Characteristics of the blue stars in the dwarf galaxies I Zw 18 and II Zw 40

Journal Article · Sat Jan 01 04:00:00 EST 1977 · Astrophys. J.; (United States)

DOI:https://doi.org/10.1086/154903· OSTI ID:7323707

Bergeron, J

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
• OSTI ID: 7323707
• Journal Information: Astrophys. J.; (United States), Journal Name: Astrophys. J.; (United States) Vol. 211:1; ISSN ASJOA
• Country of Publication: United States
• Language: English

Related Subjects

640105* -- Astrophysics &amp; Cosmology-- Galaxies
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ABUNDANCE
CHEMICAL COMPOSITION
COSMIC RADIO SOURCES
ELECTRON TEMPERATURE
EMISSION SPECTRA
GALAXIES
H2 REGIONS
IONIZATION
MAIN SEQUENCE STARS
SPECTRA
STARS