Issue |
A&A
Volume 690, October 2024
|
|
---|---|---|
Article Number | L16 | |
Number of page(s) | 7 | |
Section | Letters to the Editor | |
DOI | https://doi.org/10.1051/0004-6361/202451445 | |
Published online | 21 October 2024 |
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
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 zspec = 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 zphot ∼ 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.
Key words: galaxies: distances and redshifts / galaxies: high-redshift / galaxies: ISM / galaxies: individual: COSBO-7
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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