The importance of nebular emission for SED modeling of distant star-forming galaxies

Abstract: We highlight and discuss the importance of accounting for nebular emission in the SEDs of high redshift galaxies, as lines and continuum emission can contribute significantly or subtly to broad-band photometry. Physical parameters such as the galaxy age, mass, star-formation rate, dust attenuation and others inferred from SED fits can be affected to different extent by the treatment of nebular emission. We analyse a large sample of Lyman break galaxies from z~3-6, and show some main results illustrating e.g. the importance of nebular emission for determinations of the mass-SFR relation, attenuation and age. We suggest that a fairly large scatter in such relations could be intrinsic. We find that the majority of objects (~60-70%) is better fit with SEDs accounting for nebular emission; the remaining galaxies are found to show relatively weak or no emission lines. Our modeling, and supporting empirical evidence, suggests the existence of two categories of galaxies, "starbursts" and "post-starbursts" (lower SFR and older galaxies) among the LBG population, and relatively short star-formation timescales.