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Full-Text Articles in Physical Sciences and Mathematics
The Primordial Helium Abundance: Toward Understanding And Removing The Cosmic Scatter In The Dy/Dz Relation, D. R. Ballantyne, Gary J. Ferland, P. G. Martin
The Primordial Helium Abundance: Toward Understanding And Removing The Cosmic Scatter In The Dy/Dz Relation, D. R. Ballantyne, Gary J. Ferland, P. G. Martin
Physics and Astronomy Faculty Publications
We present results from photoionization models of low-metallicity H II regions. These nebulae form the basis for measuring the primordial helium abundance. Our models show that the helium ionization correction factor (ICF) can be nonnegligible for nebulae excited by stars with effective temperatures larger than 40,000 K. Furthermore, we find that when the effective temperature rises to above 45,000 K, the ICF can be significantly negative. This result is independent of the choice of stellar atmosphere. However, if an H II region has an [O III] λ5007/[O I] λ6300 ratio greater than 300, then our models show that, regardless of …
Neutral Hydrogen Distribution In Merging Galaxies: Differences Between Stellar And Gaseous Tidal Morphologies, J E. Hibbard, W D. Vacca, Min S. Yun
Neutral Hydrogen Distribution In Merging Galaxies: Differences Between Stellar And Gaseous Tidal Morphologies, J E. Hibbard, W D. Vacca, Min S. Yun
Min S. Yun
As part of several H I synthesis-mapping studies of merging galaxies, we have mapped the tidal gas in the three disk-disk merger systems Arp 157 (NGC 520), Arp 220, and Arp 299 (NGC 3690). These systems differ from the majority of the mergers mapped in H I in that their stellar and gaseous tidal features do not coincide. In particular, they exhibit large stellar tidal features with little if any accompanying neutral gas and large gas-rich tidal features with little if any accompanying starlight. On smaller scales, there are striking anticorrelations in which the gaseous and stellar tidal features appear …