Characteristics of the blue stars in the dwarf galaxies I Zw 18 and II Zw 40
Among dwarf blue compact galaxies, I Zw 18 and II Zw 40 have continuum and emission-line spectra similar to those of giant H II regions in external galaxies. An analysis of the emission-line spectrum shows that the emissive gas is much hotter than normal galactic H II regions and that the heavy elements Z are highly underabundant unless they are in stages of ionization higher than Z/sup 2 +/. A study of the thermal balance (i) confirms an underabundance of elements heavier than He by a factor of at least 10 compared to their cosmic values, and (ii) shows that most of the interstellar gas is ionized by hot stars, 4.0 x 10/sup 4/ < or approx. = T/sub eff/ < or approx. = 5.5 x 10/sup 4/ K. These stars must be massive young O stars, M > or approx. = =30 M/sub sun/. The He II lambda4686 line observed in emission in I Zw 18 can arise in the optically thin hot envelopes of radiation-driven winds of Of stars. These blue compact galaxies cannot be old galactic systems with a flat mass distribution of newly formed stars. (AIP)
- Research Organization:
- California Institute of Technology (CalTech), Pasadena, CA (United States)
- OSTI ID:
- 7323707
- Journal Information:
- Astrophys. J.; (United States), Vol. 211:1
- Country of Publication:
- United States
- Language:
- English
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