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 Telescope (HST) Faint Object Spectrograph (FOS) spectra and Wide Field and Planetary Camera 2 (WFPC2) V and I images to study the properties and evolutionary status of the nearby (D = 11.4 Mpc) extremely metal-deficient blue compact 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 fraction in SBS 1415+437 is Y = 0.246 ± 0.004, which agrees with the primordial helium abundance determined by Izotov & Thuan using a much larger sample of BCDs. The α 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 ~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 evolve 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 ~100 Myr on the age of SBS 1415+437. The V and I surface brightness profiles of SBS 1415+437 are well fitted by exponentials implying that the galaxy is a disklike system. The velocity distribution derived from the Hα and [O III] 5007 Å emission lines indicates a solid-body rotation with a rotational velocity of 80 km s^-1. The dynamical mass is ~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 (≲0.4 mag), indicative of a very young underlying stellar population. The (V-I) versus I color-magnitude diagrams of the resolved stellar populations in different regions of SBS 1415+437 suggest propagating star formation from the northeast side of the galaxy to the southwest. All regions in SBS 1415+437 possess very blue spectral energy distributions (SED). By comparing the observed SEDs to theoretical SEDs that include both stellar and gaseous emission, we find that the ages of the stellar populations in SBS 1415+437 to range from a few Myr to 100 Myr. Thus chemical abundances, color profiles and spectral energy distributions all say that SBS 1415+437 is a truly young galaxy that did not start to make stars until ~100 Myr ago.