A spectroscopic study of component C and the extended emission around I Zw 18

Abstract

Long-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$\hat{I}\pm$ and H${\hat{I}}^2$ 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${\hat{I}}^3$, H$\hat{I}´$ 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$\hat{I}\pm$, H${\hat{I}}^2$ emission lines and the other on H${\hat{I}}^3$, H$\hat{I}´$ 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$\hat{I}\pm$ 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.

38 pages, 11 Postscript figures, accepted for publication in the Astrophysical Journal