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29 March 2024
 
  » arxiv » astro-ph/0109257

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A spectroscopic study of component C and the extended emission around I Zw 18
Y. I. Izotov ; F. H. Chaffee ; C. B. Foltz ; T. X. Thuan ; R. F. Green ; P. Papaderos ; K. J. Fricke ; N. G. Guseva ;
Date 17 Sep 2001
Subject astro-ph
Affiliation Main Astronomical Observatory, Kyiv, Ukraine, W. M. Keck Observatory, USA, MMT Observatory, University of Arizona, USA, University of Virginia, USA, National Optical Astronomy Observatories, USA, Universitäts--Sternwarte, Göttingen, Ger
AbstractLong-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are used to investigate the stellar population and the evolutionary status of I Zw 18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18. Hydrogen H$alpha$ and H$eta$ emission lines are detected in the spectra of I Zw 18C, implying that ionizing massive stars are present. High signal-to-noise Keck II spectra of different regions in I Zw 18C reveal H$gamma$, H$delta$ and higher order hydrogen lines in absorption. Several techniques are used to constrain the age of the stellar population in I Zw 18C. Ages derived from two different methods, one based on the equivalent widths of the H$alpha$, H$eta$ emission lines and the other on H$gamma$, H$delta$ absorption lines are consistent with a 15 Myr instantaneous burst model. We find that a small extinction in the range $A_V$ = 0.20 -- 0.65 mag is needed to fit the observed spectral energy distribution of I Zw 18C with that model. In the case of constant star formation, all observed properties are consistent with stars forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available observational constraints for I Zw 18C, including those obtained from Hubble Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18 should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized gas emission around I Zw 18. H$alpha$ emission is detected as far as 30" from it. To a B surface brightness limit of ~ 27 mag arcsec$^{-2}$ we find no observational evidence for extended stellar emission in the outermost regions, at distances > 15" from I Zw 18.
Source arXiv, astro-ph/0109257
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