6 Conclusions

We present in this paper a detailed photometric and spectroscopic study of the very metal-deficient blue compact dwarf galaxy SBS 0940+544 (Z $\sim$ $Z_\odot$/27), a good young galaxy candidate. Photometric V and I data have been obtained with the 2.1 m Kitt Peak telescope, and Rimaging has been done with the 1.23 m Calar Alto telescope. Very high signal-to-noise spectra in the optical range have been obtained with the 10 m Keck II telescope and the 4.5 m MMT at two slit orientations. We have reached the following conclusions:

1.

SBS 0940+544 is a nearby BCD galaxy with cometary-like structure, i.e. a bright off-center H II region and an elongated main body. The V, R and I surface brightness profiles of the galaxy's main body are very similar, with an exponential scale length $\alpha$ $\sim$ 320 pc. The V-I colour of the brightest H II region is very blue $\sim$-0.6 mag, due to the combined effect of the young stellar population and the ionized gas emission. The colours of the main body are much redder, without significant gradients;

2.

The derived oxygen abundance 12 + log(O/H) = 7.46-7.50 is consistent within the errors with previous abundance determinations. The $\alpha$-element Ne/O, S/O, Ar/O abundance ratios are in good agreement with the mean ratios derived from previous studies of BCDs (Izotov & Thuan 1999). The nitrogen-to-oxygen abundance ratio log N/O = -1.60--1.65 lies in the narrow range obtained by Thuan et al. (1995) and Izotov & Thuan (1999) for the most metal-deficient BCDs. These abundances suggest that SBS 0940+544 is a good local young galaxy candidate;

3.

The ⁴He mass fractions Y = 0.247 $\pm$ 0.003 and 0.244 $\pm$ 0.007 derived respectively from the Keck II and MMT observations of the brightest H II region in SBS 0940+544 are in good agreement with previous determinations of the helium abundance in this galaxy and are consistent with the value of the primordial ⁴He abundance $Y_{\rm p}$ = 0.244-0.245 derived by Izotov & Thuan (1998b) and Izotov et al. (1999);

4.

The hydrogen H$\alpha$ and H$\beta$ lines in the main body are seen in emission, while higher-order Balmer lines are seen in absorption. Three methods are used to constrain the age of the stellar populations in the main body of SBS 0940+544. The first one is based on the age dependence of the H$\alpha$ and H$\beta$ emission line equivalent widths, the second one on the age dependence of the hydrogen H$\gamma$ and H$\delta$ absorption line equivalent widths, and the third one on the age dependence of the spectral energy distribution. Several star formation histories have been considered. The single instantaneous burst models do not reproduce the observed SEDs, suggesting that star formation in the main body was continuous. We find that models of continuous star formation with a constant star formation rate starting 10 Gyr ago are also excluded. However, models with continuous star formation during the period 0-10 Gyr, and with a varying star formation rate, are able to explain the observed properties. In particular, models in which the star formation rate during the last 100 Myr has increased several times can reproduce both observed equivalent widths of the emission and absorption hydrogen lines and SEDs. However, the observed spectroscopic and photometric characteristics are reproduced equally well by models in which stars were continuously formed during the last 100 Myr only, if an extinction correction as derived from the H$\alpha$/H$\beta$ flux ratio is applied to the SEDs;

5.

The age of the reddest very low-intensity southern region in the SBS 0940+544 main body gives an upper limit to the galaxy's age. Because of the region's faintness, we were not able to detect hydrogen emission and/or absorption features to derive a reliable age. Assuming no extinction, we find that the SED can be fitted by a stellar population continuously formed with a constant star formation rate for a whole Hubble time. However, some extinction is likely to exist. In particular, if an extinction coefficient C(H$\beta$) = 0.15 is assumed, the extinction-corrected spectral energy distribution of the southern region can be well fitted by a model with a stellar population continuously formed with a constant star formation rate between 100 Myr and 1 Gyr ago.

In summary, we find no compelling evidence which favours either a young or an old age for SBS 0940+544.

Acknowledgements

We thank the referee Göran Östlin for valuable comments and criticism on the manuscript. N. G. G. has been supported by DFG grant 436 UKR 17/1/00 and Y. I. I. acknowledges the Göttingen Academy of Sciences for a Gauss professorship. N. G. G. and Y. I. I. have been partially supported by INTAS 97-0033 and Swiss SCOPE 7UKPJ62178 grants. They are grateful for the hospitality of the Göttingen Observatory. Those authors, P. P. and K. J. F. acknowledge support by the Volkswagen Foundation under grant No. I/72919. Y. I. I. and T. X. T. have been partially supported by NSF grant AST-9616863. Research by P. P. and K. J. F. has been supported by the Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR) under grant 50 OR 9907 7. C. B. F. acknowledges the support of the NSF under grant AST-9803072, and K. G. N. of the Deutsche Forschungsgemeinschaft (DFG) grant FR325/50-1.