A photo-z cautionary tale: Redshift confirmation of COSBO-7 at z = 2.625

Photometric redshifts are widely used in studies of dusty star-forming galaxies (DSFGs), but catastrophic photo-z failure can undermine all redshift-dependent results. Here we report the spectroscopic redshift confirmation of COSBO-7, a strongly lensed DSFG in the COSMOS-PRIMER field. Recently, a photometric redshift solution of z & 7.0 was reported for COSBO-7 based on ten bands of James Webb Space Telescope (JWST) NIRCam and MIRI imaging data. This z value was favored by four independent spectral energy distribution (SED) fitting codes, and the result provided an appealing candidate for the most distant massive DSFG known to date. This photo-z solution was also supported by a single line detection in Atacama Large Millimeter Array (ALMA) Band 3 consistent with CO(7–6) at z = 7.46. However, our new ALMA observations robustly detect two lines in Band 6 identified as CO(7–6) and [CI](2–1) at z = 2.625, and thus the Band 3 line as CO(3–2). These three robust line detections decidedly place COSBO-7 at z = 2.625, refuting the photo-z solution. We derive physical parameters by fitting near-infrared(NIR)to-millimeter(mm) photometry and lens modeling, revealing that COSBO-7 is a main sequence galaxy. We examine possible reasons for this photo-z failure and attribute it to (1) the likely underestimation of photometric uncertainties at 0.9 µm and 1.15 µm; and (2) the lack of photometry at wavelengths beyond 20 µm. Notably, we recover a bona fide z ∼ 2.3 by including the existing MIPS 24 µm photometry, demonstrating the critical importance of mid-infrared (MIR) data in bolstering photo-z measurements. This work highlights a common challenge in modeling the SEDs of DSFGs, and provides a cautionary tale regarding the reliability of photometric redshifts as well as pseudo-spectroscopic redshifts based on single line detection.