Optical & NIR Surface Photometry of I Zw 18
Abstract
Using HST and ground-based optical and NIR data, we investigate whether the blue compact dwarf (BCD) galaxy I Zw 18 has an extended low-surface-brightness (LSB) older stellar population underlying the star-forming regions, as is the case in evolved iE/nE BCDs. Subtraction of narrow band H_alpha and [O III] exposures from R and V images shows that the filamentary LSB envelope extending out to ~2 kpc away from the starburst region, and hence the optical broad-band colors observed therein, are due mainly to ionized gas emission. Ionized gas accounts already at a galactocentric distance of 0.7 kpc for more than 80% of the R band line-of-sight intensity and contributes more than 40% of the integrated R band light of I Zw 18. The structural properties (such as the exponential scale length) of the stellar LSB component underlying the extended ionized gas emission place I Zw 18 among the most compact BCDs studied so far. Contrary to evolved nE/iE BCDs, the stellar component in I Zw 18 shows no appreciable color gradients over a range of ~8 mag in surface brightness.
- Publication:
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arXiv e-prints
- Pub Date:
- October 2001
- DOI:
- 10.48550/arXiv.astro-ph/0110040
- arXiv:
- arXiv:astro-ph/0110040
- Bibcode:
- 2001astro.ph.10040P
- Keywords:
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- Astrophysics
- E-Print:
- 4 pages, 2 figures