Ultraviolet to near-infrared spectral distributions of star-forming galaxies : metallicity and age effects

Abstract

Spectral distributions from the UV to the near-IR of a sample of 44 star-forming galaxies are used to calculate the metallicity (0/H), star-formation rate (SFR) and age of the starbursts. The oxygen abundance covers the range 8.3 < 0/H < 9.4, and nitrogen (N) is found to be mostly a product of secondary nucleosynthesis for 0/H > 8.4. Dueto its secondary origin, N/0 ratios up to ≈4 times the solar value can be obtained for metal­ rich starbursts. The SFR ranges from 0.01 to 100 M 0 yr-ˡ. The lower metallicity galaxies seem to be experiencing ao instantaneous burst of star formation, with ages ranging from under 5 x 10 6 to 10 7 yr. The highest metallicity galaxies are most probably experiencing a continuous burst. Correlations between the calculated quantities and severa! spectral features are investigated. We found a highly significant correlation between the equivalent width W(C IV λ1550)−a stellar (absorption) feature−and the oxygen abundance of the emitting gas (0/H). Thus we show for the first time that the stellar metallicity is well correlated with the gas metallicity in star-bursting galaxies. The equivalent width W(Si IV λ1400) and the emission line ratio [N II] λλ6548,84/Hα also correlate well with 0/H, and all three features can be used as metallicity indicators for star-forming galaxies. The continuum color between λ1400 and λ3500 [C(14-35)] is shown to correlate with 0/H, although it is better correlated with E(B- V). It was not possible to disentangle the metallicity from the reddening effect in C(14-35). We estimate that the reddening affecting the UV continuum is about half the one derived from the Balmer decrement of the emitting gas. The SFR correlates well with the galaxy luminosity and there is no dependence of the continuum color on the SFR. The higher metallicities are only found in the more luminous galaxies, while low metallicities are found over the whole luminosity interval (-16 < M8 < -23, H0 =50 km s-ˡ Mpc-ˡ) covered by the sample.