The young age of the extremely metal-deficient blue compact dwarf galaxy SBS 1415+437

We use Multiple Mirror Telescope (MMT) spectrophotometry and Hubble Space T elescope (HST) Faint Object Spectrograph (FOS) spectra and Wide Field and P lanetary Camera 2 (WFPC2) V and I images to study the properties and evolut ionary status of the nearby (D = 11.4 Mpc) extremely metal-deficient blue c ompact dwarf (BCD) galaxy SBS 1415 + 437 (=CG 389). The oxygen abundance in the galaxy is 12 + log (O/H) = 7.60 +/- 0.01 or Z./21. The helium mass fra ction in SBS 1415 + 437 is Y = 0.246 +/- 0.004, which agrees with the primo rdial helium abundance determined by Izotov & Thuan using a much larger sam ple of BCDs. The alpha elements-to-oxygen abundance ratios (Ne/O, S/O, Ar/O ) are in very good agreement with the mean values for other metal-deficient BCDs and are consistent with the scenario that these elements are made in massive stars. The Fe/O abundance ratio is similar to 2 times smaller than the solar ratio. The Si/O ratio is close to the solar value, implying that silicon is not significantly depleted into dust grains. The values of the N /O and C/O ratios imply that intermediate-mass stars have not had time to e volve in SBS 1415 + 437 and release their nucleosynthesis products and that both N and C in the BCD have been made by massive stars only. This sets an upper limit of similar to 100 Myr on the age of SBS 1415 + 437. The V and I surface brightness profiles of SBS 1415 + 437 are well fitted by exponent ials implying that the galaxy is a disklike system. The velocity distributi on derived from the H alpha and [O III] 5007 Angstrom emission lines indica tes a solid-body rotation with a rotational velocity of 80 km s(-1). The dy namical mass is similar to 13 times larger than the gas mass, implying that most of the mass in SBS 1415 + 437 is in the form of dark matter. The (V-I ) color of the low surface brightness component of the galaxy is blue (less than or similar to 0.4 mag), indicative of a very young underlying stellar population. The (V-I) versus I color-magnitude diagrams of the resolved st ellar populations in different regions of SBS 1415 + 437 suggest propagatin g star formation from the northeast side of the galaxy to the southwest. Al l regions in SBS 1415 + 437 possess very blue spectral energy distributions (SED). By comparing the observed SEDs to theoretical SEDs that include bot h stellar and gaseous emission, we find that the ages of the stellar popula tions in SBS 1415 + 437 to range from a few Myr to 100 Myr. Thus chemical a bundances, color profiles and spectral energy distributions ail say that SB S 1415 + 437 is a truly young galaxy that did not start to make stars until similar to 100 Myr ago.