957 research outputs found
Detecting Bimodality in Astronomical Datasets
We discuss statistical techniques for detecting and quantifying bimodality in
astronomical datasets. We concentrate on the KMM algorithm, which estimates the
statistical significance of bimodality in such datasets and objectively
partitions data into sub-populations. By simulating bimodal distributions with
a range of properties we investigate the sensitivity of KMM to datasets with
varying characteristics. Our results facilitate the planning of optimal
observing strategies for systems where bimodality is suspected.
Mixture-modeling algorithms similar to the KMM algorithm have been used in
previous studies to partition the stellar population of the Milky Way into
subsystems. We illustrate the broad applicability of KMM by analysing published
data on globular cluster metallicity distributions, velocity distributions of
galaxies in clusters, and burst durations of gamma-ray sources. PostScript
versions of the tables and figures, as well as FORTRAN code for KMM and
instructions for its use, are available by anonymous ftp from
kula.phsx.ukans.edu.Comment: 32 page
Quasi-phasematching of harmonic generation via multimode beating in waveguides
A new scheme for quasi-phasematching high harmonic generation (HHG) in gases
is proposed. In this, the rapid variation of the axial intensity resulting from
excitation of more than one mode of a waveguide is used to achieve quasi
phasematching. Numerical modeling demonstrates enhancement of the harmonic
signal over that achieved for a single coherence length by factors >10^4.Comment: 6 pages including 3 figure
Globular cluster systems as clues to galaxy evolution
We investigate the properties of systems of globular clusters in light of the hypothesis that galaxy mergers play a major role in galaxy evolution. In a previous paper, we presented a model in which the formation of globular clusters occurs during galaxy interactions and mergers. We discussed several predictions of the model, including the existence of young globular clusters in currently merging galaxies and the presence of two or more metallicity peaks in the globular clusters systems of normal elliptical galaxies. Here, we present recent observational evidence which supports both of these predictions and suggests that mergers may have a significant influence on the formation and evolution of galaxies and their globular clusters
The Globular Cluster Luminosity Function and Specific Frequency in Dwarf Elliptical Galaxies
The globular cluster luminosity function, specific globular cluster
frequency, S_N, specific globular cluster mass, T_MP, and globular cluster mass
fraction in dwarf elliptical galaxies are explored using the full 69 galaxy
sample of the HST WFPC2 Dwarf Elliptical Galaxy Snapshot Survey. The GCLFs of
the dEs are well-represented with a t_5 function with a peak at
M_{V,Z}^0(dE,HST) = -7.3 +/- 0.1. This is ~0.3 magnitudes fainter than the GCLF
peaks in giant spiral and elliptical galaxies, but the results are consistent
within the uncertainties. The bright-end slope of the luminosity distribution
has a power-law form with slope alpha = -1.9 +/- 0.1. The trend of increasing
S_N or T_MP with decreasing host galaxy luminosity is confirmed. The mean value
for T_MP in dE,N galaxies is about a factor of two higher than the mean value
for non-nucleated galaxies and the distributions of T_MP in dE,N and dE,noN
galaxies are statistically different. These data are combined with results from
the literature for a wide range of galaxy types and environments. At low host
galaxy masses the distribution of T_MP for dE,noN and dI galaxies are similar.
This supports the idea that one pathway for forming dE,noN galaxies is by the
stripping of dIs. The formation of nuclei and the larger values of T_MP in dE,N
galaxies may be due to higher star formation rates and star cluster formation
efficiencies due to interactions in galaxy cluster environments.Comment: 53 pages, 13 figures, 12 tables, accepted by the Astrophysical
Journa
HST Imaging of Globular Clusters in the Edge--on Spiral Galaxies NGC 4565 and NGC 5907
We present a study of the globular cluster systems of two edge-on spiral
galaxies, NGC4565 and NGC5907, from WFPC2 images in the F450W and F814W
filters. The globular cluster systems of both galaxies appear to be similar to
the Galactic globular cluster system. In particular, we derive total numbers of
globular clusters of N_{GC}(4565)= 204+/-38 {+87}{-53} and
N_{GC}(5907)=170+/-41 {+47}{-72} (where the first are statistical, the second
potential systematic errors) for NGC4565 and NGC5907, respectively. This
determination is based on a comparison to the Milky Way system, for which we
adopt a total number of globular clusters of 180+/-20. The specific frequency
of both galaxies is S_N~0.6: indistinguishable from the value for the Milky
Way. The similarity in the globular cluster systems of the two galaxies is
noteworthy since they have significantly different thick disks and
bulge-to-disk ratios. This would suggest that these two components do not play
a major role in the building up of a globular cluster system around late-type
galaxies.Comment: Accepted for publication in the Astronomical Journal, 17 pages incl 5
figures, AAS style two columns. Also available at
http://www.eso.org/~mkissler, Color version of figure 1 only available at
http://www.eso.org/~mkissler (B/W version included
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