Letter to the Editor

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

¹ Cosmic Dawn Center (DAWN), Denmark
² DTU Space, Technical University of Denmark, Elektrovej 327, DK-2800 Kgs. Lyngby, Denmark
³ Leiden Observatory, Leiden University, P.O. Box 9513 2300 RA Leiden, The Netherlands
⁴ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen, Denmark
⁵ Kapteyn Astronomical Institute, University of Groningen, 9700 AV, Groningen, The Netherlands
⁶ The University of Texas at Austin, 2515 Speedway Blvd Stop C1400, Austin, TX 78712, USA
⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁸ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, Garching 85748, Germany
⁹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
¹⁰ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla, 4059 Valparaíso, Chile
¹¹ Caltech/IPAC, MS 314-6, 1200 E. California Blvd, Pasadena, CA 91125, USA
¹² Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
¹³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
¹⁵ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo, Japan
¹⁶ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China
¹⁷ University of Bologna, Department of Physics and Astronomy (DIFA), Via Gobetti 93/2, I-40129 Bologna, Italy
¹⁸ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, 40129 Bologna, Italy

Received: 10 July 2024
Accepted: 1 October 2024

Abstract

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.