The blue compact dwarf galaxy I Zw 18: A comparative study of its low-surface-brightness component*,**,***
Universitäts–Sternwarte, Geismarlandstraße 11, 37083 Göttingen, Germany
2 Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Zabolotnoho str., 03680 Kyiv, Ukraine
3 Astronomy Department, University of Virginia, Charlottesville, VA 22903, USA
4 National Optical Astronomy Observatory, Tucson, AZ 85726, USA
Corresponding author: P. Papaderos, firstname.lastname@example.org
Accepted: 9 July 2002
Using HST and ground-based optical and NIR imaging data , 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 18´´ (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 ~0.65 kpc (~3 effective radii), and for 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 ~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, ~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.
Key words: galaxies: dwarf / galaxies: formation / galaxies: evolution / galaxies: structure / galaxies: starburst / galaxies: individual (I Zw 18, II Zw 70, III Zw 102, VII Zw 403, Tol 3, Henize 2-10, IC 4662, Mkn 36, Mkn 71, Mkn 178, Mkn 314, Mkn 324, Mkn 600, NGC 1705, NGC 1800, NGC 5253)
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.
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.
© ESO, 2002