Using HST Based on observations with the NASA/ESA Hubble Space Telescope , obtained at the Space Telescope Science Institute , which is operated by AURA , Inc. , under NASA contract No .
NAS 5–26555 . and ground-based optical and NIR imaging data Obtained at the German–Spanish Astronomical Center , Calar Alto , operated by the Max–Planck–Institute for Astronomy , Heidelberg , jointly with the Spanish National Commission for Astronomy . ^ { , } Obtained at the Kitt Peak National Observatory , operated by the Association of Universities for Research in Astronomy , Inc. , under cooperative agreement with the National Science Foundation . , we investigate whether the blue compact dwarf ( BCD ) galaxy I Zw 18 possesses an extended low-surface-brightness ( LSB ) old stellar population underlying its star-forming regions , as is the case in the majority of BCDs .
This question is central to the long-standing debate on the evolutionary state of I Zw 18 .
We show that the exponential intensity decrease observed in the filamentary LSB envelope of the BCD out to \ga 18″ ( \ga 1.3 kpc assuming a distance of 15 Mpc ) is not due to an evolved stellar disc underlying its star-forming regions , but rather , due to extended ionized gas emission .
Ionized gas accounts for more than 80 % of the line-of-sight emission at a galactocentric distance of \sim 0.65 kpc ( \sim 3 effective radii ) , and for \ga 30 % to 50 % of the R light of the main body of I Zw 18 .
Broad-band images reveal , after subtraction of nebular line emission , a relatively smooth stellar host extending slightly beyond the star-forming regions .
This unresolved stellar component , though very compact , is not exceptional for intrinsically faint dwarfs with respect to its structural properties .
However , being blue over a radius range of \sim 5 exponential scale lengths and showing little colour contrast to the star-forming regions , it differs strikingly from the red LSB host of standard BCDs .
This fact , together with the comparably blue colours of the faint C component , \sim 1.6 kpc away from the main body of I Zw 18 , suggests that the formation of I Zw 18 as a whole has occurred within the last 0.5 Gyr , making it a young BCD candidate .
Furthermore , we show that the ionized envelope of I Zw 18 is not exceptional among star-forming dwarf galaxies , neither by its exponential intensity fall-off nor by its scale length .
However , contrary to evolved BCDs , the stellar LSB component of I Zw 18 is much more compact than the ionized gas envelope .
In the absence of an appreciable underlying stellar population , extended ionized gas emission dominates in the outer parts of I Zw 18 , mimicking an exponential stellar disc on optical surface brightness profiles .