Long-slit Keck II W.M .
Keck Observatory is operated as a scientific partnership among the California Institute of Technology , the University of California and the National Aeronautics and Space Administration .
The Observatory was made possible by the generous financial support of the W.M .
Keck Foundation . , 4m Kitt Peak Kitt Peak National Observatory ( KPNO ) is operated by the Association of Universities for Research in Astronomy , Inc. , under cooperative agreement with the National Science Foundation . , and 4.5m MMT The MMT is a joint facility of the Smithsonian Institution and the University of Arizona . 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 \beta 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 \beta 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 \sim 10 Myr and \lesssim 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 \sim 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 \sim 27 mag arcsec ^ { -2 } we find no observational evidence for extended stellar emission in the outermost regions , at distances \gtrsim 15″ from I Zw 18 .