Broad Emission Lines in Optical Spectra of Hot, Dust-obscured Galaxies Can Contribute Significantly to JWST/NIRCam Photometry

Selecting the first galaxies at z > 7 − 10 from JWST surveys is complicated by z < 6 contaminants with degenerate photometry. For example, strong optical nebular emission lines at z < 6 may mimic JWST/NIRCam photometry of z > 7–10 Lyman-break galaxies (LBGs). Dust-obscured 3 < z < 6 galaxies in particular are potentially important contaminants, and their faint rest-optical spectra have been historically difficult to observe. A lack of optical emission line and continuum measures for 3 < z < 6 dusty galaxies now makes it difficult to test their expected JWST/NIRCam photometry for degenerate solutions with NIRCam dropouts. Toward this end, we quantify the contribution by strong emission lines to NIRCam photometry in a physically motivated manner by stacking 21 Keck II/NIRES spectra of hot, dust-obscured, massive ($\mathrm{log}{M}_{* }/{M}_{\odot }\gtrsim 10\mbox{--}11$) and infrared (IR) luminous galaxies at z ∼ 1–4. We derive an average spectrum and measure strong narrow (broad) [O iii]5007 and Hα features with equivalent widths of 130 ± 20 Å (150 ± 50 Å) and 220 ± 30 Å (540 ± 80 Å), respectively. These features can increase broadband NIRCam fluxes by factors of 1.2 − 1.7 (0.2–0.6 mag). Due to significant dust attenuation (AV ∼ 6), we find Hα+[N ii] to be significantly brighter than [O iii]+Hβ and therefore find that emission-line dominated contaminants of high −z galaxy searches can only reproduce moderately blue perceived UV continua of Sλ ∝ λβ with β > − 1.5 and z > 4. While there are some redshifts (z ∼ 3.75) where our stack is more degenerate with the photometry of z > 10 LBGs at λrest ∼ 0.3–0.8 μm , redder filter coverage beyond λobs > 3.5 μm and far-IR/submillimeter follow-up may be useful for breaking the degeneracy and making a crucial separation between two fairly unconstrained populations, dust-obscured galaxies at z ∼ 3–6 and LBGs at z > 10.