Abstract
view Abstract Citations (47) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Resolution Spectroscopy and Direct Imaging of the Dwarf Galaxy I ZW 18 Davidson, Kris ; Kinman, T. D. ; Friedman, S. D. Abstract The velocity and spatial structure of ionized gas in I Zw 18 are relevant to studies of the primordial helium abundance Y_p_. We have observed the velocity profile of Hα emission in this dwarf galaxy; it resembles the H I velocity profile reported by other authors, but our data reveal that ionized gas in the prominent NW condensation has unexpected spatial structure. CCD images confirm that the Hα appearance differs from that in the continuum; the ionized gas may form an incomplete shell or ring, or double structure, several arcseconds (roughly 200 pc) across. This calls for a reassessment of all previous spectrophotometric results, which were planned with continuum images of I Zw 18 in mind. Several fainter continuum objects fall along a line extending 1.5 arcmin northwest from the bright condensations in I Zw 18; these may be additional parts of the galaxy and merit further investigation. In order to assess the effect of Galactic interstellar Na I λ 5890 absorption on the redshifted He I λ 5876 emission from I Zw 18, we have observed the interstellar Na I lines in echelle spectra of seven appropriate stars. We find that a correction of about 10% must be applied to the apparent λ 5876 flux of I Zw 18 NW observed at low resolution, leading to a helium abundance Y~0.234 + 0.016 in this object. This agrees well with some estimates of Y_p_ based on many less extreme objects, and we argue that it has about the same weight, but we also argue that the uncertainty in Y_p_ is still very unsatisfying when possible systematic errors are acknowledged. We briefly discuss the problem of improving this situation. Publication: The Astronomical Journal Pub Date: June 1989 DOI: 10.1086/115100 Bibcode: 1989AJ.....97.1591D Keywords: Astronomical Spectroscopy; Dwarf Galaxies; Absorption Spectra; Abundance; Charge Coupled Devices; Faint Objects; H Alpha Line; Red Shift; Astrophysics; GALAXIES: INDIVIDUAL; GALAXIES: ABUNDANCES full text sources ADS | data products SIMBAD (15) NED (1)
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