345 research outputs found
Global effects of interactions on galaxy evolution
Recent observations of the evolutionary properties of paired and interacting galaxies are reviewed, with special emphasis on their global emission properties and star formation rates. Data at several wavelengths provide strong confirmation of the hypothesis, proposed originally by Larson and Tinsley, that interactions trigger global bursts of star formation in galaxies. The nature and properties of the starbursts, and their overall role in galactic evolution are also discussed
Consequences of Dust in Metal-Rich HII Regions
Dust and associated depletion of heavy elements from the gas phase can modify
the thermal properties of HII regions from the dust-free case, with significant
consequences for the emergent optical spectrum. We present the results of
theoretical calculations illustrating the effects of grains on the spectra of
giant, extragalactic HII regions, with emphasis on high metallicity systems
(i.e. solar and higher Z). Dust provides a simple explanation for the
observational absence of pure Balmer-line spectra that are expected on
theoretical grounds for dust-free, chemically enriched nebulae. Grains may also
play a role in enhancements of forbidden-line emission observed in HII regions
in the enriched nuclei of normal galaxies. In most cases, depletion introduces
the strongest perturbations to the optical spectrum. Selective absorption of
the ionizing continuum as well as heating by grain photoelectrons are important
in some instances, however, and grain heating can be particularly important for
enhancing emission in high-ionization lines. Allowing for depletion, the
presence of dust is unlikely to introduce large errors in global metallicity
indicators, although uncertainties in depletion factors coupled with the
sensitivity of infrared cooling to electron density will make accurate
calibrations difficult at high Z. The present calculations establish further
that previous relative abundance analyses that fail to take into account dust
effects in a self-consistent way (grain heating as well as depletion) may
overestimate temperature gradients in high-Z nebulae, resulting in errors in
relative abundances for different elements.Comment: 21 pages (AASTeX), plus 9 figures (uuencoded, gzipped, tar), to
appear in ApJ, December 199
HST Survey of Clusters in Nearby Galaxies. II. Statistical Analysis of Cluster Populations
We present a statistical system that can be used in the study of cluster
populations. The basis of our approach is the construction of synthetic cluster
color-magnitude-radius diagrams (CMRDs), which we compare with the observed
data using a maximum likelihood calculation. This approach permits a relatively
easy incorporation of incompleteness (a function of not only magnitude and
color, but also radius), photometry errors and biases, and a variety of other
complex effects into the calculation, instead of the more common procedure of
attempting to correct for those effects.
We then apply this procedure to our NGC 3627 data from Paper I. We find that
we are able to successfully model the observed CMRD and constrain a number of
parameters of the cluster population. We measure a power law mass function
slope of alpha = -1.50 +/- 0.07, and a distribution of core radii centered at
r_c = 1.53 +/- 0.15 pc. Although the extinction distribution is less
constrained, we measured a value for the mean extinction consistent with that
determined in Paper I from the Cepheids.Comment: 21 pages, 3 figures accepted for publication by A
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