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In this spectroscopic study of infant massive star clusters, we find thatcontinuum emission from ionized gas rivals the stellar luminosity at opticalwavelengths. In addition, we find that nebular line emission is significant inmany commonly used broad-band HST filters including the F814W I-band, the F555WV-band and the F435W B-band. Two young massive clusters (YMCs) in NGC 4449 weretargeted for spectroscopic observations after Reines et al. (2008a) discoveredan F814W I-band excess in their photometric study of radio-detected clusters inthe galaxy. The spectra were obtained with the Dual Imaging Spectrograph on the3.5 m APO telescope. We supplement these data with HST and SDSS photometry. Bycomparing our data to the Starburst99 and GALEV models, we find that nebularcontinuum emission competes with the stellar light in our observations and thatthe relative contribution is largest in the U- and I-bands, where the Balmerand Paschen jumps are located. The spectra also exhibit strong line emissionincluding the [SIII] 9069,9532 lines in the HST F814W I-band. We find that thecombination of nebular continuum and line emission can account for the F814WI-band excess found by Reines et al. (2008a). In an effort to provide abenchmark for estimating the impact of ionized gas emission on photometricobservations of YMCs, we compute the relative contributions of the stellarcontinuum, nebular continuum, and emission lines to the total flux of a 3Myr-old cluster through various HST filter/instrument combinations, includingfilters in the WFC3. We urge caution when comparing observations of YMCs toevolutionary synthesis models since nebular emission can have a large impact onmagnitudes and colors of young (< 5 Myr) clusters, significantly affectinginferred properties such as ages, masses and extinctions. (Abridged)

The Importance of Nebular Continuum and Line Emission in Observations of Young Massive Star Clusters