitudes and sizes except for faint galaxies in the -band and the
brightest galaxies in each band. However, characterization of bulge-to-total
fractions is significantly improved in the deep images. Furthermore, statistics
used to determine whether single-S\'ersic or two-component (e.g., bulge+disc)
models are required become more bimodal in the deep images. Lastly, we show
that asymmetries are enhanced in the deep images and that the enhancement is
positively correlated with the asymmetries measured in Legacy images.Comment: 27 pages, 14 figures. MNRAS accepted. Our catalogs are available in
TXT and SQL formats at
http://orca.phys.uvic.ca/~cbottrel/share/Stripe82/Catalogs
Negative-Energy Spinors and the Fock Space of Lattice Fermions at Finite Chemical Potential
Recently it was suggested that the problem of species doubling with
Kogut-Susskind lattice fermions entails, at finite chemical potential, a
confusion of particles with antiparticles. What happens instead is that the
familiar correspondence of positive-energy spinors to particles, and of
negative-energy spinors to antiparticles, ceases to hold for the Kogut-Susskind
time derivative. To show this we highlight the role of the spinorial ``energy''
in the Osterwalder-Schrader reconstruction of the Fock space of non-interacting
lattice fermions at zero temperature and nonzero chemical potential. We
consider Kogut-Susskind fermions and, for comparison, fermions with an
asymmetric one-step time derivative.Comment: 14p
The signature of dissipation in the mass-size relation: are bulges simply spheroids wrapped in a disc?
The relation between the stellar mass and size of a galaxy's structural
subcomponents, such as discs and spheroids, is a powerful way to understand the
processes involved in their formation. Using very large catalogues of
photometric bulge+disc structural decompositions and stellar masses from the
Sloan Digital Sky Survey Data Release Seven, we carefully define two large
subsamples of spheroids in a quantitative manner such that both samples share
similar characteristics with one important exception: the 'bulges' are embedded
in a disc and the 'pure spheroids' are galaxies with a single structural
component. Our bulge and pure spheroid subsample sizes are 76,012 and 171,243
respectively. Above a stellar mass of ~ M, the mass-size
relations of both subsamples are parallel to one another and are close to lines
of constant surface mass density. However, the relations are offset by a factor
of 1.4, which may be explained by the dominance of dissipation in their
formation processes. Whereas the size-mass relation of bulges in discs is
consistent with gas-rich mergers, pure spheroids appear to have been formed via
a combination of 'dry' and 'wet' mergers.Comment: Accepted for publication in MNRAS, 6 pages, 3 figure
Applications of artificial intelligence techniques to a spacecraft control problem
Artificial intelligence applied to spacecraft control proble
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