An Imperfectly Passive Nature: Bright Submillimeter Emission from Dust-obscured Star Formation in the z = 3.717 "passive" System, ZF 20115

J. M. Simpson, Ian Smail, Wei Hao Wang, D. Riechers, J. S. Dunlop, Y. Ao, N. Bourne, A. Bunker, S. C. Chapman, Chian Chou Chen, H. Dannerbauer, J. E. Geach, T. Goto, C. M. Harrison, H. S. Hwang, R. J. Ivison, Tadayuki Kodama, C. H. Lee, H. M. Lee, M. LeeC. F. Lim, M. J. Michałowski, D. J. Rosario, H. Shim, X. W. Shu, A. M. Swinbank, W. L. Tee, Y. Toba, E. Valiante, Junxian Wang, X. Z. Zheng

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34 Citations (Scopus)


The identification of high-redshift, massive galaxies with old stellar populations may pose challenges to some models of galaxy formation. However, to securely classify a galaxy as quiescent, it is necessary to exclude significant ongoing star formation, something that can be challenging to achieve at high redshifts. In this Letter, we analyze deep ALMA/870 μm and SCUBA-2/450 μm imaging of the claimed "post-starburst" galaxy ZF 20115 at z = 3.717 that exhibits a strong Balmer break and absorption lines. The rest-frame far-infrared imaging identifies a luminous starburst 0.″4 0.″1 (∼3 kpc in projection) from the position of the ultraviolet/optical emission and is consistent with lying at the redshift of ZF 20115. The star-forming component, with an obscured star formation rate of 100+15 -70M⊙ yr-1, is undetected in the rest-frame ultraviolet but contributes significantly to the lower angular resolution photometry at rest-frame wavelengths 3500. This contribution from the obscured starburst, especially in the Spitzer/IRAC wavebands, significantly complicates the determination of a reliable stellar mass for the ZF 20015 system, and we conclude that this source does not pose a challenge to current models of galaxy formation. The multi-wavelength observations of ZF 20115 unveil a complex system with an intricate and spatially varying star formation history. ZF 20115 demonstrates that understanding high-redshift obscured starbursts will only be possible with multi-wavelength studies that include high-resolution observations, available with the James Webb Space Telescope, at mid-infrared wavelengths.

Original languageEnglish
Article numberL10
JournalAstrophysical Journal Letters
Issue number1
Publication statusPublished - 2017 Jul 20


  • galaxies: high-redshift
  • galaxies: starburst


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