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

Abstract

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 ( log M * / M ⊙ ≳ 10 – 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 (A V ∼ 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 S λ ∝ λ β 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 λ rest ∼ 0.3–0.8 μm , redder filter coverage beyond λ obs > 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.

The JWST PEARLS View of the El Gordo Galaxy Cluster and of the Structure It Magnifies

“…Nonetheless, broader features such as the Ly-series continuum break should be detectable if there really is a galaxy group at this redshift. This problem is compounded by the fact that at this same redshift, the Hα+[N II] complex falls between the F200W and F277W bands and so is not recorded, while Hβ+[O III] falls within F200W, resulting in redshift degeneracies (McKinney et al 2023). These special circumstances explain why some sources at z = 3.75 might be missed, but do not explain a peak at this redshift.…”

Life beyond 30: Probing the −20 < M UV < −17 Luminosity Function at 8 < z < 13 with the NIRCam Parallel Field of the MIRI Deep Survey

“…The comparison of results obtained by analyzing photometry in different apertures can potentially help to get rid of noise or contamination by neighboring sources in the determination of the photometric redshifts, as we will explain in Section 3.1. The second difficulty is the degeneracy in the analysis of the SED for the candidates, given that large red colors consistent with the Lyα break can also be mimicked by the Balmer or 4000 Å breaks as well as by the presence of strong (i.e., high equivalent width) emission lines or dusty starbursts (Naidu et al 2022b;McKinney et al 2023;Pérez-González et al 2023;Zavala et al 2023). The third problem, directly related to the previous one, is the limited and/or biased set of templates that different programs estimating photometric redshifts use.…”

SBI++: Flexible, Ultra-fast Likelihood-free Inference Customized for Astronomical Applications

“…This has important implications for photometric redshift inference. Given the rich literature on photometric redshifts (see Newman & Gruen 2022 for a recent review), here we only reiterate that it is a well-known challenge to assign correct photometric redshift uncertainties for a simple reason: the most important information in determining a photometric redshift comes from the position of spectral breaks, (e.g., the dropout technique introduced in Steidel et al 1996), but a Balmer break can be confused with a Lyman break or strong emission lines (e.g., Dunlop et al 2007;McKinney et al 2023;Zavala et al 2023). In other words, lowz objects contaminate high-z detections when their photometry breaks in similar locations, meaning that breaking this degeneracy is very challenging and can only be done with subtle spectral features or strong priors on galaxy evolution.…”