On The Very Bright Dropouts Selected Using the James Webb Space Telescope NIRCam Instrument

Abstract

The selection of candidate high-redshift galaxies using the dropout technique targeting the Lyman-break signature sometimes yields very bright objects that are too luminous to be easily explained if they are indeed at the expected redshifts. Here we present a systematic study of very bright dropouts selected through successive bands of the NIRCam instrument onboard the James Webb Space Telescope (JWST). Using the public NIRCam data in four blank fields over 500~arcmin$^2$, 300 such objects were found. They have F356W magnitudes $<25.1$~mag or $<26.0$~mag depending on the dropout passband, and the majority of them ($>80\%$) have very red F115W$-$F356W colors $> 2.0$~mag, qualifying them as ``extremely red objects'' (EROs). We focus on 137 objects that also have mid-IR observations from the JWST MIRI instrument. Their spectral energy distribution analysis shows that these objects are dominated by low-redshift ($z\sim1$--4) galaxies ($\gtrsim67\%$). However, a non-negligible fraction ($\gtrsim7\%$) could be at high redshifts. Seven of our objects have secure spectroscopic redshifts from JWST NIRSpec identifications, and the results confirm this picture: while six are low-redshift galaxies ($z\approx3$), one is a known galaxy at $z=8.679$ {(with $M_{\rm UV}=-22.4$~mag and stellar mass $M_*=10^{9.1}M_\odot$)} recovered in our sample. In light of recent theoretical models on early galaxy formation, this confirmed high-redshift galaxy does not pose a challenge. However, as our sample contains very luminous high-redshift candidates in the regime still underexplored ($M_{\rm UV}\leq -23$~mag and $M_*>10^{10.5}M_\odot$), spectroscopic identifications are necessary to ensure they do not create tension with these new models.