The evolutionary status of the low-metallicity blue compact dwarf galaxy SBS 0940+544

We present the results of spectrophotometry and V, R, I, H alpha CCD photometry of the blue compact dwarf (BCD) galaxy SBS 0940+544. Broad-band images taken with the 2.1 m KPNO* and 1.23 m Calar Alto** telescopes reveal a compact high-surface-brightness H II region with ongoing star formation, located at the northwestern tip of the elongated low surface brightness (LSB) main body of the BCD. Very faint, patchy emission along the main body is seen in the H alpha image. High signal-to-noise 4.5 m Multiple Mirror Telescope (MMT)*** and 10 m Keck II telescope dagger long-slit spectroscopy of SBS 0940+544 is used to derive element abundances of the ionized gas in the brightest H II region and to study the stellar population in the host galaxy. The oxygen abundance in the brightest region with strong emission lines is 12 + log(O/H) = 7.46-7.50, or 1/29-1/26 solar, in agreement with earlier determinations and among the lowest for BCDs. H beta and H alpha emission lines and H delta and H gamma absorption lines are detected in a large part of the main body. Three methods are used to put constraints on the age of the stellar population at different positions along the major axis. They are based on (a) the equivalent widths of the emission lines, (b) the equivalent widths of the absorption lines and (c) the spectral energy distributions (SED). Several scenarios of star formation have been considered. We find that models with single instantaneous bursts cannot reproduce the observed SEDs, implying that star formation in the main body of SBS 0940+544 was continuous. The observed properties in the main body can be reproduced by a continuous star formation process which started not earlier than 100 Myr ago, if a small extinction is assumed. However, the observations can be reproduced equally well by a stellar population forming continuously since 10 Gyr, if the star formation rate has increased during last 100 Myr in the main body of SBS 0940+544 by at least a factor of five. We also investigate the age of the reddest southern region of the main body, where no absorption and emission lines are detected. On the assumption of zero extinction in this faint region, the observed spectrum can be fitted by a theoretical SED of a stellar population continuously formed with a constant star formation rate between 100 Myr and 10 Gyr ago. If, however, a small extinction of C(H beta) similar to 0.1 is present in this region then the observed spectrum can be fitted by a theoretical SED of a stellar population continuously formed between 100 Myr and 1 Gyr ago with a constant star formation rate. However, the poor signal-to-noise ratio of the spectrum and large photometric errors preclude reliable determination of the age of the southern region. In summary, we find no compelling evidence which favors either a young or an old age of SBS 0940+544.