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• An imperfectly passive nature : bright submillimeter emission from dust-obscured star formation in the z = 3.717 “passive” system, ZF 20115.

# An imperfectly passive nature : bright submillimeter emission from dust-obscured star formation in the z = 3.717 “passive” system, ZF 20115.

Simpson, J. M. and Smail, Ian and Wang, Wei-Hao and Riechers, D. and Dunlop, J. S. and Ao, Y. and Bourne, N. and Bunker, A. and Chapman, S. C. and Chen, Chian-Chou and Dannerbauer, H. and Geach, J. E. and Goto, T. and Harrison, C. M. and Hwang, H. S. and Ivison, R. J. and Kodama, Tadayuki and Lee, C.-H. and Lee, H.-M. and Lee, M. and Lim, C.-F. and Michałowski, M. J. and Rosario, D. J. and Shim, H. and Shu, X. W. and Swinbank, A. M. and Tee, W.-L. and Toba, Y. and Valiante, E. and Wang, Junxian and Zheng, X. Z. (2017) 'An imperfectly passive nature : bright submillimeter emission from dust-obscured star formation in the z = 3.717 “passive” system, ZF 20115.', Astrophysical journal letters, 844 (1). L10.

## Abstract

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 0farcs4 ± 0farcs1 (~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 ${100}_{-70}^{+15}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$, is undetected in the rest-frame ultraviolet but contributes significantly to the lower angular resolution photometry at rest-frame wavelengths gsim3500 Å. 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.