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

• Published in: Astronomy and astrophysics (Berlin) 2024-10, Vol.690, p.L16
• Main Authors: Jin, Shuowen, Sillassen, Nikolaj B., Hodge, Jacqueline, Magdis, Georgios E., Rizzo, Francesca, Casey, Caitlin, Koekemoer, Anton M., Valentino, Francesco, Kokorev, Vasily, Magnelli, Benjamin, Gobat, Raphael, Gillman, Steven, Franco, Maximilien, Faisst, Andreas, Kartaltepe, Jeyhan, Schinnerer, Eva, Toft, Sune, Algera, Hiddo S. B., Harish, Santosh, Lee, Minju, Liu, Daizhong, Shuntov, Marko, Talia, Margherita, Vijayan, Aswin
• Format: Article
• Language: English
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202451445

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 spec = 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 phot ∼ 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.