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Inferences from the Composition of Two Dwarf Blue Galaxies
Abstract
The emission spectra of two dwarf compact galaxies, I Zw 18 and II Zw 40, which were earlier described as "isolated eztragalactic H ri regions," have been analyzed. Oxygen and neon have lower abundances (relative to hydrogen) than does the interstellar gas near the Sun, while helium has a normal abundance. These galaxies are the first metal-poor systems of Population I to be discovered: the normal helium abundance is taken as evidence that this abundance is primordiaL It is shown that most of the mass in I Zw 18 and II Zw 40 is probably in the form of interstellar hydrogen gas. The observed colors are used with the composition data to infer that the present rate of star formation exceeds the past average rate. It is argued that the galaxies are either young (in the sense that most of their star formation has occurred in recent times) or that the star formation in them occurs in intense bursts which are separated by long quiescent periods. A statistical search for objects in the quiescent phase is suggested as a means of distinguishing between the two possibilities.
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Citations
Cited by
Conroy, C., Gunn, J. E., & White, M. (2009) · The Astrophysical Journal · doi:10.1088/0004-637x/699/1/486 · Cited by 1,891
Metallicity Effects on Mid-Infrared Colors and the 8 μm PAH Emission in Galaxies
Engelbracht, C. W., Gordon, K. D., Rieke, G. H., Werner, M. W., Dale, D. A., & Latter, W. B. (2005) · The Astrophysical Journal · doi:10.1086/432613 · Cited by 347
Di Valentino, E., Said, J. L., Riess, A., Pollo, A., Poulin, V., Gómez-Valent, A., Weltman, A., Palmese, A., Huang, C. D., Bruck, C. V. D., et al. (2025) · Physics of the Dark Universe · doi:10.1016/j.dark.2025.101965 · Cited by 319
Metadata
- Authors
- Searle, L., & Sargent, W. L. W.
- Venue
- The Astrophysical Journal
- Publisher
- American Astronomical Society
- Published
- 1972-04
- Volume
- 173
- Pages
- 25
- ISSN
- 0004-637X (ISSN-L)
- 1538-4357
- OpenAlex
- W1968297864
- Cited by
- 524
- References
- 0
- FWCI
- 9.02
- Type
- journal article
- Language
- en
- Open access
- closed
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