The Importance of Nebular Continuum and Line Emission in Observations of
  Young Massive Star Clusters
  In this spectroscopic study of infant massive star clusters, we find that
continuum emission from ionized gas rivals the stellar luminosity at optical
wavelengths. In addition, we find that nebular line emission is significant in
many commonly used broad-band HST filters including the F814W I-band, the F555W
V-band and the F435W B-band. Two young massive clusters (YMCs) in NGC 4449 were
targeted for spectroscopic observations after Reines et al. (2008a) discovered
an F814W I-band excess in their photometric study of radio-detected clusters in
the galaxy. The spectra were obtained with the Dual Imaging Spectrograph on the
3.5 m APO telescope. We supplement these data with HST and SDSS photometry. By
comparing our data to the Starburst99 and GALEV models, we find that nebular
continuum emission competes with the stellar light in our observations and that
the relative contribution is largest in the U- and I-bands, where the Balmer
and Paschen jumps are located. The spectra also exhibit strong line emission
including the [SIII] 9069,9532 lines in the HST F814W I-band. We find that the
combination of nebular continuum and line emission can account for the F814W
I-band excess found by Reines et al. (2008a). In an effort to provide a
benchmark for estimating the impact of ionized gas emission on photometric
observations of YMCs, we compute the relative contributions of the stellar
continuum, nebular continuum, and emission lines to the total flux of a 3
Myr-old cluster through various HST filter/instrument combinations, including
filters in the WFC3. We urge caution when comparing observations of YMCs to
evolutionary synthesis models since nebular emission can have a large impact on
magnitudes and colors of young (< 5 Myr) clusters, significantly affecting
inferred properties such as ages, masses and extinctions. (Abridged)
