The physical origin of extreme emission line galaxies at high redshifts: strong [OIII] emission lines produced by obscured AGNs

Abstract

We present deep Subaru/FOCAS spectra for two extreme emission line galaxies (EELGs) at z ∼ 1 with strong[O III] λ5007 emission lines, exhibiting equivalent widths (EWs) of -+ 2905 578946 Å and -+ 2000 159188 Å, comparable tothose of EELGs at high redshifts that are now routinely identified with JWST spectroscopy. Adding a similarlylarge [O III] EW ( -+ 2508 6891487 Å) EELG found at z ∼ 2 in the JWST CEERS survey to our sample, we explore thephysical origins of the large [O III] EWs of these three galaxies with the Subaru spectra and various public dataincluding JWST/NIRSpec, NIRCam, and MIRI data. While there are no clear signatures of active galactic nuclei(AGNs) identified by the optical line diagnostics, we find that two out of two galaxies covered by the MIRI datashow strong near-infrared excess in the spectral energy distributions (SEDs), indicating obscured AGNs. Becausenone of the three galaxies show clear broad Hβ lines, the upper limits on the flux ratios of broad Hβ to [O III] linesare small, 0.15 that are comparable with Seyfert 1.8–2.0 galaxies. We conduct Cloudy modeling with the stellarand AGN incident spectra, allowing a wide range of parameters, including metallicities and ionization parameters.We find that the large [O III] EWs are not self-consistently reproduced by the spectra of stars or unobscured AGNs,but obscured AGNs that efficiently produce O++ ionizing photons with weak nuclear and stellar continua that areconsistent with the SED shapes.

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