4 Extended nebular emission

Melnick et al. (1992) first obtained an H$\alpha$ image of SBS 0335-052 and found that the ionized gas extends out to $\sim$7 arcsec, or over a region of $\sim$2 kpc in diameter. The deep long-slit Keck spectra of SBS 0335-052 allow us to trace ionized gas emission over a much larger region. In Fig. 3 we show spatial distributions of the equivalent widths in two nearly perpendicular directions of the brightest emission lines H$\beta$, H$\alpha$ and [O III] $\lambda$5007. H$\alpha$ emission is detected over 32 arcsec in the direction of PA = $-30^\circ$ and over 26 arcsec in the direction of PA = 80$^\circ$. This corresponds to a linear size of 8 $\times$ 6 kpc, roughly the size of the 26 mag arcsec^-2 R band isophote (Lipovetsky et al. 1999), or more than 3 times larger than that obtained from the H$\alpha$ image by Melnick et al. (1992).

Very high equivalent widths of the emission lines in SBS 0335-052 (Fig. 3) imply that the contribution of the ionized gas to the total light is important and dominate in the regions with the largest equivalent widths. This finding shows, as already pointed out in several earlier studies (e.g. Krüger et al. 1995; Izotov et al. 1997; Papaderos et al. 1998), that both nebular continuum and line emission must be taken into account in photometric studies of stellar populations in the extended regions of BCDs. Obviously, the contribution of the emission lines to the total light depends on their relative strengths and redshift. H II region models predict a decrease of [O III] $\lambda$5007/H$\beta$ with distance from the center due to a decreasing ionization parameter. This is observed in SBS 0335-052. If one assumes that the extended emission in SBS 0335-052 is only gaseous, then the V-I colour is changed from ${\sim}-0.6$ mag in the center, where [O III] $\lambda$5007/H$\beta$ $\sim$ 3.3, to $\sim$0.0 mag in outer regions, where [O  III] $\lambda$5007/H$\beta$ $\sim$ 1.5. In other words, even without stellar emission, the distribution of gaseous emission mimics a contribution of red stars increasing with distance. For analysis of stellar populations a proper removal of ionized gas emission is therefore crucial.

Acknowledgements

Y. I. I. has been partly supported by INTAS 97-0033 grant, and acknowledges support from the "Kiev project'' of the Université Paul Sabatier of Toulouse. He is grateful for the hospitality of the Midi-Pyrénées Observatory where this work was conducted. The authors further wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible.