Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...458..524i&link_type=abstract
Astrophysical Journal v.458, p.524
Astronomy and Astrophysics
Astronomy
37
Galaxies: Abundances, Galaxies: Compact, Galaxies: Ism, Galaxies: Kinematics And Dynamics, Galaxies: Stellar Content
Scientific paper
We present spectrophotometric observations of four low-metallicity blue compact galaxies obtained with the Blue Channel Spectrograph on the Multiple Mirror Telescope. Low-intensity broad emission in strong nebular lines with a spatial extent of several kiloparsec was detected in the spectra of three of them, suggesting rapid motion of the ionized gas with velocities up to 2000-3000 km s-1. In 1256+351 ≡ NGC 4861 and 1408+551A, the detection of the broad WR emission lines N III λ4640, He II λ4686, and C IV λ5808 implies the existence of a large population of WR stars. The ratio of nitrogen WR stars to O stars and the relative number of carbon stars WC/WR suggest that an instantaneous burst of star formation may have occurred in these galaxies.
The most plausible explanation of the origin of the observed low-intensity, large-scale broad emission is that it arises from the collective influence on the interstellar medium of a large number of massive stars through supernova explosions. However, we find differences in the appearance of low-intensity broad emission lines in galaxies with different metallicities. While broad emission in the [0 III] λλ4959, 5007 emission lines has been detected in three of the galaxies studied, broad hydrogen emission was detected in only 1851 + 695, the galaxy with the lowest metallicity in our sample. On the other hand, broad hydrogen emission is not observed in 1256+351 or in 1408+551A, whose spectra show strong evidence for WR emission lines.
We argue that these differences are related to differences in evolution of massive stars with different metallicities. If broad emission originates in high-velocity gas driven by stellar winds and/or in fast-moving supernova remnants then the absence of hydrogen emission in galaxies with higher metallicity could result simply from a hydrogen deficiency in massive stars at final stages of their evolution. Conversely, in galaxies with low metallicity the efficiency of stellar winds is lower and massive stars retain their hydrogen-rich envelopes before any explosive evolution.
We confirm the previous findings by Roy et al. (1992) that while superbubble blowout is the only mechanism so far proposed capable of producing large-scale high-velocity gas in giant H II regions, the surface brightness distribution in emission lines in our sample of four galaxies cannot be explained by conventional models of superbubble evolution.
Chaffee Frederic H.
Dyak Andrew B.
Foltz Craig B.
Izotov Yuri I.
Kniazev Alexei Y.
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