Spectral Energy Distributions of Companion Galaxies to z ∼ 6 Quasars

C. Mazzucchelli, R. Decarli, E. P. Farina, E. Bañados, B. P. Venemans, M. A. Strauss, F. Walter, M. Neeleman, F. Bertoldi, X. Fan, D. Riechers, H. W. Rix, R. Wang

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Massive, quiescent galaxies are already observed at redshift z ∼ 4, i.e., ∼1.5 Gyr after the big bang. Current models predict them to be formed via massive, gas-rich mergers at z > 6. Recent ALMA observations of the cool gas and dust in z 6 quasars have discovered [C ii]- and far-infrared-bright galaxies adjacent to several quasars. In this work, we present sensitive imaging and spectroscopic follow-up observations, with HST/WFC3, Spitzer/IRAC, VLT/MUSE, Magellan/FIRE, and LBT/LUCI-MODS, of ALMA-detected, dust-rich companion galaxies of four quasars at z 6, specifically acquired to probe their stellar content and unobscured star formation rate. Three companion galaxies do not show significant emission in the observed optical/IR wavelength range. The photometric limits suggest that these galaxies are highly dust-enshrouded, with unobscured star formation rates SFRUV < few M o yr-1, and a stellar content of M < 1010 M o yr-1. However, the companion to PJ167-13 shows bright rest-frame UV emission (F140W AB = 25.48). Its spectral energy distribution resembles that of a star-forming galaxy with a total SFR ∼ 50 M o yr-1 and M ∼ 9 × 109 M o. All the companion sources are consistent with residing on the galaxy main sequence at z ∼ 6. Additional, deeper data from future facilities, such as the James Webb Space Telescope, are needed in order to characterize these gas-rich sources in the first gigayear of cosmic history.

Original languageEnglish (US)
Article number163
JournalAstrophysical Journal
Issue number2
StatePublished - Aug 20 2019

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • cosmology: observations
  • early universe
  • galaxies: high-redshift
  • quasars: general


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