Downloaded from orbit.dtu.dk on: Oct 09, 2021 Discovery of a Dark, Massive, ALMA-only Galaxy at z ~ 5–6 in a Tiny 3 mm Survey Williams, Christina C.; Labbe, Ivo; Spilker, Justin; Stefanon, Mauro; Leja, Joel; Whitaker, Katherine; Bezanson, Rachel; Narayanan, Desika; Oesch, Pascal; Weiner, Benjamin Published in: The Astrophysical Journal Link to article, DOI: 10.3847/1538-4357/ab44aa Publication date: 2019 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Williams, C. C., Labbe, I., Spilker, J., Stefanon, M., Leja, J., Whitaker, K., Bezanson, R., Narayanan, D., Oesch, P., & Weiner, B. (2019). Discovery of a Dark, Massive, ALMA-only Galaxy at z ~ 5–6 in a Tiny 3 mm Survey. The Astrophysical Journal, 884(2), [154]. https://doi.org/10.3847/1538-4357/ab44aa General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. The Astrophysical Journal, 884:154 (13pp), 2019 October 20 https://doi.org/10.3847/1538-4357/ab44aa © 2019. The American Astronomical Society. All rights reserved. Discovery of a Dark, Massive, ALMA-only Galaxy at z∼5–6 in a Tiny 3 mm Survey Christina C. Williams1,11 , Ivo Labbe2, Justin Spilker3 , Mauro Stefanon4 , Joel Leja5,11 , Katherine Whitaker6 , Rachel Bezanson7 , Desika Narayanan8 , Pascal Oesch9,10 , and Benjamin Weiner1 1 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA 2 Centre for Astrophysics & Supercomputing, Swinburne University of Technology, P.O. Box 218, Hawthorn, VIC 3112, Australia 3 Department of Astronomy, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA 4 Leiden Observatory, Leiden University, NL-2300 RA Leiden, The Netherlands 5 Harvard-Smithsonian Center for Astrophysics, 60 Garden St. Cambridge, MA 02138, USA 6 Department of Physics, University of Connecticut, 2152 Hillside Road, Unit 3046, Storrs, CT 06269, USA 7 Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA, 15260, USA 8 Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611, USA 9 Department of Astronomy, University of Geneva, 51 Ch. des Maillettes, 1290 Versoix, Switzerland 10 International Associate, Cosmic Dawn Center (DAWN) at the Niels Bohr Institute, University of Copenhagen and DTU-Space, Technical University of Denmark, Denmark Received 2019 May 24; revised 2019 September 10; accepted 2019 September 13; published 2019 October 22 Abstract We report the serendipitous detection of two 3 mm continuum sources found in deep ALMA Band 3 observations to study intermediate-redshift galaxies in the COSMOS field. One is near a foreground galaxy at 1 3, but is a previously unknown dust-obscured star-forming galaxy (DSFG) at probable zCO=3.329, illustrating the risk of misidentifying shorter wavelength counterparts. The optical-to-millimeter spectral energy distribution (SED) favors a gray λ−0.4 attenuation curve and results in significantly larger stellar mass and SFR compared to a Calzetti starburst law, suggesting caution when relating progenitors and descendants based on these quantities. The other source is missing from all previous optical/near-infrared/submillimeter/radio catalogs (“ALMA-only”), and remains undetected even in stacked ultradeep optical (>29.6 AB) and near-infrared (>27.9 AB) images. Using the ALMA position as a prior reveals faint signal-to-noise ratio∼3 measurements in stacked IRAC 3.6+4.5, ultradeep SCUBA2 850 μm, and VLA 3 GHz, indicating the source is real. The SED is robustly reproduced by a * 10.8 11 −1 massive M =10 Me and Mgas=10 Me, highly obscured AV∼4, star-forming SFR∼300 Me yr galaxy at redshift z=5.5±1.1. The ultrasmall 8 arcmin2 survey area implies a large yet uncertain contribution to the −2 −1 −3 cosmic star formation rate density CSFRD(z=5)∼0.9×10 Me yr Mpc , comparable to all ultraviolet- selected galaxies combined. These results indicate the existence of a prominent population of DSFGs at z>4, below the typical detection limit of bright galaxies found in single-dish submillimeter surveys, but with larger space densities ∼3×10−5 Mpc−3, higher duty cycles of 50%–100%, contributing more to the CSFRD, and potentially dominating the high-mass galaxy stellar mass function. Key words: galaxies: evolution – galaxies: high-redshift – galaxies: starburst 1. Introduction (Spilker et al. 2016;Zavalaetal.2018b), the lensing correction In past decades, single-dish submillimeter surveys have and selection effects make it challenging to establish their identified populations of massive, dusty star-forming galaxies at contribution to the CSFRD. Progress is hampered by the limited > ( ) sensitivity and low spatial resolution of single-dish submillimeter z 1 e.g., Casey et al. 2014 . While these galaxies are rare even fi ( < z < ) observations and the dif culty of associating detections with at Cosmic Noon 1 3 when the star formation activity in – ( ) the universe peaks, their contribution to the cosmic star formation counterparts in the optical near-infrared NIR . Ultradeep ∼ 2 rate density (CSFRD) equals that of all optical and near-infrared SCUBA surveys over moderate 100 arcmin are now pushing “ ” ( −1 selected galaxies combined (Madau & Dickinson 2014).How- into the range of normal SFRs several 100 Me yr ,main- ever, at z>3 the situation is much less certain. A tail of sequence galaxies; e.g., Koprowski et al. 2016;Cowieetal. ) > submillimeter-selected galaxies have been confirmed beyond 2017, 2018 and extending to z 4, but the analysis is often z>4, but they trace only the very tip of the star formation rate limited by the ability to identify counterparts at other wavelengths (SFR) distribution at early times (e.g., Cooray et al. 2014;Strandet and derive accurate redshifts. et al. 2017; Marrone et al. 2018). The total contribution of dust- ALMA has opened an avenue for addressing this issue obscured star formation, and therefore the census of star formation through surveys at superior sensitivity and spatial resolution. in the early universe, is unknown. Despite the strongly negative ALMA deep fields at ∼1mm(Aravena et al. 2016; Dunlop k-correction allowing sources to be found to z=10, the et al. 2017) have probed to extremely deep flux limits over 2 overwhelming majority of (sub)millimeter-selected galaxies small areas (<5 arcmin ). Progress has still been limited likely continue to be confirmed at z<3 with Atacama Large because dust-obscured star formation preferentially occurs in Millimeter/submillimeter Array (ALMA) spectroscopy (Brisbin massive galaxies (Whitaker et al. 2017), which are clustered −2 et al. 2017; Danielson et al. 2017). While some dusty galaxies and relatively rare (∼0.1 arcmin at log()MM > 10.8 and have been discovered beyond z>5 through gravitational lensing z∼4; Davidzon et al. 2017). Wider (10ʼs arcmin2) and shallower (∼100–200 μJy) ALMA surveys at ∼1mm(Franco 11 NSF Fellow. et al. 2018; Hatsukade et al. 2018) are now approaching large 1 The Astrophysical Journal, 884:154 (13pp), 2019 October 20 Williams et al. enough areas to identify tentative massive candidates at z>4 2.2. ALMA Source Detection (e.g., Yamaguchi et al. 2019). Interferometric maps without correction for the primary A promising development is to select at longer wavelengths (> ) beam response have uniform, normally distributed noise 2mm, which optimizes the selection to dusty star formation fi fi at redshift z>4 (Béthermin et al. 2015; Casey et al. 2018). properties across the eld, and source detection signi cance However, the current state-of-the-art ALMA Spectroscopic is straightforward to measure from such maps. Two blind 3 mm Survey (ASPECS) at 3 mm (González-López et al. 2019), still continuum sources were apparent in this map, located 24 6 and only covered ∼5 arcmin2, and identified 6 continuum sources, 38 2 from the phase center, corresponding to primary beam all at z<3. Larger archival studies of ALMA 3 mm response levels of 0.57 and 0.29, respectively. Each source is ( / ) ∼ observations to find high-redshift candidates report some detected at a peak signal-to-noise ratio S N of 8; the fi fl spectroscopic confirmations, but like the 1 mm redshift probability of nding a Gaussian noise uctuation of this z< ( ) magnitude given the number of independent beams in the distribution, the majority lie at 3 Zavala et al. 2018a . −9 Recent advances with IRAM/GISMO provide evidence that images is exceedingly low (<10 ). Both sources thus are real. 2 mm surveys favor selecting higher-redshift sources (Magnelli The 3 mm flux densities of these sources, corrected for the et al. 2019), although counterpart identification continues to be primary beam response, are 155±20 μJy and 75±10, problematic due to large beam sizes. Overall, the number of hereafter referred to as 3MM-1 and 3MM-2, respectively. strong candidates for dust-obscured sources at z>4 remains Neither source is spatially resolved, based on a comparison of small and as a result, the contribution of dust-obscured star the peak pixel values and the integrated flux densities. formation in the early universe is poorly constrained.