A low [CII]/[NII] ratio in the center of a massive galaxy at z = 3.7: Evidence for a transition to quiescence at high redshift?

¹ Astrophysics, Department of Physics, Keble Road, Oxford OX1 3RH, UK
e-mail: cschreib@orange.fr
² Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia
³ George P. and Cynthia W. Mitchell Institute for Fundamental Physics and Astronomy, Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA
⁴ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Ejército Libertador 441, Santiago 8320000, Chile
⁵ AIM-Paris-Saclay, CEA/DSM/Irfu – CNRS – Université Paris Diderot, CEA-Saclay, Pt Courrier 131, 91191 Gif-sur-Yvette, France
⁶ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
⁷ Department of Astronomy, University of Geneva, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
⁸ Faculty of Physics, Ludwig-Maximilians Universität, Scheinerstr. 1, 81679 Munich, Germany
⁹ Research Centre for Astronomy, Astrophysics & Astrophotonics, Macquarie University, Sydney, NSW 2109, Australia
¹⁰ Department of Physics & Astronomy, Macquarie University, Sydney, NSW 2109, Australia
¹¹ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹² Australia Telescope National Facility, CSIRO Astronomy and Space Science, PO Box 76 Epping, NSW 1710, Australia
¹³ School of Physics, University of New South Wales, Sydney, NSW 2052, Australia
¹⁴ Institute of Astronomy, The University of Tokyo, Osawa, Mitaka, Tokyo 181-0015, Japan
¹⁵ National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
¹⁶ Chinese Academy of Sciences South America Center for Astronomy (CASSACA), National Astronomical Observatories, CAS, Beijing 100101, PR China
¹⁷ Institute of Astrophysics, Foundation for Research and Technology–Hellas (FORTH), Heraklion 70013, Greece

Received: 5 August 2019
Accepted: 26 November 2020

Abstract

Understanding the process of quenching is one of the major open questions in galaxy evolution and crucial insights may be obtained by studying quenched galaxies at high redshifts at epochs when the Universe and the galaxies were younger and simpler to model. However, establishing the degree of quiescence in high-redshift galaxies is a challenging task. One notable example is Hyde, a recently discovered galaxy at z_spec = 3.709. Equally as compact (r_1/2 ∼ 0.5 kpc) and massive (M_* ∼ 10¹¹ M_⊙) as its quenched neighbor Jekyll, it is also extremely obscured yet only moderately luminous in the sub-millimeter. Panchromatic modeling has suggested it could be the first galaxy found in transition to quenching at z >  3, however, the data are also consistent with a broad range of star-formation activity, from fully quenched to moderate star-formation rates (SFR) in the lower scatter of the galaxy main-sequence. Here, we describe Atacama Large Millimeter Array observations of the [C II] 157 μm and [N II] 205 μm far-infrared emission lines. The [C II] emission within the half-light radius is dominated by ionized gas, while the outskirts are dominated by photo-dissociation regions or neutral gas. This suggests that the ionization in the center is not primarily powered by ongoing star formation, and is instead coming from remnant stellar populations formed in an older burst or from a moderate active galactic nucleus . Accounting for this information in the multi-wavelength modeling provides a tighter constraint on the star formation rate of SFR = 50₋₁₈⁺²⁴ M_⊙ yr⁻¹ yr⁻¹. This rules out fully quenched solutions and favors SFRs more than factor of two lower than expected for a main-sequence galaxy, confirming the nature of Hyde as a transition galaxy. These results suggest that quenching happens from inside-out and starts before the galaxy expels or consumes all its gas reservoirs. Similar observations of a sample of massive and obscured galaxies would determine whether this is an isolated case or the norm for quenching at high redshift.